Unable to make field private java.lang.String java.util.regex.Pattern.pattern accessible: module java.base does not "opens java.util.regex" to unnamed module

[Dacesilian]

With Java 16, toJson(obj) produces error.

java.lang.reflect.InaccessibleObjectException: Unable to make field private java.lang.String java.util.regex.Pattern.pattern accessible: module java.base does not "opens java.util.regex" to unnamed module @223191a6
	at java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:357) ~[?:?]
	at java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297) ~[?:?]
	at java.lang.reflect.Field.checkCanSetAccessible(Field.java:177) ~[?:?]
	at java.lang.reflect.Field.setAccessible(Field.java:171) ~[?:?]
	at com.google.gson.internal.reflect.UnsafeReflectionAccessor.makeAccessible(UnsafeReflectionAccessor.java:44) ~[gson-2.8.7.jar:?]
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:159) ~[gson-2.8.7.jar:?]
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102) ~[gson-2.8.7.jar:?]
	at com.google.gson.Gson.getAdapter(Gson.java:458) ~[gson-2.8.7.jar:?]
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.createBoundField(ReflectiveTypeAdapterFactory.java:117) ~[gson-2.8.7.jar:?]
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:166) ~[gson-2.8.7.jar:?]
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102) ~[gson-2.8.7.jar:?]
	at com.google.gson.Gson.getAdapter(Gson.java:458) ~[gson-2.8.7.jar:?]
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.createBoundField(ReflectiveTypeAdapterFactory.java:117) ~[gson-2.8.7.jar:?]
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:166) ~[gson-2.8.7.jar:?]
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102) ~[gson-2.8.7.jar:?]
	at com.google.gson.Gson.getAdapter(Gson.java:458) ~[gson-2.8.7.jar:?]
	at com.google.gson.Gson.toJson(Gson.java:696) ~[gson-2.8.7.jar:?]
	at com.google.gson.Gson.toJson(Gson.java:683) ~[gson-2.8.7.jar:?]
	at com.google.gson.Gson.toJson(Gson.java:638) ~[gson-2.8.7.jar:?]
	at com.google.gson.Gson.toJson(Gson.java:618) ~[gson-2.8.7.jar:?]
	at com.mycode...

This produces error when working with obj of Class which has private Pattern myRegex field inside.
Workaround is to use private **transient** Pattern myRegex modifier.

[Marcono1234]

The reason for this is that you are trying to serialize a JDK class (java.util.regex.Pattern) using Gson's reflection-based type adapter. That adapter is used if no built-in adapter exists and you have not provided your own. Since Java 16 applications can by default no longer access JDK internals using reflection, see JDK-8256358.

This issue is similar to #1216; you should avoid using (implicitly) the reflective type adapter for third party classes because you make yourself dependent on the internal implementation of these classes (which could change at any point).
To solve this you should use a custom TypeAdapter for java.util.regex.Pattern.

However, if making the Pattern myRegex field transient in your case solves the issue, then that sounds like the proper solution because your application is apparently not dependent on the myRegex field in the JSON anyways.

[Marcono1234]

Closing because as mentioned above you should not rely on Gson's reflection-based serialization for third-party classes.