Skip to content

How to hide variables from Java’s JDI?

I am instrumenting some classes and introducing some new local variables. Now, when the user places a breakpoint in the code, and execution is stopped, the newly introduced local variables can be seen inside Intellij IDEA’s debugger window. How can I hide them?

UPDATE: I will have to somehow remove debug info from the instrumented code, but not sure how to do it.

UPDATE 2: I am using the ASM library for instrumentation.

 public void visitCode() {;, "MyAgentClass", "loadData", "(Ljava/lang/String;)LDataClass;", false);, this.getDataIndex());

public void visitMaxs(int maxStack, int maxLocals) {
     if (this.myStartLabel != null && this.myEndLabel != null) {"__my__data__", "Ljava/lang/Object;", (String) null, this.myStartLabel, this.myEndLabel, this.getDataIndex());

       super.visitMaxs(maxStack, maxLocals);

__my__data__ is shown inside Intellij IDEA.



Based on the JVM specs, you can remove your local variables from LocalVariableTable. Javassist does that automatically in generated code and I cannot see the variable secretCode during the run:

enter image description here

Decompiling the result class can show that there are no LocalVariableTable entries for it:

  public void run();
    descriptor: ()V
    flags: ACC_PUBLIC
      stack=2, locals=3, args_size=1
         0: ldc           #39                 // int -889275714
         2: istore_1
         3: iload_1
         4: invokestatic  #43                 // Method org/example/App.test:(I)Ljava/lang/Integer;
         7: astore_2
         8: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
        11: aload_2
        12: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
        15: new           #2                  // class SecreFoo
        18: dup
        19: invokespecial #3                  // Method "<init>":()V
        22: astore_1
        23: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
        26: aload_1
        27: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
        30: return
        line 14: 15
        line 15: 23
        line 16: 30
        Start  Length  Slot  Name   Signature
            0      31     0  this   LSecreFoo;
           23       8     1 fCopy   LSecreFoo;

So when you are instrumenting your class, drop all your local variables from the table (or do not add them).