RE: ASM-based class transformation affect class loading?

I'm not entirely sure if this is your problem or not, but one lesser known
things about ClassFileTransformer agents is that they are not reentrant.
If the course of instrumenting a class (ie. within your  transform()
method) you cause another class load then the transformer will be skipped.
If your transform loads other types (for verification or other purposes)
then you might miss types. 

 

For the classes that are missing you might want to set a breakpoint on
your c lassloader to see if it is being loaded in the context of
transforming some other type. Once you know the call stack causing it you
can work on trying to fix it :-) 

 

 

 

From: Sai Zhang [mailto:[email protected]] 
Sent: Tuesday, August 03, 2010 8:49 AM
To: asm
Subject: [asm] ASM-based class transformation affect class loading?

 

Hi all:

I met a strange problem when using ASM to transform my client code at load
time (excluding all JDK code already).

It seems that my transformer is highly likely to affect the class loading
process. Please refer to the simple code below
for illustration:

Here is my simple ClassFileTransformer for load time class transformation,
all transformation operations are
in the treeAPITransform(byte[]) method. It is actually a fairly simple
intra-method transformation based on the "tree api" 
of ASM (just adding tracing method at the begin/end of each method call)


@Override
    public byte[] transform(ClassLoader loader, String className,
Class<?> clazz, ProtectionDomain domain,   byte[] bytes) {
         //code omitted here, for excluding all JDK classes

          
           /*I print each loaded class for debugging */
(1)   System.err.println("---- load class: " + className);
        
        try {
(2)          return this.treeAPITransform(bytes);   //my simple
transformation implementation
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

The weird thing is that statement (1) does not print out some obviously
loaded classes (which has definitely been initialized and executed).

 

If I remove statement (2), or change that to "return null", every thing
OK. all classes loaded are printed out at statement (1).

 

I feel very confused on this point. I am wondering will ASM-based
intra-method transformation affect the class loading? or

did I instrument some extra classes by accident? or I should not use the
tree api? or there is any tricks on this point?

 

P.S. I use the ClassCheckerAdapter in ASM to check the validity of
transformed bytecode after transformation. and no error messages

are issued.

 

thanks. Any comments are highly appreciated!!!

 

-Sai

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