Question

While instrumenting a class for its different methods In order to make a method do a write operation in a text file. I first stored the string in a local variable 3160 explicitly defined. How to choose these variables to prevent conflicts with already existing variables.

Like in this snippet The code does the work of writing the class name to a text file every time it enters any method. In order to do so the string s had to be loaded on stack using variable 3160 ( value kept large so that the already defined variable names do not conflict with variable s (3160). My Question is how to define a local variables in a method during instrumentation with ASM Library. This question may seem kind of premature to many but that is because I am a beginner.

    String s= className;
    mv.visitLdcInsn(s);
    mv.visitVarInsn(Opcodes.ASTORE, 3160);
    mv.visitTypeInsn(Opcodes.NEW, "java/lang/StringBuilder");
    mv.visitInsn(Opcodes.DUP);
    mv.visitVarInsn(Opcodes.ALOAD, 3160);
    mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/String", "valueOf", "(Ljava/lang/Object;)Ljava/lang/String;");
    mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "(Ljava/lang/String;)V");
    mv.visitMethodInsn(Opcodes.INVOKESTATIC, "com/me/database/dataCollectionFile/Info", "callMeAnyTime", "()Ljava/lang/String;");
    mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;");
    mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;");

Answer 1

You should use LocalVariablesSorter adapter (either extend your own visitor from it, or add it to the visitors chain before MethodWriter). Then when you’ll need a new variable you can call LocalVariablesSorter.newLocal() method to get new variable slot allocated. Also see ASM guide for more details.