From d570a023e6e024e9c30ab13dee2532eb15681588 Mon Sep 17 00:00:00 2001
From: Iwao AVE! バッチ更新用に JDBC ドライバ内に蓄積されたステートメントを任意のタイミングでデータベースへフラッシュ(実行)するメソッドがあります。このメソッドは、 トランザクションのスコープを制御するメソッドは4つあります。当然ですが、auto-commit を使用する場合や、外部のトランザクションマネージャーを使っている場合、これらのメソッドは効果がありません。しかし、Connection のインスタンスによって管理されている JDBC トランザクションマネージャーを利用している場合は便利なメソッドです。 デフォルトでは、データベースが insert, update, delete メソッドの実行によって変更されない限り MyBatis は commit を実行しません。何らかの理由でこれらのメソッドを使わずにデータを変更した場合は確実にコミットされるように commit メソッドに引数 true を渡してください(ただし、auto-commit モードのセッションや外部のトランザクションマネージャーを使っている場合は true を渡してもコミットされません)。commit が実行されない場合、MyBatis がロールバックを実行するので、通常明示的に rollback() メソッドを呼び出す必要はありません。しかし、一つのセッションの中で複数のコミットやロールバックが必要とされるようなケースでは、rollback() メソッドを使ってより細かい制御を行うことが可能です。 デフォルトでは、データベースの変更を伴うメソッド insert, update, delete, NOTE Mybatis-Spring と MyBatis-Guice では宣言的トランザクションがサポートされています。詳細は各サブプロジェクトのドキュメントを参照してください。
+ 例外として、INSERT, UPDATE, DELETE 文から ResultSet を返す SQL 文(PostgreSQL, MariaDB の
+ * e.g. RETURNING of PostgreSQL or OUTPUT of MS SQL Server.
+ *
+ * @return {@code true} if this select affects DB data; {@code false} if otherwise
+ * @since 3.5.12
+ */
+ boolean affectData() default false;
+
/**
* The container annotation for {@link Select}.
* @author Kazuki Shimizu
diff --git a/src/main/java/org/apache/ibatis/annotations/SelectProvider.java b/src/main/java/org/apache/ibatis/annotations/SelectProvider.java
index f109ebb43de..282374c6459 100644
--- a/src/main/java/org/apache/ibatis/annotations/SelectProvider.java
+++ b/src/main/java/org/apache/ibatis/annotations/SelectProvider.java
@@ -99,6 +99,15 @@
*/
String databaseId() default "";
+ /**
+ * Returns whether this select affects DB data.
+ * e.g. RETURNING of PostgreSQL or OUTPUT of MS SQL Server.
+ *
+ * @return {@code true} if this select affects DB data; {@code false} if otherwise
+ * @since 3.5.12
+ */
+ boolean affectData() default false;
+
/**
* The container annotation for {@link SelectProvider}.
* @author Kazuki Shimizu
diff --git a/src/main/java/org/apache/ibatis/builder/MapperBuilderAssistant.java b/src/main/java/org/apache/ibatis/builder/MapperBuilderAssistant.java
index 993031c997d..4c9341a16a9 100644
--- a/src/main/java/org/apache/ibatis/builder/MapperBuilderAssistant.java
+++ b/src/main/java/org/apache/ibatis/builder/MapperBuilderAssistant.java
@@ -261,7 +261,8 @@ public MappedStatement addMappedStatement(
String keyColumn,
String databaseId,
LanguageDriver lang,
- String resultSets) {
+ String resultSets,
+ boolean dirtySelect) {
if (unresolvedCacheRef) {
throw new IncompleteElementException("Cache-ref not yet resolved");
@@ -285,7 +286,8 @@ public MappedStatement addMappedStatement(
.resultSetType(resultSetType)
.flushCacheRequired(flushCache)
.useCache(useCache)
- .cache(currentCache);
+ .cache(currentCache)
+ .dirtySelect(dirtySelect);
ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
if (statementParameterMap != null) {
@@ -344,12 +346,24 @@ public MappedStatement addMappedStatement(String id, SqlSource sqlSource, Statem
SqlCommandType sqlCommandType, Integer fetchSize, Integer timeout, String parameterMap, Class> parameterType,
String resultMap, Class> resultType, ResultSetType resultSetType, boolean flushCache, boolean useCache,
boolean resultOrdered, KeyGenerator keyGenerator, String keyProperty, String keyColumn, String databaseId,
- LanguageDriver lang) {
+ LanguageDriver lang, String resultSets) {
return addMappedStatement(
id, sqlSource, statementType, sqlCommandType, fetchSize, timeout,
parameterMap, parameterType, resultMap, resultType, resultSetType,
flushCache, useCache, resultOrdered, keyGenerator, keyProperty,
- keyColumn, databaseId, lang, null);
+ keyColumn, databaseId, lang, null, false);
+ }
+
+ public MappedStatement addMappedStatement(String id, SqlSource sqlSource, StatementType statementType,
+ SqlCommandType sqlCommandType, Integer fetchSize, Integer timeout, String parameterMap, Class> parameterType,
+ String resultMap, Class> resultType, ResultSetType resultSetType, boolean flushCache, boolean useCache,
+ boolean resultOrdered, KeyGenerator keyGenerator, String keyProperty, String keyColumn, String databaseId,
+ LanguageDriver lang) {
+ return addMappedStatement(
+ id, sqlSource, statementType, sqlCommandType, fetchSize, timeout,
+ parameterMap, parameterType, resultMap, resultType, resultSetType,
+ flushCache, useCache, resultOrdered, keyGenerator, keyProperty,
+ keyColumn, databaseId, lang, null);
}
private ExecutorType
として ExecutorType.BATCH
を使用している場合に使用することができます。トランザクションを制御するメソッド
+ トランザクションを制御するメソッド
affectData
を有効化した select が実行されない限り MyBatis は commit を実行しません。何らかの理由でこれらのメソッドを使わずにデータを変更した場合は確実にコミットされるように commit メソッドに引数 true を渡してください(ただし、auto-commit モードのセッションや外部のトランザクションマネージャーを使っている場合は true を渡してもコミットされません)。commit が実行されない場合、MyBatis がロールバックを実行するので、通常明示的に rollback() メソッドを呼び出す必要はありません。しかし、一つのセッションの中で複数のコミットやロールバックが必要とされるようなケースでは、rollback() メソッドを使ってより細かい制御を行うことが可能です。ローカルキャッシュ
diff --git a/src/site/ja/xdoc/sqlmap-xml.xml b/src/site/ja/xdoc/sqlmap-xml.xml
index 27b7b92f571..eae6ca58207 100644
--- a/src/site/ja/xdoc/sqlmap-xml.xml
+++ b/src/site/ja/xdoc/sqlmap-xml.xml
@@ -233,6 +233,11 @@ ps.setInt(1,id);]]>
複数の ResultSet を利用する場合にのみ有効です。ステートメントが返す ResultSet にそれぞれ任意の名前を付けてリストアップします。名前はカンマで区切ります。
+
+
+ affectData
ResultSet を返す INSERT, UPDATE, DELETE 文を記述する場合に true をセットします。これによりトランザクション制御が正しく実行されるようになります。トランザクションを制御するメソッド も参照してください。 デフォルト:
+ false
(3.5.12 以降)
+ RETURNING
, MS SQL Server の OUTPUT
など)で結果をマップするためには ]]>
を使用する必要があります。
+ ExecutorType
을 ExecutorType.BATCH
로 설정한 경우 사용가능하다.
트랜잭션을 제어하기 위해 4개의 메소드가 있다. 물론 자동커밋을 선택하였거나 외부 트랜잭션 관리자를 사용하면 영향이 없다. 어쨌든 Connection인스턴스에 의해 관리되고 JDBC 트랜잭션 관리자를 사용하면 이 4개의 메소드를 사용할 수 있다.
@@ -324,7 +324,7 @@ public interface ResultHandler기본적으로 마이바티스는 insert, update 또는 delete 를 호출하여 데이터베이스가 변경된 것으로 감지하지 않는 한 실제로 커밋하지 않는다. +
기본적으로 마이바티스는 insert, update, delete 또는 affectData
가 활성화된select 를 호출하여 데이터베이스가 변경된 것으로 감지하지 않는 한 실제로 커밋하지 않는다.
이러한 메소드 호출없이 변경되면 커밋된 것으로 보장하기 위해 commit 와 rollback 메소드에 true 값을 전달한다.
참고 MyBatis-Spring 과 MyBatis-Guice는 선언적인 트랜잭션 관리기법을 제공한다. 그래서 스프링이나 쥬스와 함께 마이바티스를 사용한다면 해당되는 메뉴얼을 꼭 참고하길 바란다.
diff --git a/src/site/ko/xdoc/sqlmap-xml.xml b/src/site/ko/xdoc/sqlmap-xml.xml index 70a6615a425..4ddde0df518 100644 --- a/src/site/ko/xdoc/sqlmap-xml.xml +++ b/src/site/ko/xdoc/sqlmap-xml.xml @@ -188,6 +188,11 @@ ps.setInt(1,id);]]> 디폴트값은false
이다.
+ affectData
false
(since 3.5.12)
+
+ As an irregular case, some databases allow INSERT, UPDATE or DELETE statement to return result set (e.g. RETURNING
clause of PostgreSQL and MariaDB or OUTPUT
clause of MS SQL Server). This type of statement must be written as ]]>
to map the returned data.
+
There is method for flushing (executing) batch update statements that are stored in a JDBC driver class at any time. This method can be used when the ExecutorType
is ExecutorType.BATCH
.
There are four methods for controlling the scope of a transaction. Of course, these have no effect if you've chosen to use auto-commit or if you're using an external transaction manager. However, if you're using the JDBC transaction manager, managed by the Connection
instance, then the four methods that will come in handy are:
By default MyBatis does not actually commit unless it detects that the database has been changed by a call to insert
, update
or delete
. If you've somehow made changes without calling these methods, then you can pass true
into the commit
and rollback
methods to guarantee that they will be committed (note, you still can't force a session in auto-commit mode, or one that is using an external transaction manager). Most of the time you won't have to call rollback()
, as MyBatis will do that for you if you don't call commit. However, if you need more fine-grained control over a session where multiple commits and rollbacks are possible, you have the rollback option there to make that possible.
By default MyBatis does not actually commit unless it detects that the database has been changed by a call to insert
, update
, delete
or select
with affectData
enabled. If you've somehow made changes without calling these methods, then you can pass true
into the commit
and rollback
methods to guarantee that they will be committed (note, you still can't force a session in auto-commit mode, or one that is using an external transaction manager). Most of the time you won't have to call rollback()
, as MyBatis will do that for you if you don't call commit. However, if you need more fine-grained control over a session where multiple commits and rollbacks are possible, you have the rollback option there to make that possible.
NOTE MyBatis-Spring and MyBatis-Guice provide declarative transaction handling. So if you are using MyBatis with Spring or Guice please refer to their specific manuals.
affectData
false
(since 3.5.12)
+
+ As an irregular case, some databases allow INSERT, UPDATE or DELETE statement to return result set (e.g. RETURNING
clause of PostgreSQL and MariaDB or OUTPUT
clause of MS SQL Server). This type of statement must be written as ]]>
to map the returned data.
