Generating bytecode using asm

Question

I have a code like this:

tracker.setValue(false);

for which I need to generate the bytecode via ASM. So using the tool, I found the bytecode instructions for the above line is:

ALOAD 0
GETFIELD example/MethodAdapter.tracker : Lexample/Tracker;
ICONST_0
INVOKEVIRTUAL example/Tracker.setValue (Z)V

so when I convert this to into an ASM code, which looks like this:

mv.visitVarInsn(ALOAD, 0);
mv.visitFieldInsn(GETFIELD, "example/MethodAdapter", "tracker", "Lexample/Tracker;");
mv.visitInsn(ICONST_0);
mv.visitMethodInsn(INVOKEVIRTUAL, "example/Tracker", "setValue", "(Z)V", false);

(this code snippet is written into visitMethodInsn ) however running the above ASM with -javaagent for a test class , I get an exception saying:

Exception Details:
  Location:
    TestClass.main([Ljava/lang/String;)V @20: getfield
  Reason:
    Type '[Ljava/lang/String;' (current frame, stack[0]) is not assignable to 'example/MethodAdapter'
  Current Frame:
    bci: @20
    flags: { }
    locals: { '[Ljava/lang/String;', 'TestClass' }
    stack: { '[Ljava/lang/String;' }
  Bytecode:
    0000000: b800 02bb 0003 59b7 0004 4c2b b600 052b
    0000010: b600 062a b400 2a03 b600 30b8 0007 b1  

    at java.lang.Class.getDeclaredMethods0(Native Method)
    at java.lang.Class.privateGetDeclaredMethods(Class.java:2688)
    at java.lang.Class.getMethod0(Class.java:2937)
    at java.lang.Class.getMethod(Class.java:1771)
    at sun.launcher.LauncherHelper.validateMainClass(Unknown Source)
    at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)

I found the issue is happening with this line:

mv.visitVarInsn(ALOAD, 0);
mv.visitFieldInsn(GETFIELD, "example/MethodAdapter", "tracker", "Lexample/Tracker;");

looks like ALOAD of 0 is not pulling out the proper this or am I missing something here?

Answer 1

You're inserting the code into your main function, which is static, and hence has no this argument. The first argument is the string array.