How to catch runtime exception in bytecode through ASM

Question

I am trying to catch runtime exception through exception. I am able to capture usual method exiting events. But , the control never reaches in opcode==Opcodes.ATHROW .

I think I am doing something wrong in calling the events.

Here is my sample code :

 public void visitCode() { 
//          mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;"); 
//          mv.visitLdcInsn("Entering method "  + fQMethodName); 
//          mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream","println","(Ljava/lang/String;)V"); 
            }


     @Override
    public void visitInsn(int opcode)
    {



         if (opcode == Opcodes.ATHROW)
         {
                          mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "err", "Ljava/io/PrintStream;");

          mv.visitLdcInsn("Exiting on exception " );         
             mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
                    "(Ljava/lang/String;)V");
         }


        super.visitInsn(opcode);
    }


         public void visitMethodInsn(int opcode, String owner, String name,
                   String desc) {


                  super.visitMethodInsn(opcode, owner, name, desc);
                    //                
                  if (opcode == Opcodes.ATHROW)
                  {
                                              mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "err", "Ljava/io/PrintStream;");

                   mv.visitLdcInsn("Exiting on exception " + name);         
                      mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
                            "(Ljava/lang/String;)V");
                  }
                  else if (!name.equals("println")
                    && !name.equals("<init>")
                    && (opcode == Opcodes.INVOKEVIRTUAL || opcode == Opcodes.INVOKESPECIAL
                      || opcode == Opcodes.INVOKESTATIC || opcode == Opcodes.INVOKEDYNAMIC)) {




                      this.currentMethod = name;

                   onFinally(opcode);
                  }
                 }

                 private void onFinally(int opcode) {
                                          mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "err",
                    "Ljava/io/PrintStream;");
                  mv.visitLdcInsn("Returning to " + fQMethodName + " from " + currentMethod);
                  mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
                    "(Ljava/lang/String;)V");
                 }

At the time of runtime exception , the existing stack content is cleared and only the throwable object remains. Can I get the stack top element ( i.e stack frame basically ) through ASM ?

Thanks in adavance.

EDIT :

Here is my modified program after adding try - catch block.

@Holger : Thanks for your help. Really appreciate it.

So , I am trying to insert dynamic try-catch block in MethodVisitor of ASM.

Here is my code :

public void visitCode() 
    {

        super.visitCode();

        this.visitTryCatchBlock(lblTryBlockStart, lblTryBlockEnd, lblCatchExceptionBlockStart, "java/lang/Exception");
        this.visitLabel(lblTryBlockStart);

    }  



    public void visitMaxs(int maxStack, int maxLocals)
    {
        // visit try block end label
        this.visitLabel(lblTryBlockEnd);
        // visit normal execution exit block
        this.visitJumpInsn(Opcodes.GOTO, exitBlock);

        // visit catch exception block
        this.visitLabel(lblCatchExceptionBlockStart);
        // store the exception
        this.visitVarInsn(Opcodes.ASTORE, 1);
        // load the exception
        this.visitVarInsn(Opcodes.ALOAD, 1);
        // call printStackTrace()
        //this.visitInsn(Opcodes.ATHROW);
        this.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/Exception", "printStackTrace", "()V");


        // exit from this dynamic block
        this.visitLabel(exitBlock);

        super.visitMaxs(maxStack+2, maxLocals+2);

    }

My intention is to add dynamic try - catch block , and if any exception is thrown , print it.

Answer 1

RuntimeExceptions do not need an athrow instruction to be thrown. An idiv or irem instruction can throw an ArithmeticException, a getfield, putfield or invoke… instruction can throw a NullPointerException, just to name some examples. So you don’t find an athrow instruction at these places.

The only way to intercept all of them is to inject an exception handler which processes your code, and re-throws the exception if you wish.