Add a field to java.lang.Object

Question

I added a field to Object class, as in :

class Object {
   ...

   private Object _objInfo;
}

I changed java.lang.Object's source code and recompiled OpenJDK 6. I get the following exception when the VM boots:

Error occurred during initialization of VM
    java.lang.IllegalStateException
    at java.lang.Throwable.initCause(Throwable.java:337)
    at java.lang.ExceptionInInitializerError.<init>(ExceptionInInitializerError.java:79)

The same problem occurs when I define my own Object class and prepended it to bootclasspath, as in:

java -Xbootclasspath/p:<path to my Object class>

Thanks, Horatiu

Answer 1

Don't modify Object. Don't modify anything in java.lang. I don't know if it's technically possible, but it is definitely an exceptionally bad idea, and basically breaks the Java platform ("Q: What's the contract of Object.equals()? A: It depends what the custom modifications to the JVM make it do...") - you wouldn't be able to get anything done.

Think about what you're doing - you're adding this class (and possible behaviour) to every object. ClassLoaders, Strings, Threads, InputStreams, Throwables, XMLGregorianCalendar, everything. This is almost certainly not what you intended.

Instead, an alternative approach would be to add your modifications to an abstract class AppnameSuperObject, and extend this for the classes that you really want to add this behaviour to.

---

On the other hand, if you really do want to do this for all objects for some kind of logging/profiling/etc kind of work, look at using aspect-oriented programming to weave the extra fields onto the classes at runtime.

Answer 2

Error occurred during initialization of VM java.lang.IllegalStateException at java.lang.Throwable.initCause(Throwable.java:337) at java.lang.ExceptionInInitializerError.(ExceptionInInitializerError.java:79)

The java.lang.IllegalStateException is thrown if initCause() is called more than once. Sounds like your modification of Object is causing an exception and when the JVM tries to create an Exception object (which is a subclass of Object) it gets into a recursive loop and attempts to call initCause() more than once on the same Exception object.

Why do you want to modify the definition of Object?