+
当你将 ExecutorType
设置为 ExecutorType.BATCH
时,可以使用这个方法清除(执行)缓存在 JDBC 驱动类中的批量更新语句。
有四个方法用来控制事务作用域。当然,如果你已经设置了自动提交或你使用了外部事务管理器,这些方法就没什么作用了。然而,如果你正在使用由 Connection 实例控制的 JDBC 事务管理器,那么这四个方法就会派上用场:
@@ -253,7 +253,7 @@ public interface ResultHandler默认情况下 MyBatis 不会自动提交事务,除非它侦测到调用了插入、更新或删除方法改变了数据库。如果你没有使用这些方法提交修改,那么你可以在 commit 和 rollback 方法参数中传入 true 值,来保证事务被正常提交(注意,在自动提交模式或者使用了外部事务管理器的情况下,设置 force 值对 session 无效)。大部分情况下你无需调用 rollback(),因为 MyBatis 会在你没有调用 commit 时替你完成回滚操作。不过,当你要在一个可能多次提交或回滚的 session 中详细控制事务,回滚操作就派上用场了。
+默认情况下 MyBatis 不会自动提交事务,除非它侦测到调用了插入、更新、删除或 select with affectData
enabled 方法改变了数据库。如果你没有使用这些方法提交修改,那么你可以在 commit 和 rollback 方法参数中传入 true 值,来保证事务被正常提交(注意,在自动提交模式或者使用了外部事务管理器的情况下,设置 force 值对 session 无效)。大部分情况下你无需调用 rollback(),因为 MyBatis 会在你没有调用 commit 时替你完成回滚操作。不过,当你要在一个可能多次提交或回滚的 session 中详细控制事务,回滚操作就派上用场了。
提示 MyBatis-Spring 和 MyBatis-Guice 提供了声明式事务处理,所以如果你在使用 Mybatis 的同时使用了 Spring 或者 Guice,请参考它们的手册以获取更多的内容。
affectData
false
(since 3.5.12)
+
+ As an irregular case, some databases allow INSERT, UPDATE or DELETE statement to return result set (e.g. RETURNING
clause of PostgreSQL and MariaDB or OUTPUT
clause of MS SQL Server). This type of statement must be written as ]]>
to map the returned data.
+
false
.
+ resultSets
affectData
false
(since 3.5.12)
diff --git a/src/site/ko/xdoc/sqlmap-xml.xml b/src/site/ko/xdoc/sqlmap-xml.xml
index 4ddde0df518..d49d9cd7ef2 100644
--- a/src/site/ko/xdoc/sqlmap-xml.xml
+++ b/src/site/ko/xdoc/sqlmap-xml.xml
@@ -188,6 +188,12 @@ ps.setInt(1,id);]]>
디폴트값은 false
이다.
resultSets
affectData
false
(since 3.5.12)
From 5911cb6476a707992536c8c7d1c1785131bac409 Mon Sep 17 00:00:00 2001
From: Iwao AVE! MyBatis incluye dos tipos de TransactionManager (ej. type=”[JDBC|MANAGED]”):
마이바티스는 두 가지 타입의 TransactionManager를 제공한다.
- Every MyBatis application centers around an instance of - SqlSessionFactory. A SqlSessionFactory instance can be acquired by - using the SqlSessionFactoryBuilder. SqlSessionFactoryBuilder can - build a SqlSessionFactory instance from an XML configuration file, - or from a custom prepared instance of the Configuration class. -
-- Building a SqlSessionFactory instance from an XML file is very - simple. It is recommended that you use a classpath resource for - this configuration, but you could use any InputStream instance, - including one created from a literal file path or a file:// URL. - MyBatis includes a utility class, called Resources, that contains - a number of methods that make it simpler to load resources from - the classpath and other locations. -
- -- The configuration XML file contains settings for the core of the - MyBatis system, including a DataSource for acquiring database - Connection instances, as well as a TransactionManager for - determining how transactions should be scoped and controlled. The - full details of the XML configuration file can be found later in - this document, but here is a simple example: -
- -- While there is a lot more to the XML configuration file, the - above example points out the most critical parts. Notice the - XML header, required to validate the XML document. The body - of the environment element contains the environment - configuration for transaction management and connection - pooling. The mappers element contains a list of mappers – - the XML files and/or annotated Java interface classes that - contain the SQL code and mapping definitions. -
-- If you prefer to directly build the configuration from Java, rather - than XML, or create your own configuration builder, MyBatis provides - a complete Configuration class that provides all of the same - configuration options as the XML file. -
- -- Notice in this case the configuration is adding a mapper class. - Mapper classes are Java classes that - contain SQL Mapping Annotations - that avoid the need for XML mapping. However, due - to some limitations of - Java Annotations and the complexity of some MyBatis mappings, XML - mapping is still required for the - most advanced mappings (e.g. - Nested Join Mapping). For this reason, - MyBatis will automatically - look - for and load a peer XML file if it exists (in this case, - BlogMapper.xml would be loaded based on the - classpath and name of - BlogMapper.class). More on this later. -
-- Now that you have a SqlSessionFactory, as the name suggests, you - can acquire an instance of - SqlSession. The SqlSession contains absolutely every method needed to execute - SQL commands against - the database. You can execute mapped SQL statements directly against - the SqlSession instance. For - example: -
- -- While this approach works, and is familiar to users of previous - versions of MyBatis, there is now a - cleaner approach. Using an interface (e.g. BlogMapper.class) that properly - describes the parameter and - return value for a given statement, you can now execute cleaner and more - type safe code, without error - prone string literals and casting. -
-- For example: -
- -- Now let's explore what exactly is being executed here. -
-- At this point you may be wondering what exactly is being executed by - the SqlSession or Mapper class. The topic of Mapped SQL Statements - is a big one, and that topic will likely dominate the majority of - this documentation. But to give you an idea of what exactly is being - run, here are a couple of examples. -
-- In either of the examples above, the statements could have been - defined by either XML or Annotations. Let's take a look at XML - first. The full set of features provided by MyBatis can be realized - by using the XML based mapping language that has made MyBatis - popular over the years. If you've used MyBatis before, the concept - will be familiar to you, but there have been numerous improvements - to the XML mapping documents that will become clear later. Here is - an example of an XML based mapped statement that would satisfy the - above SqlSession calls. -
- -- While this looks like a lot of overhead for this simple example, it is - actually very light. You can define as many mapped statements in a - single mapper XML file as you like, so you get a lot of mileage out - of the XML header and doctype declaration. The rest of the file is - pretty self explanatory. It defines a name for the mapped statement - “selectBlog”, in the namespace “org.mybatis.example.BlogMapper”, - which would allow you to call it by specifying the fully qualified - name of “org.mybatis.example.BlogMapper.selectBlog”, as we did above - in the following example: -
- -- Notice how similar this is to calling a method on a fully qualified Java - class, and there's a reason for that. This name can be directly - mapped to a Mapper class of the same name as the namespace, with a - method that matches the name, parameter, and return type as the - mapped select statement. This allows you to very simply call the - method against the Mapper interface as you saw above, but here it is - again in the following example: -
- -- The second approach has a lot of advantages. First, it doesn't depend - on a string literal, so it's much safer. Second, if your IDE has - code completion, you can leverage that when navigating your mapped - SQL statements. -
-- NOTE - A note about namespaces. -
-- Namespaces - were optional in previous versions of MyBatis, which was confusing - and unhelpful. Namespaces are now required and have a purpose beyond - simply isolating statements with longer, fully-qualified names. -
-- Namespaces enable the interface bindings as you see here, and even if you - don’t think you’ll use them today, you should follow these practices - laid out here in case you change your mind. Using the namespace - once, and putting it in a proper Java package namespace will clean - up your code and improve the usability of MyBatis in the long term. -
-- Name Resolution: To reduce the amount of typing, MyBatis uses the - following name resolution rules for all named configuration - elements, including statements, result maps, caches, etc. -
-- There's one more trick to Mapper classes like BlogMapper. Their mapped - statements don't need to be mapped with XML at all. Instead they can - use Java Annotations. For example, the XML above could be eliminated - and replaced with: -
- -- The annotations are a lot cleaner for simple statements, however, Java - Annotations are both limited and messier for more complicated - statements. Therefore, if you have to do anything complicated, - you're better off with XML mapped statements. -
-- It will be up to you and your project team to determine which is right - for you, and how important it is to you that your mapped statements - be defined in a consistent way. That said, you're never locked into - a single approach. You can very easily migrate Annotation based - Mapped Statements to XML and vice versa. -
-- It's very important to understand the various scopes and lifecycles - classes we've discussed so far. Using them incorrectly can cause - severe concurrency problems. -
- -NOTE - Object lifecycle and Dependency Injection Frameworks -
-- Dependency Injection frameworks can create thread safe, transactional SqlSessions and mappers - and inject them directly into your beans so you can just forget about their lifecycle. - You may want to have a look at MyBatis-Spring or - MyBatis-Guice sub-projects to know more about using MyBatis with DI frameworks. -
-- This class can be instantiated, used and thrown away. There is no need - to keep it around once you've created your SqlSessionFactory. - Therefore the best scope for instances of SqlSessionFactoryBuilder - is method scope (i.e. a local method variable). You can reuse the - SqlSessionFactoryBuilder to build multiple SqlSessionFactory - instances, but it's still best not to keep it around to ensure that - all of the XML parsing resources are freed up for more important - things. -
-- Once created, the SqlSessionFactory should exist for the duration of - your application execution. There should be little or no reason to - ever dispose of it or recreate it. It's a best practice to not - rebuild the SqlSessionFactory multiple times in an application run. - Doing so should be considered a “bad smell”. Therefore the best - scope of SqlSessionFactory is application scope. This can be - achieved a number of ways. The simplest is to use a Singleton - pattern or Static Singleton pattern. -
-- Each thread should have its own instance of SqlSession. Instances of - SqlSession are not to be shared and are not thread safe. Therefore - the best scope is request or method scope. Never keep references to - a SqlSession instance in a static field or even an instance field - of a class. Never keep references to a SqlSession in any sort of - managed scope, such as HttpSession of the Servlet framework. If - you're using a web framework of any sort, consider the SqlSession - to follow a similar scope to that of an HTTP request. In other - words, upon receiving an HTTP request, you can open a SqlSession, - then upon returning the response, you can close it. Closing the - session is very important. You should always ensure that it's - closed within a finally block. The following is the standard - pattern for ensuring that SqlSessions are closed: -
- -- Using this pattern consistently throughout your code will ensure that - all database resources are properly closed. -
-- Mappers are interfaces that you create to bind to your mapped statements. - Instances of the mapper interfaces are acquired from the - SqlSession. As such, technically the broadest scope of any mapper - instance is the same as the SqlSession from which they were - requested. However, the best scope for mapper instances is method - scope. That is, they should be requested within the method that - they are used, and then be discarded. They do not need to be closed - explicitly. While it's not a problem to keep them around throughout - a request, similar to the SqlSession, you might find that managing - too many resources at this level will quickly get out of hand. Keep - it simple, keep Mappers in the method scope. The following example - demonstrates this practice. -
- - -- MyBatis is a first class persistence framework with support for - custom SQL, - stored procedures and advanced mappings. MyBatis - eliminates almost - all of the JDBC code and manual setting of - parameters and retrieval - of results. MyBatis can use simple XML or - Annotations for - configuration and map primitives, Map interfaces and - Java POJOs - (Plain Old Java Objects) to database records. -
-If you are completely new to database access in Java,https://www.marcobehler.com/guides/a-guide-to-accessing-databases-in-java contains a good overview of the various parts, pieces and options and shows you where exactly MyBatis fits in. -
-- If you find this documentation lacking in any way, or missing - documentation for a feature, then the best thing to do is learn - about it and then write the documentation yourself! -
-- Sources of this manual are available in xdoc format at - project's Git - Fork the repository, update them and send a pull request. -
-- You’re the best author of this documentation, people like you have - to read it! -
-Users can read about MyBatis in following translations:
- -Do you want to read about MyBatis in your own native language? File an issue providing patches with your - mother tongue documentation!
-Now that you know how to configure MyBatis and create mappings, you're ready for the good stuff. The MyBatis Java API is where you get to reap the rewards of your efforts. As you'll see, compared to JDBC, MyBatis greatly simplifies your code and keeps it clean, easy to understand and maintain. MyBatis 3 has introduced a number of significant improvements to make working with SQL Maps even better.
- -Before we dive in to the Java API itself, it's important to understand the best practices surrounding directory structures. MyBatis is very flexible, and you can do almost anything with your files. But as with any framework, there's a preferred way.
-Let's look at a typical application directory structure:
-/my_application - /bin - /devlib - /lib <-- MyBatis *.jar files go here. - /src - /org/myapp/ - /action - /data <-- MyBatis artifacts go here, including, Mapper Classes, XML Configuration, XML Mapping Files. - /mybatis-config.xml - /BlogMapper.java - /BlogMapper.xml - /model - /service - /view - /properties <-- Properties included in your XML Configuration go here. - /test - /org/myapp/ - /action - /data - /model - /service - /view - /properties - /web - /WEB-INF - /web.xml-
Remember, these are preferences, not requirements, but others will thank you for using a common directory structure.
-The rest of the examples in this section will assume you're following this directory structure.
