Is there any way to print the object ref that called an instance/Static method, Using byte-code instrumentation - java-bytecode-asm

Question

I read somewhere that when ever a method is called by "invokevirtual",
the object reference is fetched from the top of stack, followed by arguments.
I need to somehow print the object reference. Is it possible?

Answer 1

So, I'm not going to do it for you, because the actual code is annoying and tedious and if you're really genuinely interested you should learn how to do it yourself. But I will attempt to be helpful and provide you with some direction.
Firstly, you're going to want to read the ASM tutorials here.
The byte code format i'm going to write below comes from ASMIfier because it's much more clear. I'm going to completely ignore javap because it's even more pedantic and detailed, but if you want to know what it is actually showing you, then you should read about the Java ClassFile format.
Actually, you should do that first anyway, just to make sure that your background knowledge is somewhat filled out.
So, here's the nutshell of what you're going to want to do. You're going to want to write a ClassWriter that looks for instances of the INVOKEVIRTUAL opcode.
invokevirtual pops the values from the stack in reverse order, so last parameter first and the object you're invoking against last. the #38 you are referring too is not the object, its a reference to the constant pool which contains a method name and method descriptor pair which is used as metadata because the JVM is typesafe.
Lets assume you have this code:
package sample;
public class JavaSimpleHelloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
If you run ASMIFier against it, you'll get something like this for just the main method ( cutting the context down for brevity )
public static main([Ljava/lang/String;)V
L0
LINENUMBER 6 L0
GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
LDC "Hello World"
INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
L1
LINENUMBER 7 L1
RETURN
L2
LOCALVARIABLE args [Ljava/lang/String; L0 L2 0
MAXSTACK = 2
MAXLOCALS = 1
so, you implement some sort of static dump method ( public static final dump( Object o ) ) , and write a class visitor that reorganizes your byte code.
You can use the method descriptor to figure out how deep in the pervious stack push instructions ( ALOAD, LDC, ) you need to insert the the DUP/INVOKE to print your methods object target. For example the Method Descriptor for System.out.println is [Ljava/lang/String;]V Which means the method takes an array of Strings and returns void. So you need to go 1 back in the stack to find the object target. Your bytecode would, in turn, look like this:
Happy byte code twiddling.
public static main([Ljava/lang/String;)V
L0
GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
DUP
INVOKESTATIC my/staticutil/ClassThatDumps.dump (Ljava/lang/Object;)V
LDC "Hello World"
INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
RETURN
L1
LOCALVARIABLE args [Ljava/lang/String; L0 L1 0
MAXSTACK = 2
MAXLOCALS = 1