Changing Core System Library Classes

[Alec Lee]

I want to temporarily change the code to say, java.lang.Class for debugging/monitoring. I know the java.exe has a -Xbootclasspath/p:path option allowing us to prepend our own version of the core api classes to the classpath and override those found in JRE's rt.jar. Furthermore, the source code for the core api classes are provided by the jdk. But the problem is it seems that the javac compiler does not accept a class name in the java.* package and I was not able to recompile from the modified source code directly. As a work around, I used byte code instrumentation tools like Javassist/BCEL to inject code to existing system classes and saved it into the rt.jar of the JRE. But is it possible to modify the source code directly and recompile them without having to use byte code instrumenation?

[Ulf Dittmer]

The JRE license prohibits patching the class libraries, so there are probably no tools that support it. Javassist/BCEL may be your best option, or you could check out AspectJ for something a bit less hackish.

[Alec Lee]

Just wondering why, then, they provide the -XBootclasspath/p: option and the source code for the core system library but just disable the compiler.

[Ulf Dittmer]

That mechanism can be used to replace libraries that ship with the JRE (like JAXB and JAX-WS) with newer versions.

The source code is still useful to have for learning how particular classes/methods work.