-The primary Java interface for working with MyBatis is the SqlSession. Through this interface you can execute commands, get mappers and manage transactions. We'll talk more about SqlSession itself shortly, but first we have to learn how to acquire an instance of SqlSession. SqlSessions are created by a SqlSessionFactory instance. The SqlSessionFactory contains methods for creating instances of SqlSessions all different ways. The SqlSessionFactory itself is created by the SqlSessionFactoryBuilder that can create the SqlSessonFactory from XML, annotations or hand coded Java configuration.
-NOTE When using MyBatis with a dependency injection framework like Spring or Guice, SqlSessions are created and injected by the DI framework so you don't need to use the SqlSessionFactoryBuilder or SqlSessionFactory and can go directly to the SqlSession section. Please refer to the MyBatis-Spring or MyBatis-Guice manuals for further info.
-The SqlSessionFactoryBuilder has five build() methods, each which allows you to build a SqlSessionFactory from a different source.
- - -The first four methods are the most common, as they take an InputStream instance that refers to an XML document, or more specifically, the mybatis-config.xml file discussed above. The optional parameters are environment and properties. Environment determines which environment to load, including the datasource and transaction manager. For example:
- - -If you call a build method that takes the environment parameter, then MyBatis will use the configuration for that environment. Of course, if you specify an invalid environment, you will receive an error. If you call one of the build methods that does not take the environment parameter, then the default environment is used (which is specified as default="development" in the example above).
-If you call a method that takes a properties instance, then MyBatis will load those properties and make them available to your configuration. Those properties can be used in place of most values in the configuration using the syntax: ${propName}
-Recall that properties can also be referenced from the mybatis-config.xml file, or specified directly within it. Therefore it's important to understand the priority. We mentioned it earlier in this document, but here it is again for easy reference:
- -If a property exists in more than one of these places, MyBatis loads them in the following order.
-Thus, the highest priority properties are those passed in as a method parameter, followed by resource/url attributes and finally the properties specified in the body of the properties element.
-So to summarize, the first four methods are largely the same, but with overrides to allow you to optionally specify the environment and/or properties. Here is an example of building a SqlSessionFactory from an mybatis-config.xml file.
- - - -Notice that we're making use of the Resources utility class, which lives in the org.apache.ibatis.io package. The Resources class, as its name implies, helps you load resources from the classpath, filesystem or even a web URL. A quick look at the class source code or inspection through your IDE will reveal its fairly obvious set of useful methods. Here's a quick list:
- - -The final build method takes an instance of Configuration. The Configuration class contains everything you could possibly need to know about a SqlSessionFactory instance. The Configuration class is useful for introspecting on the configuration, including finding and manipulating SQL maps (not recommended once the application is accepting requests). The configuration class has every configuration switch that you've learned about already, only exposed as a Java API. Here's a simple example of how to manually a Configuration instance and pass it to the build() method to create a SqlSessionFactory.
- - -Now you have a SqlSessionFactory that can be used to create SqlSession instances.
- -SqlSessionFactory has six methods that are used to create SqlSession instances. In general, the decisions you'll be making when selecting one of these methods are:
-The set of overloaded openSession() method signatures allow you to choose any combination of these options that makes sense.
- - -The default openSession() method that takes no parameters will create a SqlSession with the following characteristics:
-Connection
object will be acquired from the DataSource instance configured by the active environment.Most of the methods are pretty self explanatory. To enable auto-commit, pass a value of true
to the
- optional autoCommit
parameter.
- To provide your own connection, pass an instance of Connection
to the connection
parameter. Note that
- there's no override to set both the Connection
and autoCommit
, because MyBatis will use whatever setting
- the provided connection object is currently using. MyBatis uses a Java enumeration wrapper for transaction isolation
- levels, called TransactionIsolationLevel
, but otherwise they work as expected and have the 5 levels
- supported by JDBC (NONE
, READ_UNCOMMITTED
, READ_COMMITTED
,
- REPEATABLE_READ
, SERIALIZABLE
).
The one parameter that might be new to you is ExecutorType
. This enumeration defines 3 values:
ExecutorType.SIMPLE
: This type of executor does nothing special. It creates a new PreparedStatement for each execution of a statement.ExecutorType.REUSE
: This type of executor will reuse PreparedStatements.ExecutorType.BATCH
: This executor will batch all update statements and demarcate them as necessary if SELECTs are executed between them, to ensure an easy-to-understand behavior.NOTE There's one more method on the SqlSessionFactory that we didn't mention, and that is getConfiguration(). This method will return an instance of Configuration that you can use to introspect upon the MyBatis configuration at runtime.
-NOTE If you've used a previous version of MyBatis, you'll recall that sessions, transactions and batches were all something separate. This is no longer the case. All three are neatly contained within the scope of a session. You need not deal with transactions or batches separately to get the full benefit of them.
- -As mentioned above, the SqlSession instance is the most powerful class in MyBatis. It is where you'll find all of the methods to execute statements, commit or rollback transactions and acquire mapper instances.
-There are over twenty methods on the SqlSession class, so let's break them up into more digestible groupings.
- -These methods are used to execute SELECT, INSERT, UPDATE and DELETE statements that are defined in your SQL Mapping XML files. They are pretty self explanatory, each takes the ID of the statement and the Parameter Object, which can be a primitive (auto-boxed or wrapper), a JavaBean, a POJO or a Map.
- -The difference between selectOne
and selectList
is only in that selectOne
must return exactly one object or null
(none). If any more than one, an exception will be thrown. If you don't know how many objects are expected, use selectList
. If you want to check for the existence of an object, you're better off returning a count (0 or 1). The selectMap
is a special case in that it is designed to convert a list of results into a Map
based on one of the properties in the resulting objects. Because not all statements require a parameter, these methods are overloaded with versions that do not require the parameter object.
The value returned by the insert
, update
and delete
methods indicate the number of rows affected by the statement.
A Cursor
offers the same results as a List, except it fetches data lazily using an Iterator
.
Finally, there are three advanced versions of the select
methods that allow you to restrict the range of rows to return, or provide custom result handling logic, usually for very large data sets.
The RowBounds
parameter causes MyBatis to skip the number of records specified, as well as limit the number of results returned to some number. The RowBounds
class has a constructor to take both the offset
and limit
, and is otherwise immutable.
Different drivers are able to achieve different levels of efficiency in this regard. For the best performance, use result set types of SCROLL_SENSITIVE
or SCROLL_INSENSITIVE
(in other words: not FORWARD_ONLY
).
The ResultHandler
parameter allows you to handle each row however you like. You can add it to a List
, create a Map
, Set
, or throw each result away and instead keep only rolled up totals of calculations. You can do pretty much anything with the ResultHandler
, and it's what MyBatis uses internally itself to build result set lists.
Since 3.4.6, ResultHandler
passed to a CALLABLE
statement is used on every REFCURSOR
output parameter of the stored procedure if there is any.
The interface is very simple.
- - -The ResultContext
parameter gives you access to the result object itself, a count of the number of result objects created, and a Boolean stop()
method that you can use to stop MyBatis from loading any more results.
Using a ResultHandler
has two limitations that you should be aware of:
ResultHandler
will not be cached.resultMap
s MyBatis will probably require several rows to build an object. If a ResultHandler
is used you may be given an object whose associations or collections are not yet filled.There is method for flushing (executing) batch update statements that are stored in a JDBC driver class at any time. This method can be used when the ExecutorType
is ExecutorType.BATCH
.
There are four methods for controlling the scope of a transaction. Of course, these have no effect if you've chosen to use auto-commit or if you're using an external transaction manager. However, if you're using the JDBC transaction manager, managed by the Connection
instance, then the four methods that will come in handy are:
By default MyBatis does not actually commit unless it detects that the database has been changed by a call to insert
, update
, delete
or select
with affectData
enabled. If you've somehow made changes without calling these methods, then you can pass true
into the commit
and rollback
methods to guarantee that they will be committed (note, you still can't force a session in auto-commit mode, or one that is using an external transaction manager). Most of the time you won't have to call rollback()
, as MyBatis will do that for you if you don't call commit. However, if you need more fine-grained control over a session where multiple commits and rollbacks are possible, you have the rollback option there to make that possible.
NOTE MyBatis-Spring and MyBatis-Guice provide declarative transaction handling. So if you are using MyBatis with Spring or Guice please refer to their specific manuals.
- -MyBatis uses two caches: a local cache and a second level cache.
-Each time a new session is created MyBatis creates a local cache and attaches it to the session. Any query executed within the session will be stored in the local cache so further executions of the same query with the same input parameters will not hit the database. The local cache is cleared upon update
, commit
, rollback
and close
.
By default local cache data is used for the whole session duration. This cache is needed to resolve circular references and to speed up repeated nested queries, so it can never be completely disabled but you can configure the local cache to be used just for the duration of a statement execution by setting localCacheScope=STATEMENT
.
Note that when the localCacheScope
is set to SESSION
, MyBatis returns references to the same objects which are stored in the local cache. Any modification of the returned objects (lists etc.) influences the local cache contents and subsequently the values which are returned from the cache in the lifetime of the session. Therefore, as best practice, do not to modify the objects returned by MyBatis.
You can clear the local cache at any time by calling:
- - -The most important thing you must ensure is to close any session that you open. The best way to ensure this is to use the following unit of work pattern:
- -NOTE Just like SqlSessionFactory
, you can get the instance of Configuration
that the SqlSession
is using by calling the getConfiguration()
method.
While the various insert
, update
, delete
and select
methods above are powerful, they are also very verbose, not type safe and not as helpful to your IDE or unit tests as they could be. We've already seen an example of using Mapper
s in the Getting Started section above.
Therefore, a more common way to execute mapped statements is to use Mapper
classes. A Mapper
class is simply an interface with method definitions that match up against the SqlSession
methods. The following example class demonstrates some method signatures and how they map to the SqlSession
.
In a nutshell, each Mapper
method signature should match that of the SqlSession
method that it's associated to, but without the String
parameter ID. Instead, the method name must match the mapped statement ID.
In addition, the return type must match that of the expected result type for single results or an array or collection for multiple results or Cursor
. All of the usual types are supported, including: Primitives, Maps
, POJOs and JavaBeans
.
NOTE Mapper interfaces do not need to implement any interface or extend any class. As long as the method signature can be used to uniquely identify a corresponding mapped statement.
-NOTE Mapper interfaces can extend other interfaces. Be sure that you have the statements in the appropriate namespace when using XML binding to Mapper
interfaces. Also, the only limitation is that you cannot have the same method signature in two interfaces in a hierarchy (a bad idea anyway).
You can pass multiple parameters to a mapper method. If you do, they will be named by the literal "param" followed by their position in the parameter list by default, for example: #{param1}
, #{param2}
etc. If you wish to change the name of the parameters (multiple only), then you can use the @Param("paramName")
annotation on the parameter.
You can also pass a RowBounds
instance to the method to limit query results.
Since the very beginning, MyBatis has been an XML driven framework. The configuration is XML based, and the Mapped Statements are defined in XML. With MyBatis 3, there are new options available. MyBatis 3 builds on top of a comprehensive and powerful Java based Configuration API. This Configuration API is the foundation for the XML based MyBatis configuration, as well as the new annotation-based configuration. Annotations offer a simple way to implement simple mapped statements without introducing a lot of overhead.
-NOTE Java annotations are unfortunately limited in their expressiveness and flexibility. Despite a lot of time spent in investigation, design and trials, the most powerful MyBatis mappings simply cannot be built with annotations – without getting ridiculous that is. C# Attributes (for example) do not suffer from these limitations, and thus MyBatis.NET will enjoy a much richer alternative to XML. That said, the Java annotation-based configuration is not without its benefits.
-The annotations are as follows:
-Annotation | -Target | -XML equivalent | -Description | -
---|---|---|---|
@CacheNamespace |
- Class |
- <cache> |
-
- Configures the cache for the given namespace (i.e. class). Attributes: implementation ,
- eviction , flushInterval , size , readWrite ,
- blocking , properties .
- |
-
@Property |
- N/A | -<property> |
- Specifies the property value or placeholder(can replace by configuration properties that defined at the mybatis-config.xml ). Attributes: name , value . (Available on MyBatis 3.4.2+) |
-
@CacheNamespaceRef |
- Class |
- <cacheRef> |
-
- References the cache of another namespace to use. Note that caches declared in an XML mapper file are considered
- a separate namespace, even if they share the same FQCN. Attributes: value and name .
