Merge pull request #151 from Hearen/advice-class-en

advice-class en

site/src/site/sphinx/en/advice-class.md

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+Critical Fields in Expressions
+==============================
+
+There is a very fundamental class `Advice` for the expressions used in filtering, tracing or monitoring and other aspects in commands.  
+
+```java
+public class Advice {
+
+    private final ClassLoader loader;
+    private final Class<?> clazz;
+    private final ArthasMethod method;
+    private final Object target;
+    private final Object[] params;
+    private final Object returnObj;
+    private final Throwable throwExp;
+    private final boolean isBefore;
+    private final boolean isThrow;
+    private final boolean isReturn;
+    ...
+    // getter/setter  
+}  
+```
+
+|Name|Specification|
+|---:|:---|
+|loader|class loader of the class|
+|clazz|the reference of the class|
+|method|the reflective reference of the method|
+|target|the instance of the class|
+|params|the parameters of the method, which is an array (when there is no argument in the method, it will be an empty array)|
+|returnObj|the return value of the method - only when `isReturn==true`, it's a valid result but if the return value is `void` then it will be a `null`|
+|throwExp|the exceptions thrown by the method invoking - only when `isThrow==true`, it's a valid thrown exception|
+|isBefore|assistant checking flag used in [`before-watching` point](watch.md)  and at this very moment: `isBefore==true`, `isThrow==false` and `isReturn==false` since it's before we invoke the method|
+|isThrow|assistant checking flag: whether the current method invoking ends with exceptions|
+|isReturn|assistant checking flag: whether the method invoking exits normally without exceptions|
+
+F.Y.I
+1. all the *fields* mentioned in the table above can be used directly in the `expressions`; 
+2. if the expressions are [invalid OGNL](https://en.wikipedia.org/wiki/OGNL), the command will be cancelled automatically with hints to correct the expressions;
+3. [typical use cases](https://github.com/alibaba/arthas/issues/71);
+4. [OGNL official usage guide](https://commons.apache.org/proper/commons-ognl/language-guide.html).
+
+
+