[Fwd: JVM -instrumentanting the JDK classes]

[Christian Wimmer]

Hi

>> In order for our system to work, we need to change the code of the  
>> Java libraries - all of them. The changes imply adding a field  
>> (potentially two), and adding some code to the beginning and end of  
>> every method.
>>  Using Soot, I got to add the field and methods to the classes.  
>> However, there are exactly two classes (Object and String) that,  
>> when added a field, make the JVM crash on startup. I'm starting the  
>> Sun JVM with the parameter Xbootclasspath  pointing to the  
>> instrumented libraries.

The layout of some core classes is known by the HotSpot VM, for  
example the class String. The reason is that HotSpot must allocate  
String objects even from non-Java code, for example when a String  
constant is loaded from a class file's constant pool. Other classes  
are for example the weak references, which are specially handled in  
the garbage collector.
When you add fields to Object, you implicitly add to all other  
classes, so this also fails.

All known classes should be listed in the file src/share/vm/classfile/ 
javaClasses.hpp of the HotSpot source code, look for example at http://hg.openjdk.java.net/jdk7/jdk7/hotspot/file/42ca4002efc2/src/share/vm/classfile/javaClasses.hpp

Christian