- If you use this annotation, you should be specified either value or name attribute.
- For the value attribute specify a java type indicating the namespace(the namespace name become a FQCN of specified java type),
- and for the name attribute(this attribute is available since 3.4.2) specify a name indicating the namespace.
- |
-
@ConstructorArgs |
- Method |
- <constructor> |
- Collects a group of results to be passed to a result object constructor. Attributes: value , which is an array of Arg s. |
-
@Arg |
- N/A | -
-
|
- A single constructor argument that is part of a ConstructorArgs collection. Attributes: id ,
- column , javaType , jdbcType , typeHandler , select ,
- resultMap . The id attribute is a boolean value that identifies the property to be used for comparisons,
- similar to the <idArg> XML element. Since 3.5.4, it can be used as repeatable annotation. |
-
@TypeDiscriminator |
- Method |
- <discriminator> |
- A group of value cases that can be used to determine the result mapping to perform. Attributes: column ,
- javaType , jdbcType , typeHandler , cases . The cases attribute is an array of Case s. |
-
@Case |
- N/A | -<case> |
- A single case of a value and its corresponding mappings. Attributes: value , type ,
- results . The results attribute is an array of Results, thus this Case Annotation is
- similar to an actual ResultMap , specified by the Results annotation below. |
-
@Results |
- Method |
- <resultMap> |
- A list of Result mappings that contain details of how a particular result column is mapped to a property or field.
- Attributes: value , id . The value attribute is an array of Result annotations. The id attribute is the name of the result mapping. |
-
@Result |
- N/A | -
-
|
- A single result mapping between a column and a property or field. Attributes: id , column ,
- property , javaType , jdbcType , typeHandler , one ,
- many . The id attribute is a boolean value that indicates that the property should be used for comparisons
- (similar to <id> in the XML mappings).
- The one attribute is for single associations, similar to <association> , and the many attribute
- is for collections, similar to <collection> . They are named as they are to avoid class naming conflicts.
- Since 3.5.4, it can be used as repeatable annotation. |
-
@One |
- N/A | -<association> |
- A mapping to a single property value of a complex type. Attributes: select , which is the fully
- qualified name of a mapped statement (i.e. mapper method) that can load an instance of the appropriate type.
- fetchType , which supersedes the global configuration parameter lazyLoadingEnabled for this
- mapping.
- resultMap (available since 3.5.5), which is the fully qualified name of a result map that map to
- a single container object from select result.
- columnPrefix (available since 3.5.5), which is column prefix for grouping select columns at nested result map.
- NOTE You will notice that join mapping is not supported via the Annotations API.
- This is due to the limitation in Java Annotations that does not allow for circular references. |
-
@Many |
- N/A | -<collection> |
- A mapping to a collection property of a complex type. Attributes: select , which is the fully
- qualified name of a mapped statement (i.e. mapper method) that can load a collection of instances of the appropriate
- types. fetchType , which supersedes the global configuration parameter lazyLoadingEnabled for this
- mapping.
- resultMap (available since 3.5.5), which is the fully qualified name of a result map that map to
- collection object from select result.
- columnPrefix (available since 3.5.5), which is column prefix for grouping select columns at nested result map.
- NOTE You will notice that join mapping is not supported via the
- Annotations API. This is due to the limitation in Java Annotations that does not allow for circular references. |
-
@MapKey |
- Method |
- - | This is used on methods which return type is a Map. It is used to convert a List of result objects as a Map
- based on a property of those objects. Attributes: value , which is a property used as the key of the map. |
-
@Options |
- Method |
- Attributes of mapped statements. | -This annotation provides access to the wide range of switches and configuration options that are normally
- present on the mapped statement as attributes. Rather than complicate each statement annotation, the
- Options annotation provides a consistent and clear way to access these. Attributes:
- useCache=true , flushCache=FlushCachePolicy.DEFAULT , resultSetType=DEFAULT ,
- statementType=PREPARED , fetchSize=-1 , timeout=-1 ,
- useGeneratedKeys=false , keyProperty="" , keyColumn="" , resultSets=""
- and databaseId="" .
- It's important to understand that with Java Annotations, there is no way to specify null as a value.
- Therefore, once you engage the Options annotation, your statement is subject to all of the default
- values. Pay attention to what the default values are to avoid unexpected behavior.
- The databaseId (Available since 3.5.5), in case there is a configured DatabaseIdProvider ,
- the MyBatis use the Options with no databaseId attribute or with a databaseId
- that matches the current one. If found with and without the databaseId the latter will be discarded.- Note that keyColumn is only required in certain databases (like Oracle and PostgreSQL).
- See the discussion about keyColumn and keyProperty above in the discussion of the
- insert statement for more information about allowable values in these attributes.
- |
-
-
|
- Method |
-
-
|
- Each of these annotations represents the actual SQL that is to be executed. They each take an array of strings
- (or a single string will do). If an array of strings is passed, they are concatenated with a single space between
- each to separate them. This helps avoid the "missing space" problem when building SQL in Java code. However,
- you're also welcome to concatenate together a single string if you like.
- Attributes: value , which is the array of Strings to form the single SQL statement.
- The databaseId (Available since 3.5.5), in case there is a configured DatabaseIdProvider ,
- the MyBatis use a statement with no databaseId attribute or with a databaseId
- that matches the current one. If found with and without the databaseId the latter will be discarded.
- |
-
-
|
- Method |
-
-
|
- Allows for creation of dynamic SQL. These alternative SQL annotations allow you to specify a class and
- a method name that will return the SQL to run at execution time
- (Since 3.4.6, you can specify the CharSequence instead of String as a method return type).
- Upon executing the mapped statement, MyBatis will instantiate the class, and execute the method, as specified by the provider.
- You can pass objects that passed to arguments of a mapper method, "Mapper interface type", "Mapper method" and "Database ID"
- via the ProviderContext (available since MyBatis 3.4.5 or later) as method argument.
- (In MyBatis 3.4 or later, it's allow multiple parameters)
- Attributes: value , type , method and databaseId .
- The value and type attribute is a class
- (The type attribute is alias for value , you must be specify either one.
- But both attributes can be omit when specify the defaultSqlProviderType as global configuration).
- The method is the name of the method on that class
- (Since 3.5.1, you can omit method attribute, the MyBatis will resolve a target method via the
- ProviderMethodResolver interface.
- If not resolve by it, the MyBatis use the reserved fallback method that named provideSql ).
- The databaseId (Available since 3.5.5), in case there is a configured DatabaseIdProvider ,
- the MyBatis will use a provider method with no databaseId attribute or with a databaseId
- that matches the current one. If found with and without the databaseId the latter will be discarded.
-
- NOTE
- Following this section is a discussion about the class, which can help build dynamic SQL in a cleaner, easier to read way. |
-
@Param |
- Parameter |
- N/A | -If your mapper method takes multiple parameters, this annotation can be applied to a mapper method parameter
- to give each of them a name. Otherwise, multiple parameters will be named by their position prefixed with "param"
- (not including any RowBounds parameters). For example #{param1} , #{param2} etc. is the default.
- With @Param("person") , the parameter would be named #{person} . |
-
@SelectKey |
- Method |
- <selectKey> |
- This annotation duplicates the <selectKey> functionality for methods annotated with
- @Insert , @InsertProvider , @Update , or @UpdateProvider .
- It is ignored for other methods. If you specify a
- @SelectKey annotation, then MyBatis will ignore any generated key properties set via the
- @Options annotation, or configuration properties.
- Attributes: statement an array of strings which is the SQL statement to execute, keyProperty which
- is the property of the parameter object that will be updated with the new value, before which must be either
- true or false to denote if the SQL statement should be executed before or after the insert,
- resultType which is the Java type of the keyProperty , and statementType is a type of the statement that is any one of STATEMENT , PREPARED or CALLABLE that is mapped to Statement , PreparedStatement and CallableStatement respectively. The default is PREPARED .
- The databaseId (Available since 3.5.5), in case there is a configured DatabaseIdProvider ,
- the MyBatis will use a statement with no databaseId attribute or with a databaseId
- that matches the current one. If found with and without the databaseId the latter will be discarded.
- |
-
@ResultMap |
- Method |
- N/A | -This annotation is used to provide the id of a <resultMap> element in an XML mapper to a
- @Select or @SelectProvider annotation. This allows annotated selects to reuse resultmaps
- that are defined in XML. This annotation will override any @Results or @ConstructorArgs
- annotation if both are specified on an annotated select. |
-
@ResultType |
- Method |
- N/A | -This annotation is used when using a result handler. In that case, the return type is void
- so MyBatis must have a way to determine the type of object to construct for each row.
- If there is an XML result map, use the @ResultMap annotation. If the result type is
- specified in XML on the <select> element, then no other annotation is
- necessary. In other cases, use this annotation. For example, if a @Select annotated method
- will use a result handler, the return type must be void and this annotation (or @ResultMap)
- is required. This annotation is ignored unless the method return type is void. |
-
@Flush |
- Method |
- N/A | -If this annotation is used, it can be called the SqlSession#flushStatements() via method defined at a Mapper interface.(MyBatis 3.3 or above) |
-
This example shows using the @SelectKey annotation to retrieve a value from a sequence before an insert:
- - -This example shows using the @SelectKey annotation to retrieve an identity value after an insert:
- - -This example shows using the @Flush
annotation to call the SqlSession#flushStatements()
:
These examples show how to name a ResultMap by specifying id attribute of @Results annotation.
- - -This example shows solo parameter using the SelectProvider annotation:
- - -This example shows multiple parameters using the Sql Provider annotation:
- - -This example shows usage that share an sql provider class to all mapper methods using global configuration(Available since 3.5.6):
- - - -This example shows usage the default implementation of ProviderMethodResolver
(available since MyBatis 3.5.1 or later):
This example shows usage the databaseId
attribute on the statement annotation(Available since 3.5.5):
MyBatis provides logging information through the use of an - internal log factory. The internal log factory will delegate logging - information to one of the following log implementations: -
-The logging solution chosen is based on runtime introspection by - the internal MyBatis log factory. The MyBatis log factory will use - the first logging implementation it finds (implementations are - searched in the above order). If MyBatis finds none of the above - implementations, then logging will be disabled. -
-Many environments ship Commons Logging as a part of the application server - classpath (good examples include Tomcat and WebSphere). It is - important to know that in such environments, MyBatis will use Commons Logging as - the logging implementation. In an environment like WebSphere this - will mean that your Log4J configuration will be ignored because - WebSphere supplies its own proprietary implementation of Commons Logging. This - can be very frustrating because it will appear that MyBatis is - ignoring your Log4J configuration (in fact, MyBatis is ignoring your - Log4J configuration because MyBatis will use Commons Logging in such - environments). If your application is running in an environment - where Commons Logging is included in the classpath but you would - rather use one of the other logging implementations you can select a - different logging implementation by adding a setting in mybatis-config.xml file as follows: -
- -Valid values are SLF4J, LOG4J, LOG4J2, JDK_LOGGING, COMMONS_LOGGING, STDOUT_LOGGING, NO_LOGGING or
- a full qualified class name that implements org.apache.ibatis.logging.Log
and gets
- an string as a constructor parameter.
-
You can also select the implementation by calling one of the following methods: -
- -If you choose to call one of these methods, you should do so - before calling any other MyBatis method. Also, these methods - will only switch to the requested log implementation if that - implementation is available on the runtime classpath. For example, if - you try to select Log4J2 logging and Log4J2 is not available at runtime, - then MyBatis will ignore the request to use Log4J2 and will use it's - normal algorithm for discovering logging implementations. -
-The specifics of SLF4J, Apache Commons Logging, Apache Log4J and the JDK - Logging API are beyond the scope of this document. However the - example configuration below should get you started. If you would like - to know more about these frameworks, you can get more information from - the following locations: -
-To see MyBatis logging statements you may enable logging on a - package, a mapper fully qualified class name, a namespace - o a fully qualified statement name. -
-Again, how you do this is dependent on the logging implementation
- in use. We'll show how to do it with SLF4J(Logback). Configuring the
- logging services is simply a matter of including one or more extra
- configuration files (e.g. logback.xml
) and sometimes a new JAR file.
