Why am I getting java.lang.IncompatibleClassChangeError when using Java JARs?

Incompatible Class Changes in Java JARs

Encountering java.lang.IncompatibleClassChangeError errors while invoking methods from a packaged Java JAR can be perplexing. These errors often appear sporadically, leaving developers wondering about the underlying cause.

Causes of Incompatible Class Change Errors

As outlined in the Java Language Specification §13, incompatible class changes arise from alterations to the library's binary format:

• Modifying non-static, non-private fields or methods to become static or vice versa

Resolving the Issue

The solution to this issue is straightforward: recompile the client code against the updated library. Once recompiled, the client code will be compatible with the new binary structure and the errors should dissipate.

Maintaining Binary Compatibility

When publishing public libraries, it's crucial to preserve binary backward compatibility by avoiding incompatible changes. Ideally, updating dependency JARs shouldn't disrupt the application or its build. If unavoidable, it's advisable to increment the major version number (e.g., from 1.x.y to 2.0.0) before releasing the change to signify the potential for binary incompatibilities.

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