java agent - Instrument selected files (exclude all inbuilt java classes and methods)

Question

I am writing my first java agent. I am trying to instrument a project that I picked from git repo (not developed by me).

I have written my Agent class with the premain method and implement some logging ( logs the number of lines executed using the ASM bytecode manipulation framework)

However, I see that even the inbuilt java function calls/ classes are getting instrumented which is incorrect. I only want the files in my project to be instrumented. To achieve this, I added a filter shown below -

  public static void premain(String agentArgs, Instrumentation inst) {

    System.out.println("Premain called");

    inst.addTransformer(new ClassFileTransformer() {
      public byte[] transform(ClassLoader classLoader, 
                              String className,
                              Class<?> classBeingRedefined, 
                              ProtectionDomain protectionDomain, 
                              byte[] bytes)throws IllegalClassFormatException {


        // ASM Code
        if(className.startsWith("org/mytestpackage/")){
          ClassReader reader = new ClassReader(bytes);
          ClassWriter writer = new ClassWriter(reader, 0);
          ClassTransformVisitor visitor = new ClassTransformVisitor(writer);
          reader.accept(visitor, 0);
          return writer.toByteArray();
        } 

        return null;

      }
    });

  }

After I added this filter, the premain is being called but I am getting some exception

initializationError(org.mytestpackage.TestAllPackages)  Time elapsed: 0.002 sec  <<< ERROR!
java.lang.VerifyError: (class: org/mytestpackage/TestAllPackages, method: main signature: ([Ljava/lang/String;)V) Stack size too large
        at java.lang.Class.getDeclaredMethods0(Native Method)
        at java.lang.Class.privateGetDeclaredMethods(Class.java:2701)
        at java.lang.Class.privateGetMethodRecursive(Class.java:3048)
        at java.lang.Class.getMethod0(Class.java:3018)
        at java.lang.Class.getMethod(Class.java:1784)

Before I added this filter ( if condition based on the className), I was able to see my logic work for inbuilt java classes and methods. If fails after adding the filter.

Appreciate any help, TIA.

Answer 1

Your ClassTransformVisitor seems to break the code of your transformed case. In your particular case, it seems to not adjust the stack size of methods in your class org.mytestpackage.TestAllPackages.

The verifier error Stack size too large indicates that you push more values to a method's operand stack than what you specified as slots for this stack. You can ask ASM to fix this for you by specifying new ClassWriter(reader, ClassWriter.COMPUTE_MAXS).