- The following example configuration will
- configure full logging services using SLF4J(Logback) as a provider. There
- are 2 steps.
-
Because we are using SLF4J(Logback), we will need to ensure its - JAR file is available to our application. To use SLF4J(Logback), you need to - add the JAR file to your application classpath. -
-For web or enterprise applications you can add the logback-classic.jar
- ,logback-core.jar
and slf4j-api.jar
to
- your WEB-INF/lib
directory, or for a standalone application you can
- simply add it to the JVM -classpath
startup parameter.
-
If you use the maven, you can download jar files by adding following settings on your pom.xml
.
-
Configuring Logback is simple. Suppose you want to enable the log for this mapper: -
- -Create a file called logback.xml
- as shown below and place it in your classpath:
-
- The above file will cause SLF4J(Logback) to report detailed logging for
- org.mybatis.example.BlogMapper
- and just errors for the rest of the classes of your application.
-
- If you want to tune the logging at a finer level you can turn logging
- on for specific statements instead of the whole mapper file.
- The following line will enable logging just for the selectBlog
- statement:
-
By the contrary you may want want to enable logging for a group of mappers. - In that case you should add as a logger the root package where your mappers reside:
- - - -There are queries that can return huge result sets. In that cases you may want to see the - SQL statement but not the results. For that purpose SQL statements are logged at the DEBUG level - (FINE in JDK logging) and results at the TRACE level (FINER in JDK logging), so in case - you want to see the statement but not the result, set the level to DEBUG. -
- - - -But what about if you are not using mapper interfaces but mapper XML files like this one? -
- - - -In that case you can enable logging for the whole XML file by adding a logger for the namespace as shown below:
- - - -Or for an specific statement:
- - - -Yes, as you may have noticed, there is no difference in configuring - logging for mapper interfaces or for XML mapper files.
- -NOTE If you are using SLF4J or Log4j 2 MyBatis will call it using the marker MYBATIS
.
The remaining configuration in the logback.xml
file is used
- to configure the appenders, which is beyond the scope of this
- document. However, you can find more information at the Logback website.
- Or, you could simply experiment with it to see what effects the different configuration options have.
-
pom.xml
log4j2.xml
pom.xml
log4j.properties
logging.properties
- The true power of MyBatis is in the Mapped Statements. This is where the magic happens. - For all of their power, the Mapper XML files are relatively simple. Certainly if you were - to compare them to the equivalent JDBC code, you would immediately see a savings of 95% of the code. - MyBatis was built to focus on the SQL, and does its best to stay out of your way. -
-The Mapper XML files have only a few first class elements (in the order that they should be defined): -
-cache
- – Configuration of the cache for a given namespace.
- cache-ref
- – Reference to a cache configuration from another namespace.
- resultMap
- – The most complicated and powerful element that describes how to load your objects from the
- database result sets.
- parameterMap
- – Deprecated! Old-school way to map parameters. Inline parameters are preferred and this element
- may be removed in the future. Not documented here.
- sql
- – A reusable chunk of SQL that can be referenced by other statements.
- insert
- – A mapped INSERT statement.
- update
- – A mapped UPDATE statement.
- delete
- – A mapped DELETE statement.
- select
- – A mapped SELECT statement.
- The next sections will describe each of these elements in detail, starting with the statements - themselves. -
- -- The select statement is one of the most popular elements that you'll use in MyBatis. - Putting data in a database isn't terribly valuable until you get it back out, so most - applications query far more than they modify the data. For every insert, update or delete, - there are probably many selects. This is one of the founding principles of MyBatis, and is the - reason so much focus and effort was placed on querying and result mapping. The select element is - quite simple for simple cases. For example: -
- - - -- This statement is called selectPerson, takes a parameter of type int (or Integer), and returns a - HashMap - keyed by column names mapped to row values. -
- -- Notice the parameter notation: -
- - - -- This tells MyBatis to create a PreparedStatement parameter. With JDBC, such a parameter would be - identified by a "?" in SQL passed to a new PreparedStatement, something like this: -
- - - -- Of course, there's a lot more code required by JDBC alone to extract the results and map them to an - instance of an object, which is what MyBatis saves you from having to do. There's a lot more to know - about parameter and result mapping. Those details warrant their own section, which follows later in - this section. -
- -- The select element has more attributes that allow you to configure the details of how each statement - should behave. -
- - - -Attribute | -Description | -
---|---|
id |
- A unique identifier in this namespace that can be used to reference this statement. | -
parameterType |
- The fully qualified class name or alias for the parameter that will be passed into this
- statement. This attribute is optional because MyBatis can calculate the TypeHandler to use out of
- the actual parameter passed to the statement. Default is unset .
- |
-
- parameterMap |
-
- parameterMap . Use
- inline parameter mappings and the parameterType attribute.
- |
-
resultType |
- The fully qualified class name or alias for the expected type that will be returned from
- this statement. Note that in the case of collections, this should be the type that the
- collection contains, not the type of the collection itself. Use resultType OR
- resultMap , not both.
- |
-
resultMap |
- A named reference to an external resultMap . Result maps are the most powerful feature of
- MyBatis, and with a good understanding of them, many difficult mapping cases can be
- solved. Use resultMap OR resultType , not both.
- |
-
flushCache |
- Setting this to true will cause the local and 2nd level caches to be flushed whenever this statement is
- called. Default: false for select statements.
- |
-
useCache |
- Setting this to true will cause the results of this statement to be cached in 2nd level cache. Default:
- true for select statements.
- |
-
timeout |
- This sets the number of seconds the driver will wait for the database to return from a
- request, before throwing an exception. Default is unset (driver dependent).
- |
-
fetchSize |
- This is a driver hint that will attempt to cause the driver to return results in batches
- of rows numbering in size equal to this setting. Default is unset (driver dependent).
- |
-
statementType |
- Any one of STATEMENT , PREPARED or CALLABLE .
- This causes MyBatis to use Statement ,
- PreparedStatement or CallableStatement respectively. Default: PREPARED .
- |
-
resultSetType |
- Any one of FORWARD_ONLY |SCROLL_SENSITIVE |SCROLL_INSENSITIVE |DEFAULT (same as unset).
- Default is unset (driver dependent).
- |
-
databaseId |
- In case there is a configured databaseIdProvider, MyBatis will load all statements with no databaseId
- attribute or with a databaseId that matches the current one. If case the same statement
- if found with and without the databaseId the latter will be discarded.
- |
-
resultOrdered |
- This is only applicable for nested result select statements: If this is true, it
- is assumed that nested results are contained or grouped together such that when a
- new main result row is returned, no references to a previous result row will occur
- anymore. This allows nested results to be filled much more memory friendly. Default:
- false .
- |
-
resultSets |
- This is only applicable for multiple result sets. It lists the result sets that will - be returned by the statement and gives a name to each one. Names are separated by commas. - | -
affectData |
- Set this to true when writing a INSERT, UPDATE or DELETE statement that returns data so that the transaction is controlled properly. Also see Transaction Control Method. Default: false (since 3.5.12)
- |
-
The data modification statements insert, update and delete are very similar in their implementation: -
- - - -Attribute | -Description | -
---|---|
id |
- A unique identifier in this namespace that can be used to reference this statement. | -
parameterType |
- The fully qualified class name or alias for the parameter that will be passed into this
- statement. This attribute is optional because MyBatis can calculate the TypeHandler to use out of
- the actual parameter passed to the statement. Default is unset .
- |
-
- parameterMap |
-
- |
-
flushCache |
- Setting this to true will cause the 2nd level and local caches to be flushed whenever this statement is
- called. Default: true for insert, update and delete statements.
- |
-
timeout |
- This sets the maximum number of seconds the driver will wait for the database to return from a
- request, before throwing an exception. Default is unset (driver dependent).
- |
-
statementType |
- Any one of STATEMENT , PREPARED or CALLABLE .
- This causes MyBatis to use Statement ,
- PreparedStatement or CallableStatement respectively. Default: PREPARED .
- |
-
useGeneratedKeys |
- (insert and update only) This tells MyBatis to use the JDBC getGeneratedKeys method to retrieve
- keys generated internally by the database (e.g. auto increment fields in RDBMS like
- MySQL or SQL Server). Default: false .
- |
-
keyProperty |
- (insert and update only) Identifies a property into which MyBatis will set the key value returned
- by getGeneratedKeys , or by a selectKey child element of the insert statement.
- Default: unset . Can be a comma separated list of property names if multiple
- generated columns are expected.
- |
-
keyColumn |
- (insert and update only) Sets the name of the column in the table with a generated key. This is - only required in certain databases (like PostgreSQL) when the key column is not the - first column in the table. Can be a comma separated list of columns names if multiple - generated columns are expected. - | -
databaseId |
- In case there is a configured databaseIdProvider, MyBatis will load all statements with no databaseId
- attribute or with a databaseId that matches the current one. If case the same statement
- if found with and without the databaseId the latter will be discarded.
- |
-
The following are some examples of insert, update and delete statements.
- - - -- As mentioned, insert is a little bit more rich in that it has a few extra attributes and - sub-elements that allow it to deal with key generation in a number of ways. -
- -
- First, if your database supports auto-generated key fields (e.g. MySQL and SQL Server),
- then you can simply set useGeneratedKeys="true"
and set the keyProperty
to the target property
- and you're done. For example, if the Author
table above had used an auto-generated column type
- for the id, the statement would be modified as follows:
-
- If your database also supports multi-row insert, you can pass a list or an array of Author
s and retrieve the auto-generated keys.
-
- MyBatis has another way to deal with key generation for databases that don't support auto-generated - column types, or perhaps don't yet support the JDBC driver support for auto-generated keys. -
- -- Here's a simple (silly) example that would generate a random ID (something you'd likely never do, - but this demonstrates the flexibility and how MyBatis really doesn't mind): -
- -
- In the example above, the selectKey statement would be run first, the Author
id property would be
- set, and then the insert statement would be called. This gives you a similar behavior to an
- auto-generated key in your database without complicating your Java code.
-
- The selectKey element is described as follows: -
- - -Attribute | -Description | -
---|---|
keyProperty |
- The target property where the result of the selectKey statement should be set.
- Can be a comma separated list of property names if multiple generated columns
- are expected.
- |
-
keyColumn |
- The column name(s) in the returned result set that match the properties. - Can be a comma separated list of column names if multiple generated columns - are expected. - | -
resultType |
- - The type of the result. MyBatis can usually figure this out, but it doesn't hurt to - add it to be sure. MyBatis allows any simple type to be used as the key, - including Strings. If you are expecting multiple generated columns, then - you can use an Object that contains the expected properties, or a Map. - | -
order |
-
- This can be set to BEFORE or AFTER . If set to BEFORE , then it will select the key
- first, set the keyProperty and then execute the insert statement. If set to AFTER ,
- it runs the insert statement and then the selectKey statement – which is common with
- databases like Oracle that may have embedded sequence calls inside of insert statements.
- |
-
statementType |
-
- Same as above, MyBatis supports STATEMENT , PREPARED and CALLABLE statement types that
- map to Statement , PreparedStatement and CallableStatement respectively.
- |
-
- As an irregular case, some databases allow INSERT, UPDATE or DELETE statement to return result set (e.g. RETURNING
clause of PostgreSQL and MariaDB or OUTPUT
clause of MS SQL Server). This type of statement must be written as ]]>
to map the returned data.
-
- This element can be used to define a reusable fragment of SQL code that can be - included in other statements. It can be statically (during load phase) parametrized. Different property values can - vary in include instances. For example: -
- - - -- The SQL fragment can then be included in another statement, for example: -
- - - -- Property value can be also used in include refid attribute or property values inside include clause, for example: -
- - -- In all of the past statements, you've seen examples of simple parameters. Parameters are very - powerful elements in MyBatis. For simple situations, probably 90% of the cases, there's not much - to them, for example: -
- - - -
- The example above demonstrates a very simple named parameter mapping. The parameterType is set to
- int
, so therefore the parameter could be named anything. Primitive or simple data types such as
- Integer
and String
have no relevant properties, and thus will replace the full value of the
- parameter entirely. However, if you pass in a complex object, then the behavior is a little
- different. For example:
-
- If a parameter object of type User was passed into that statement, the id, username and password
- property would be looked up and their values passed to a PreparedStatement
parameter.
