What causes java.lang.IncompatibleClassChangeError?

Solution 1:

This means that you have made some incompatible binary changes to the library without recompiling the client code. Java Language Specification §13 details all such changes, most prominently, changing non-static non-private fields/methods to be static or vice versa.

Recompile the client code against the new library, and you should be good to go.

UPDATE: If you publish a public library, you should avoid making incompatible binary changes as much as possible to preserve what's known as "binary backward compatibility". Updating dependency jars alone ideally shouldn't break the application or the build. If you do have to break binary backward compatibility, it's recommended to increase the major version number (e.g. from 1.x.y to 2.0.0) before releasing the change.

Solution 2:

Your newly packaged library is not backward binary compatible (BC) with old version. For this reason some of the library clients that are not recompiled may throw the exception.

This is a complete list of changes in Java library API that may cause clients built with an old version of the library to throw java.lang.IncompatibleClassChangeError if they run on a new one (i.e. breaking BC):

1. Non-final field become static,
2. Non-constant field become non-static,
3. Class become interface,
4. Interface become class,
5. if you add a new field to class/interface (or add new super-class/super-interface) then a static field from a super-interface of a client class C may hide an added field (with the same name) inherited from the super-class of C (very rare case).

Note: There are many other exceptions caused by other incompatible changes: NoSuchFieldError, NoSuchMethodError, IllegalAccessError, InstantiationError, VerifyError, NoClassDefFoundError and AbstractMethodError.

The better paper about BC is "Evolving Java-based APIs 2: Achieving API Binary Compatibility" written by Jim des Rivières.

There are also some automatic tools to detect such changes:

• japi-compliance-checker
• clirr
• japitools
• sigtest
• japi-checker

Usage of japi-compliance-checker for your library:

japi-compliance-checker OLD.jar NEW.jar

Usage of clirr tool:

java -jar clirr-core-0.6-uber.jar -o OLD.jar -n NEW.jar

Good luck!