-
- That's nice and simple for passing parameters into statements. But there are a lot of other - features of parameter maps. -
- -- First, like other parts of MyBatis, parameters can specify a more specific data type. -
- - - -
- Like the rest of MyBatis, the javaType can almost always be determined from the parameter object,
- unless that object is a HashMap
. Then the javaType
should be specified to ensure the correct
- TypeHandler
is used.
-
- NOTE The JDBC Type is required by JDBC for all nullable
- columns, if null
is passed as a value. You can investigate this yourself by reading the JavaDocs
- for the PreparedStatement.setNull()
method.
-
- To further customize type handling, you can also specify a specific TypeHandler
- class (or alias), for example:
-
- So already it seems to be getting verbose, but the truth is that you'll rarely set any of these. -
- -
- For numeric types there's also a numericScale
for determining how many decimal places are relevant.
-
- Finally, the mode attribute allows you to specify IN
, OUT
or INOUT
parameters. If a parameter is
- OUT
or INOUT
, the actual value of the parameter object property will be changed, just as you would
- expect if you were calling for an output parameter. If the mode=OUT
(or INOUT
) and the
- jdbcType=CURSOR
(i.e. Oracle REFCURSOR), you must specify a resultMap
to map the ResultSet
to the
- type of the parameter. Note that the javaType
attribute is optional here, it will be automatically
- set to ResultSet
if left blank with a CURSOR
as the jdbcType
.
-
- MyBatis also supports more advanced data types such as structs, but you must tell the statement - the type name when registering the out parameter. For example (again, don't break lines like - this in practice): -
- - - -
- Despite all of these powerful options, most of the time you'll simply specify the property name,
- and MyBatis will figure out the rest. At most, you'll specify the jdbcType
for nullable columns.
-
- By default, using the #{}
syntax will cause MyBatis to generate PreparedStatement
properties and
- set the values safely against the PreparedStatement
parameters (e.g. ?). While this is safer,
- faster and almost always preferred, sometimes you just want to directly inject an unmodified string
- into the SQL Statement. For example, for ORDER BY, you might use something like this:
-
- Here MyBatis won't modify or escape the string. -
- -
- String Substitution can be very useful when the metadata(i.e. table name or column name) in the sql statement is dynamic,
- for example, if you want to select
from a table by any one of its columns, instead of writing code like:
-
- you can just write:
-
- in which the ${column}
will be substituted directly and the #{value}
will be "prepared".
- Thus you can just do the same work by:
-
-
- This idea can be applied to substitute the table name as well. -
- -- NOTE It's not safe to accept input from a user and supply - it to a statement unmodified in this way. This leads to potential SQL Injection attacks and - therefore you should either disallow user input in these fields, or always perform your own escapes - and checks. -
-
- The resultMap
element is the most important and powerful element in MyBatis. It's what allows you to
- do away with 90% of the code that JDBC requires to retrieve data from ResultSet
s, and in some cases
- allows you to do things that JDBC does not even support. In fact, to write the equivalent code for
- something like a join mapping for a complex statement could probably span thousands of lines of
- code. The design of the ResultMap
s is such that simple statements don't require explicit result
- mappings at all, and more complex statements require no more than is absolutely necessary to
- describe the relationships.
-
- You've already seen examples of simple mapped statements that don't have an explicit resultMap
. For
- example:
-
- Such a statement simply results in all columns being automatically mapped to the keys of a HashMap
,
- as specified by the resultType
attribute. While useful in many cases, a HashMap
doesn't make a very
- good domain model. It's more likely that your application will use JavaBeans or POJOs (Plain Old
- Java Objects) for the domain model. MyBatis supports both. Consider the following JavaBean:
-
- Based on the JavaBeans specification, the above class has 3 properties: id, username, and - hashedPassword. These match up exactly with the column names in the select statement. -
- -
- Such a JavaBean could be mapped to a ResultSet
just as easily as the HashMap
.
-
- And remember that TypeAliases are your friends. Use them so that you don't have to keep typing the - fully qualified path of your class out. For example: -
- - - -
- In these cases MyBatis is automatically creating a ResultMap
behind the scenes to auto-map the columns to
- the JavaBean properties based on name.
- If the column names did not match exactly, you could employ
- select clause aliases (a standard SQL feature) on the column names to make the labels match. For
- example:
-
- The great thing about ResultMap
s is that you've already learned a lot about them, but you haven't
- even seen one yet! These simple cases don't require any more than you've seen here. Just for example
- sake, let's see what this last example would look like as an external resultMap
, as that is another
- way to solve column name mismatches.
-
- And the statement that references it uses the resultMap
attribute to do so (notice we removed the
- resultType
attribute). For example:
-
- Now if only the world was always that simple. -
- -- MyBatis was created with one idea in mind: Databases aren't always what you want or need them to be. - While we'd love every database to be perfect 3rd normal form or BCNF, they aren't. And it would be - great if it was possible to have a single database map perfectly to all of the applications that use - it, it's not. Result Maps are the answer that MyBatis provides to this problem. -
- -For example, how would we map this statement?
- - - -- You'd probably want to map it to an intelligent object model consisting of a Blog that was written - by an Author, and has many Posts, each of which may have zero or many Comments and Tags. The - following is a complete example of a complex ResultMap (assume Author, Blog, Post, Comments and Tags - are all type aliases). Have a look at it, but don't worry, we're going to go through each step. - While it may look daunting at first, it's actually very simple. -
- - - -
- The resultMap
element has a number of sub-elements and a structure worthy of some discussion. The
- following is a conceptual view of the resultMap
element.
-
constructor
- used for injecting results into the constructor of a class upon instantiation
- idArg
- ID argument; flagging results as ID will help improve overall performancearg
- a normal result injected into the constructorid
– an ID result; flagging results as ID will help improve overall performanceresult
– a normal result injected into a field or JavaBean propertyassociation
– a complex type association; many results will roll up into this type
- resultMap
s themselves, or can refer to one
- collection
– a collection of complex types
- resultMap
s themselves, or can refer to onediscriminator
– uses a result value to determine which resultMap
to use
- case
– a case is a result map based on some value
- Attribute | -Description | -
---|---|
id |
- A unique identifier in this namespace that can be used to reference this result map. | -
type |
- A fully qualified Java class name, or a type alias (see the table above for the list of built-in type aliases). - | -
autoMapping |
- If present, MyBatis will enable or disable the automapping for this ResultMap. - This attribute overrides the global autoMappingBehavior. Default: unset. - | -
- Best Practice Always build ResultMaps incrementally. Unit
- tests really help out here. If you try to
- build a gigantic resultMap
like the one above all at once, it's likely you'll get it wrong and it
- will be hard to work with. Start simple, and evolve it a step at a time. And unit test! The downside
- to using frameworks is that they are sometimes a bit of a black box (open source or not). Your best
- bet to ensure that you're achieving the behaviour that you intend, is to write unit tests. It also
- helps to have them when submitting bugs.
-
- The next sections will walk through each of the elements in more detail. -
- -- These are the most basic of result mappings. Both id and - result map a single column value to a - single property or field of a simple data type (String, int, double, Date, etc.). -
- -- The only difference between the two is that id - will flag the result as an identifier property to be - used when comparing object instances. This helps to improve general performance, but especially - performance of caching and nested result mapping (i.e. join mapping). -
- -- Each has a number of attributes: -
- -Attribute | -Description | -
---|---|
property |
-
- The field or property to map the column result to. If a matching JavaBeans property
- exists for
- the given name, then that will be used. Otherwise, MyBatis will look for a field of the
- given
- name. In both cases you can use complex property navigation using the usual dot
- notation. For
- example, you can map to something simple like: username , or to something more
- complicated
- like: address.street.number .
- |
-
column |
-
- The column name from the database, or the aliased column label. This is the same string
- that
- would normally be passed to resultSet.getString(columnName) .
- |
-
javaType |
- - A fully qualified Java class name, or a type alias (see the table above for the list of - built-in type aliases). MyBatis can usually figure out the type if you're mapping to a - JavaBean. However, if you are mapping to a HashMap, then you should specify the javaType - explicitly to ensure the desired behaviour. - | -
jdbcType |
- - The JDBC Type from the list of supported types that follows this table. The JDBC type is - only required for nullable columns upon insert, update or delete. This is a JDBC - requirement, not a MyBatis one. So even if you were coding JDBC directly, you'd need to - specify this type – but only for nullable values. - | -
typeHandler |
- - We discussed default type handlers previously in this documentation. Using this property - you can override the default type handler on a mapping-by-mapping basis. The value is - either a fully qualified class name of a TypeHandler implementation, or a type alias. - | -
- For future reference, MyBatis supports the following JDBC Types via the included JdbcType - enumeration. -
- -BIT |
- FLOAT |
- CHAR |
- TIMESTAMP |
- OTHER |
- UNDEFINED |
-
TINYINT |
- REAL |
- VARCHAR |
- BINARY |
- BLOB |
- NVARCHAR |
-
SMALLINT |
- DOUBLE |
- LONGVARCHAR |
- VARBINARY |
- CLOB |
- NCHAR |
-
INTEGER |
- NUMERIC |
- DATE |
- LONGVARBINARY |
- BOOLEAN |
- NCLOB |
-
BIGINT |
- DECIMAL |
- TIME |
- NULL |
- CURSOR |
- ARRAY |
-
- While properties will work for most Data Transfer Object (DTO) type classes, and likely most of your - domain model, there may be some cases where you want to use immutable classes. Often tables that - contain reference or lookup data that rarely or never changes is suited to immutable classes. - Constructor injection allows you to set values on a class upon instantiation, without exposing - public methods. MyBatis also supports private properties and private JavaBeans properties to achieve - this, but some people prefer Constructor injection. The - constructor - element enables this. -
- -- Consider the following constructor: -
- - - -
- In order to inject the results into the constructor, MyBatis needs to identify the constructor for somehow.
- In the following example, MyBatis searches a constructor declared with three parameters: java.lang.Integer
, java.lang.String
and int
in this order.
-
- When you are dealing with a constructor with many parameters, maintaining the order of arg elements is error-prone.
- Since 3.4.3, by specifying the name of each parameter, you can write arg elements in any order. To reference constructor parameters by their names, you can either add @Param
annotation to them or compile the project with '-parameters' compiler option and enable useActualParamName
(this option is enabled by default).
- The following example is valid for the same constructor even though the order of the second and the third parameters does not match with the declared order.
-
- javaType
can be omitted if there is a property with the same name and type.
-
- The rest of the attributes and rules are the same as for the regular id and result elements. -
- -Attribute | -Description | -
---|---|
column |
-
- The column name from the database, or the aliased column label. This is the same string
- that would normally be passed to resultSet.getString(columnName) .
- |
-
javaType |
- - A fully qualified Java class name, or a type alias (see the table above for the list of - built-in type aliases). MyBatis can usually figure out the type if you're mapping to a - JavaBean. However, if you are mapping to a HashMap, then you should specify the javaType - explicitly to ensure the desired behaviour. - | -
jdbcType |
- - The JDBC Type from the list of supported types that follows this table. The JDBC type is - only required for nullable columns upon insert, update or delete. This is a JDBC - requirement, not an MyBatis one. So even if you were coding JDBC directly, you'd need to - specify this type – but only for nullable values. - | -
typeHandler |
-
- We discussed default type handlers previously in this documentation. Using this property
- you can override the default type handler on a mapping-by-mapping basis. The value is
- either a fully qualified class name of a TypeHandler implementation, or a type alias.
- |
-
select |
- - The ID of another mapped statement that will load the complex type required by this - property mapping. The values retrieved from columns specified in the column attribute - will be passed to the target select statement as parameters. See the Association element - for more. - | -
resultMap |
-
- This is the ID of a ResultMap that can map the nested results of this argument into an
- appropriate object graph. This is an alternative to using a call to another select
- statement. It allows you to join multiple tables together into a single ResultSet . Such
- a ResultSet will contain duplicated, repeating groups of data that needs to be
- decomposed and mapped properly to a nested object graph. To facilitate this, MyBatis
- lets you "chain" result maps together, to deal with the nested results. See the
- Association element below for more.
- |
-
name |
- - The name of the constructor parameter. Specifying name allows you to write arg elements in any order. See the above explanation. Since 3.4.3. - | -
- The association element deals with a "has-one" type relationship. For example, in our example, a
- Blog has one Author. An association mapping works mostly like any other result. You specify the
- target property, the javaType
of the property (which MyBatis
- can figure out most of the time), the jdbcType if necessary and a typeHandler if you want to
- override the retrieval of the result values.
-
- Where the association differs is that you need to tell MyBatis how to load the association. MyBatis - can do so in two different ways: -
- -- First, let's examine the properties of the element. As you'll see, it differs from a normal result - mapping only by the select and resultMap attributes. -
- -Attribute | -Description | -
---|---|
property |
-
- The field or property to map the column result to. If a matching JavaBeans property
- exists for the given name, then that will be used. Otherwise, MyBatis will look for a
- field of the given name. In both cases you can use complex property navigation using the
- usual dot notation. For example, you can map to something simple like: username , or to
- something more complicated like: address.street.number .
- |
-
javaType |
-
- A fully qualified Java class name, or a type alias (see the table above for the list of
- built- in type aliases). MyBatis can usually figure out the type if you're mapping to a
- JavaBean. However, if you are mapping to a HashMap , then you should specify the javaType
- explicitly to ensure the desired behaviour.
- |
-
jdbcType |
- - The JDBC Type from the list of supported types that follows this table. The JDBC type is - only required for nullable columns upon insert, update or delete. This is a JDBC - requirement, not an MyBatis one. So even if you were coding JDBC directly, you'd need to - specify this type – but only for nullable values. - | -
typeHandler |
- - We discussed default type handlers previously in this documentation. Using this property - you can override the default type handler on a mapping-by-mapping basis. The value is - either a fully qualified class name of a TypeHandler implementation, or a type alias. - | -
Attribute | -Description | -
---|---|
column |
-
- The column name from the database, or the aliased column label that holds the value that will be passed to the nested
- statement as an input parameter.
- This is the same string that would normally be passed to resultSet.getString(columnName) .
- Note: To deal with composite keys, you can specify multiple column names to pass to the
- nested select statement by using the syntax column="{prop1=col1,prop2=col2}" . This will
- cause prop1 and prop2 to be set against the parameter object for the target nested
- select statement.
- |
-
select |
-
- The ID of another mapped statement that will load the complex type required by this
- property mapping. The values retrieved from columns specified in the column attribute
- will be passed to the target select statement as parameters. A detailed example follows
- this table.
- Note: To deal with composite keys, you can specify multiple column names to pass to the
- nested select statement by using the syntax column="{prop1=col1,prop2=col2}" . This will
- cause prop1 and prop2 to be set against the parameter object for the target nested
- select statement.
- |
-
fetchType |
-
- Optional. Valid values are lazy and eager . If present, it supersedes
- the global configuration parameter lazyLoadingEnabled for this mapping.
- |
-
- For example: -
- - - -
- That's it. We have two select statements: one to load the Blog, the other to load the Author, and
- the Blog's resultMap describes that the selectAuthor
statement should be used to load its author
- property.
-
- All other properties will be loaded automatically assuming their column and property names match. -
- -- While this approach is simple, it will not perform well for large data sets or lists. This problem - is known as the "N+1 Selects Problem". In a nutshell, the N+1 selects problem is caused like this: -
- -- This problem could result in hundreds or thousands of SQL statements to be executed. This is not - always desirable. -
- -- The upside is that MyBatis can lazy load such queries, thus you might be spared the cost of these - statements all at once. However, if you load such a list and then immediately iterate through it to - access the nested data, you will invoke all of the lazy loads, and thus performance could be very - bad. -
- -- And so, there is another way. -
- -Attribute | -Description | -
---|---|
resultMap |
- - This is the ID of a ResultMap that can map the nested results of this association into - an appropriate object graph. This is an alternative to using a call to another select - statement. It allows you to join multiple tables together into a single ResultSet. Such - a ResultSet will contain duplicated, repeating groups of data that needs to be - decomposed and mapped properly to a nested object graph. To facilitate this, MyBatis - lets you "chain" result maps together, to deal with the nested results. An example will - be far easier to follow, and one follows this table. - | -
columnPrefix |
- - When joining multiple tables, you would have to use column alias to avoid duplicated - column names in the ResultSet. Specifying columnPrefix allows you to map such columns - to an external resultMap. Please see the example explained later in this section. - | -
notNullColumn |
- - By default a child object is created only if at least one of the columns mapped to the child's properties - is non null. With this attribute you can change this behaviour by specifiying which columns must have a value - so MyBatis will create a child object only if any of those columns is not null. Multiple column names can be - specified using a comma as a separator. Default value: unset. - | -
autoMapping |
- If present, MyBatis will enable or disable automapping when mapping the result to this property.
- This attribute overrides the global autoMappingBehavior.
- Note that it has no effect on an external resultMap, so it is pointless to use it with select or resultMap attribute. Default value: unset.
- |
-
- You've already seen a very complicated example of nested associations above. The following is a far - simpler example to demonstrate how this works. Instead of executing a separate statement, we'll join - the Blog and Author tables together, like so: -
- - - -- Notice the join, as well as the care taken to ensure that all results are aliased with a unique and - clear name. This makes mapping far easier. Now we can map the results: -
- - - -- In the example above you can see at the Blog's "author" association delegates to the "authorResult" - resultMap to load the Author instance. -
- -- Very Important: id elements play a very important role in Nested - Result mapping. You should always - specify one or more properties that can be used to uniquely identify the results. The truth is that - MyBatis will still work if you leave it out, but at a severe performance cost. Choose as few - properties as possible that can uniquely identify the result. The primary key is an obvious choice - (even if composite). -
- -- Now, the above example used an external resultMap element to map the association. This makes the - Author resultMap reusable. However, if you have no need to reuse it, or if you simply prefer to - co-locate your result mappings into a single descriptive resultMap, you can nest the association - result mappings. Here's the same example using this approach: -
- - - -- What if the blog has a co-author? - The select statement would look like: -
- - - -- Recall that the resultMap for Author is defined as follows. -
- - - -
- Because the column names in the results differ from the columns defined in the resultMap,
- you need to specify columnPrefix
to reuse the resultMap for mapping co-author results.
-
Attribute | -Description | -
---|---|
column |
-
- When using multiple resultset this attribute specifies the columns (separated by commas) that will be correlated
- with the foreignColumn to identify the parent and the child of a relationship.
- |
-
foreignColumn |
-
- Identifies the name of the columns that contains the foreign keys which values will be matched against the
- values of the columns specified in the column attibute of the parent type.
- |
-
resultSet |
- - Identifies the name of the result set where this complex type will be loaded from. - | -
Starting from version 3.2.3 MyBatis provides yet another way to solve the N+1 problem.
- -Some databases allow stored procedures to return more than one resultset or execute more than one statement - at once and return a resultset per each one. This can be used to hit the database just once - and return related data without using a join.
- -In the example, the stored procedure executes the following queries and returns two result sets. - The first will contain Blogs and the second Authors.
- - - -A name must be given to each result set by adding a
- resultSets
attribute to the mapped statement with a list of names separated by commas.
- Now we can specify that the data to fill the "author" association comes in the "authors" result set: -
- - - -- You've seen above how to deal with a "has one" type association. But what about "has many"? That's - the subject of the next section. -
- -- The collection element works almost identically to the association. In fact, it's so similar, to - document the similarities would be redundant. So let's focus on the differences. -
- -- To continue with our example above, a Blog only had one Author. But a Blog has many Posts. On the - blog class, this would be represented by something like: -
- - - -- To map a set of nested results to a List like this, we use the collection element. Just like the - association element, we can use a nested select, or nested results from a join. -
- -- First, let's look at using a nested select to load the Posts for the Blog. -
- - - -- There are a number things you'll notice immediately, but for the most part it looks very similar to - the association element we learned about above. First, you'll notice that we're using the collection - element. Then you'll notice that there's a new "ofType" attribute. This attribute is necessary to - distinguish between the JavaBean (or field) property type and the type that the collection contains. - So you could read the following mapping like this: -
- - - -- Read as: "A collection of posts in an ArrayList of type Post." -
- -
- The javaType
attribute is really unnecessary, as MyBatis will figure this out for you in most cases.
- So you can often shorten this down to simply:
-
- By this point, you can probably guess how nested results for a collection will work, because it's
- exactly the same as an association, but with the same addition of the ofType
attribute applied.
-
First, let's look at the SQL:
- - - -- Again, we've joined the Blog and Post tables, and have taken care to ensure quality result column - labels for simple mapping. Now mapping a Blog with its collection of Post mappings is as simple as: -
- - - -- Again, remember the importance of the id elements here, or read the association section above if you - haven't already. -
- -- Also, if you prefer the longer form that allows for more reusability of your result maps, you can - use the following alternative mapping: -
- - -- As we did for the association, we can call a stored procedure that executes two queries and returns two result sets, one with Blogs - and another with Posts: -
- - - -A name must be given to each result set by adding a
- resultSets
attribute to the mapped statement with a list of names separated by commas.
We specify that the "posts" collection will be filled out of data contained in the result set named "posts":
- - - -- NOTE There's no limit to the depth, breadth or - combinations of the associations and collections that you map. - You should keep performance in mind when mapping them. Unit testing and performance - testing of your application goes a long way toward discovering the best approach for your - application. The nice thing is that MyBatis lets you change your mind later, with very little (if - any) impact to your code. -
- -- Advanced association and collection mapping is a deep subject. Documentation can only get you so - far. With a little practice, it will all become clear very quickly. -
- -- Sometimes a single database query might return result sets of many different (but hopefully somewhat - related) data types. The discriminator element was designed to deal with this situation, and others, - including class inheritance hierarchies. The discriminator is pretty simple to understand, as it - behaves much like a switch statement in Java. -
- -- A discriminator definition specifies column and javaType attributes. The column is where MyBatis - will look for the value to compare. The javaType is required to ensure the proper kind of equality - test is performed (although String would probably work for almost any situation). For example: -
- - - -
- In this example, MyBatis would retrieve each record from the result set and compare its vehicle type
- value. If it matches any of the discriminator cases, then it will use the resultMap
specified by the
- case. This is done exclusively, so in other words, the rest of the resultMap is ignored (unless it
- is extended, which we talk about in a second). If none of the cases match, then MyBatis simply uses
- the resultMap as defined outside of the discriminator block. So, if the carResult was declared as
- follows:
-
- Then ONLY the doorCount property would be loaded. This is done to allow completely independent - groups of discriminator cases, even ones that have no relationship to the parent resultMap. In this - case we do of course know that there's a relationship between cars and vehicles, as a Car is-a - Vehicle. Therefore, we want the rest of the properties loaded too. One simple change to the - resultMap and we're set to go. -
- - - -- Now all of the properties from both the vehicleResult and carResult will be loaded. -
- -- Once again though, some may find this external definition of maps somewhat tedious. Therefore - there's an alternative syntax for those that prefer a more concise mapping style. For example: -
- - - -- NOTE Remember - that these are all Result Maps, and if you don't specify any results at all, then MyBatis will - automatically match up columns and properties for you. So most of these examples are more verbose - than they really need to be. That said, most databases are kind of complex and it's unlikely that - we'll be able to depend on that for all cases. -
- -- As you have already seen in the previous sections, in simple cases MyBatis can auto-map the results for you - and in others you will need to build a result map. - But as you will see in this section you can also mix both strategies. - Let's have a deeper look at how auto-mapping works. -
- -- When auto-mapping results MyBatis will get the column name and look for a property with the same name ignoring case. That means that if - a column named ID and property named id are found, MyBatis will set the id property with the ID column value. -
- -
- Usually database columns are named using uppercase letters and underscores between words and java properties often follow the camelcase
- naming covention. To enable the auto-mapping between them set the setting mapUnderscoreToCamelCase
to true.
-
- Auto-mapping works even when there is an specific result map. When this happens, for each result map, all columns that are present in the - ResultSet that have not a manual mapping will be auto-mapped, then manual mappings will be processed. - In the following sample id and userName columns will be auto-mapped and hashed_password column will be mapped.
- - - - - -- There are three auto-mapping levels: -
- -NONE
- disables auto-mapping. Only manually mapped properties will be set.
- PARTIAL
- will auto-map results except those that have nested result mappings defined inside (joins).
- FULL
- auto-maps everything.
-
- The default value is PARTIAL
, and it is so for a reason. When FULL
is used auto-mapping will
- be performed when processing join results and joins retrieve data of several different entities in the same row
- hence this may result in undesired mappings. To understand the risk have a look at the following sample:
-
- With this result map both Blog and Author will be auto-mapped. But note that Author has an id
- property and there is a column named id in the ResultSet so Author's id will be filled with Blog's id, and that is not
- what you were expecting. So use the FULL
option with caution.
-
- Regardless of the auto-mapping level configured you can enable or disable the automapping for an specific ResultMap
- by adding the attribute autoMapping
to it:
-
- MyBatis includes a powerful transactional query caching feature which is very configurable and customizable. - A lot of changes have been made in the MyBatis 3 cache implementation to make it both more powerful and far easier to configure. -
- -- By default, just local session caching is enabled that is used solely to cache data for the duration of a session. - To enable a global second level of caching you simply need to add one line to your SQL Mapping file: -
- - - -- Literally that's it. The effect of this one simple statement is as follows: -
- -- NOTE The cache will only apply to statements declared in the mapping file - where the cache tag is located. If you are using the Java API in conjunction with the XML mapping files, then - statements declared in the companion interface will not be cached by default. You will need to refer to the - cache region using the @CacheNamespaceRef annotation. -
- -- All of these properties are modifiable through the attributes of the cache element. For example: -
- - - -- This more advanced configuration creates a FIFO cache that flushes once every 60 seconds, stores up - to 512 references to result objects or lists, and objects returned are considered read-only, thus - modifying them could cause conflicts between callers in different threads. -
- -- The available eviction policies available are: -
- -LRU
– Least Recently Used: Removes objects that haven't been used for the longst period of
- time.
- FIFO
– First In First Out: Removes objects in the order that they entered the cache.
- SOFT
– Soft Reference: Removes objects based on the garbage collector state and the rules of
- Soft References.
- WEAK
– Weak Reference: More aggressively removes objects based on the garbage collector state
- and rules of Weak References.
- The default is LRU.
- -- The flushInterval can be set to any positive integer and should represent a reasonable amount of - time specified in milliseconds. The default is not set, thus no flush interval is used and the cache - is only flushed by calls to statements. -
- -- The size can be set to any positive integer, keep in mind the size of the objects your caching and - the available memory resources of your environment. The default is 1024. -
- -- The readOnly attribute can be set to true or false. A read-only cache will return the same instance - of the cached object to all callers. Thus such objects should not be modified. This offers a - significant performance advantage though. A read-write cache will return a copy (via serialization) - of the cached object. This is slower, but safer, and thus the default is false. -
- -- NOTE Second level cache is transactional. That means that it is updated - when a SqlSession finishes with commit or when it finishes with rollback but no inserts/deletes/updates - with flushCache=true where executed. -
- -- In addition to customizing the cache in these ways, you can also completely override the cache - behavior by implementing your own cache, or creating an adapter to other 3rd party caching - solutions. -
- - - -- This example demonstrates how to use a custom cache implementation. The class specified in the type - attribute must implement the org.apache.ibatis.cache.Cache interface and provide a constructor that gets - an String id as an argument. This interface is one of the more complex in the MyBatis framework, but simple given what it does. -
- - - -
- To configure your cache, simply add public JavaBeans properties to your Cache implementation, and
- pass properties via the cache Element, for example, the following would call a method called
- setCacheFile(String file)
on your Cache implementation:
-
- You can use JavaBeans properties of all simple types, MyBatis will do the conversion.
- And you can specify a placeholder(e.g. ${cache.file}
) to replace value defined at configuration properties.
-
- Since 3.4.2, the MyBatis has been supported to call an initialization method after it's set all properties.
- If you want to use this feature, please implements the org.apache.ibatis.builder.InitializingObject
- interface on your custom cache class.
-
NOTE - Settings of cache (like eviction strategy, read write..etc.) in section above are not applied - when using Custom Cache. -
- -- It's important to remember that a cache configuration and the cache instance are bound to the - namespace of the SQL Map file. Thus, all statements in the same namespace as the cache are bound by - it. Statements can modify how they interact with the cache, or exclude themselves completely by - using two simple attributes on a statement-by-statement basis. By default, statements are configured - like this: -
- - - -- Since that's the default, you obviously should never explicitly configure a statement that way. - Instead, only set the flushCache and useCache attributes if you want to change the default behavior. - For example, in some cases you may want to exclude the results of a particular select statement from - the cache, or you might want a select statement to flush the cache. Similarly, you may have some - update statements that don't need to flush the cache upon execution. -
- -- Recall from the previous section that only the cache for this particular namespace will be used or - flushed for statements within the same namespace. There may come a time when you want to share the - same cache configuration and instance between namespaces. In such cases you can reference another - cache by using the cache-ref element. -
- - -One of the nastiest things a Java developer will ever have to do is embed SQL in Java code. Usually this is - done because the SQL has to be dynamically generated - otherwise you could externalize it in a file or a - stored proc. As you've already seen, MyBatis has a powerful answer for dynamic SQL generation in its - XML mapping features. However, sometimes it becomes necessary to build a SQL statement string inside of Java - code. In that case, MyBatis has one more feature to help you out, before reducing yourself to the typical mess - of plus signs, quotes, newlines, formatting problems and nested conditionals to deal with extra commas or - AND conjunctions. Indeed, dynamically generating SQL code in Java can be a real nightmare. For example: -
- - -MyBatis 3 offers a convenient utility class to help with the problem. - With the SQL class, you simply create an instance that lets you call methods against it to build a SQL statement - one step at a time. The example problem above would look like this when rewritten with the SQL class: -
- - - -What is so special about that example? Well, if you look closely, it doesn't have to worry about - accidentally duplicating "AND" keywords, or choosing between "WHERE" - and "AND" or none at all. The SQL class takes care of understanding where "WHERE" - needs to go, where an "AND" should be used and all of the String concatenation. -
- -Here are some examples:
- - - -Method | -Description | -
---|---|
-
|
- Starts or appends to a
- SELECT
- clause. Can be called more than once, and parameters will be appended to
- the
- SELECT
- clause. The parameters are usually a comma separated list of columns and aliases, but can be
- anything acceptable to the driver.
- |
-
-
|
- Starts or appends to a
- SELECT
- clause, also adds the
- DISTINCT
- keyword to the generated query.
- Can be called more than once, and parameters will be appended to the
- SELECT
- clause. The parameters
- are usually a comma separated list of columns and aliases, but can be anything acceptable to the driver.
- |
-
-
|
- Starts or appends to a
- FROM
- clause. Can be called more than once, and parameters will be appended to the
- FROM
- clause. Parameters are usually a table name and an alias, or anything acceptable to the driver.
- |
-
-
|
- Adds a new
- JOIN
- clause of the appropriate type, depending on the method called. The parameter can include
- a standard join consisting of the columns and the conditions to join on.
- |
-
-
|
- Appends a new
- WHERE
- clause condition, concatenated byAND . Can be called multiple times, which causes it
- to concatenate the new conditions each time withAND . Use
- OR()
- to split with anOR .
- |
-
- OR()
- |
- Splits the current
- WHERE
- clause conditions with anOR . Can be called more than once, but calling more
- than once in a row will generate erraticSQL .
- |
-
- AND()
- |
- Splits the current
- WHERE
- clause conditions with anAND . Can be called more than once, but calling more
- than once in a row will generate erraticSQL . Because
- WHERE
- and
- HAVING
- both automatically concatenate with
- AND , this is a very uncommon method to use and is only really included for completeness.
- |
-
-
|
- Appends a new
- GROUP BY
- clause elements, concatenated by a comma. Can be called multiple times, which
- causes it to concatenate the new conditions each time with a comma.
- |
-
-
|
- Appends a new
- HAVING
- clause condition, concatenated by AND. Can be called multiple times, which causes it
- to concatenate the new conditions each time withAND . Use
- OR()
- to split with anOR .
- |
-
-
|
- Appends a new
- ORDER BY
- clause elements, concatenated by a comma. Can be called multiple times, which
- causes it to concatenate the new conditions each time with a comma.
- |
-
-
|
-
- Appends a LIMIT clause.
- This method valid when use together with SELECT(), UPDATE() and DELETE().
- And this method is designed to use together with OFFSET() when use SELECT(). (Available since 3.5.2)
- |
-
-
|
-
- Appends a OFFSET clause.
- This method valid when use together with SELECT().
- And this method is designed to use together with LIMIT(). (Available since 3.5.2)
- |
-
-
|
-
- Appends a OFFSET n ROWS clause.
- This method valid when use together with SELECT().
- And this method is designed to use together with FETCH_FIRST_ROWS_ONLY(). (Available since 3.5.2)
- |
-
-
|
-
- Appends a FETCH FIRST n ROWS ONLY clause.
- This method valid when use together with SELECT().
- And this method is designed to use together with OFFSET_ROWS(). (Available since 3.5.2)
- |
-
- DELETE_FROM(String)
- |
- Starts a delete statement and specifies the table to delete from. Generally this should be followed by - a - WHERE statement! - | -
- INSERT_INTO(String)
- |
- Starts an insert statement and specifies the table to insert into. This should be followed by one or - more VALUES() or INTO_COLUMNS() and INTO_VALUES() calls. - | -
-
|
- Appends to the "set" list for an update statement. | -
- UPDATE(String)
- |
- Starts an update statement and specifies the table to update. This should be followed by one or more - SET() calls, and usually a WHERE() call. - | -
- VALUES(String, String)
- |
- Appends to an insert statement. The first parameter is the column(s) to insert, the second parameter - is - the value(s). - | -
- INTO_COLUMNS(String...)
- |
- - Appends columns phrase to an insert statement. - This should be call INTO_VALUES() with together. - | -
- INTO_VALUES(String...)
- |
- - Appends values phrase to an insert statement. - This should be call INTO_COLUMNS() with together. - | -
- ADD_ROW()
- |
- - Add new row for bulk insert. (Available since 3.5.2) - | -
- NOTE
- It is important to note that SQL class writes LIMIT
, OFFSET
, OFFSET n ROWS
and FETCH FIRST n ROWS ONLY
clauses into the generated statement as is.
- In other words, the library does not attempt to normalize those values for databases that don’t support these clauses directly.
- Therefore, it is very important for users to understand whether or not the target database supports these clauses.
- If the target database does not support these clauses, then it is likely that using this support will create SQL that has runtime errors.
-
Since version 3.4.2, you can use variable-length arguments as follows:
- - - -Since version 3.5.2, you can create insert statement for bulk insert as follow:
- - - -Since version 3.5.2, you can create select statement for limiting search result rows clause as follow:
- - - -- Before version 3.2 we took a bit of a different approach, by utilizing a ThreadLocal variable to mask some of the - language limitations that make Java DSLs a bit cumbersome. However, this approach is now deprecated, as modern - frameworks have warmed people to the idea of using builder-type patterns and anonymous inner classes for such - things. Therefore the SelectBuilder and SqlBuilder classes have been deprecated. -
-- The following methods apply to only the deprecated SqlBuilder and SelectBuilder classes. -
-Method | -Description | -
---|---|
- BEGIN()
- /
- RESET()
- |
- These methods clear the ThreadLocal state of the SelectBuilder class, and prepare it for a new
- statement to be
- built.
- BEGIN()
- reads best when starting a new statement.
- RESET()
- reads best when clearing a
- statement in the middle of execution for some reason (perhaps if the logic demands a completely
- different
- statement under some conditions).
- |
-
- SQL()
- |
- This returns the generated
- SQL()
- and resets the
- SelectBuilder
- state (as if
- BEGIN()
- or
- RESET()
- were called).
- Thus, this method can only be called ONCE!
- |
-
- The SelectBuilder and SqlBuilder classes are not magical, but it's important to know how they work. - SelectBuilder and SqlBuilder use a combination of - Static Imports and a ThreadLocal variable to enable a clean syntax that can be easily interlaced with - conditionals. To use them, you statically import the methods from the classes like this (one or the other, - not both): -
- - - - -This allows us to create methods like these:
- - - - - -