thierry-janaudys-powerbook-g4-17:~/Easily.co.uk/janaudy-ws/byteslayer thierryjanaudy$

ByteSlayer
[Sept, 10th 2005]

I became quite bored lately and I decided to write a Java decompiler. 
Before I could do that, I needed to fully understand the Java byte-code instructions; 
so I decided to write my own javap tool. It is part of a framework called ByteSlayer. 
I like the name ;-).

So, what does it do? It does exactly what it says on the can, it gets a Java .class 
and spits out the byte code instructions, exactly like javap.

Here is a stupid Java class:


package org.byteslayer.test;

import java.awt.Point;

public abstract class Shape {
	private Point location = null;
	public Shape() {this.location = new Point(0, 0);}
	public Shape(Shape s) {this.location = s.getLocation();}
	public abstract void paint();
	public native void renderOpenGL();
	public Point getLocation() {return this.location;}
}


The official javap tools produces:

thierry-janaudys-powerbook-g4-17:~/Projects/ByteSlayer/classes thierryjanaudy$ javap -c -private -classpath . org.byteslayer.test.Shape
HotSpot not at correct virtual address. Sharing disabled.

Compiled from "Shape.java"
public abstract class org.byteslayer.test.Shape extends java.lang.Object{
private java.awt.Point location;

public org.byteslayer.test.Shape();
  Code:
   0:   aload_0
   1:   invokespecial   #11; //Method java/lang/Object."":()V
   4:   aload_0
   5:   aconst_null
   6:   putfield        #13; //Field location:Ljava/awt/Point;
   9:   aload_0
   10:  new     #15; //class java/awt/Point
   13:  dup
   14:  iconst_0
   15:  iconst_0
   16:  invokespecial   #18; //Method java/awt/Point."":(II)V
   19:  putfield        #13; //Field location:Ljava/awt/Point;
   22:  return

public org.byteslayer.test.Shape(org.byteslayer.test.Shape);
  Code:
   0:   aload_0
   1:   invokespecial   #11; //Method java/lang/Object."":()V
   4:   aload_0
   5:   aconst_null
   6:   putfield        #13; //Field location:Ljava/awt/Point;
   9:   aload_0
   10:  aload_1
   11:  invokevirtual   #27; //Method getLocation:()Ljava/awt/Point;
   14:  putfield        #13; //Field location:Ljava/awt/Point;
   17:  return

public abstract void paint();

public native void renderOpenGL();

public java.awt.Point getLocation();
  Code:
   0:   aload_0
   1:   getfield        #13; //Field location:Ljava/awt/Point;
   4:   areturn

}

ByteSlayer produces this:


package org.byteslayer.test;

public abstract class Shape extends java.lang.Object {
  private  java.awt.Point  location;

  public  void Shape() {
    0:	(0x2a aload_0)
    1:	(0xb7 invokespecial) 0 b -> Method :()V in class java.lang.Object
    4:	(0x2a aload_0)
    5:	(0x1 aconst_null)
    6:	(0xb5 putfield) 0 d -> Method location:Ljava/awt/Point; in class org.byteslayer.test.Shape
    9:	(0x2a aload_0)
    10:	(0xbb new) 0 f -> java.awt.Point
    13:	(0x59 dup)
    14:	(0x3 iconst_0)
    15:	(0x3 iconst_0)
    16:	(0xb7 invokespecial) 0 12 -> Method :(II)V in class java.awt.Point
    19:	(0xb5 putfield) 0 d -> Method location:Ljava/awt/Point; in class org.byteslayer.test.Shape
    22:	(0xb1 return)
  }

  public  void Shape(org.byteslayer.test.Shape local1) {
    0:	(0x2a aload_0)
    1:	(0xb7 invokespecial) 0 b -> Method :()V in class java.lang.Object
    4:	(0x2a aload_0)
    5:	(0x1 aconst_null)
    6:	(0xb5 putfield) 0 d -> Method location:Ljava/awt/Point; in class org.byteslayer.test.Shape
    9:	(0x2a aload_0)
    10:	(0x2b aload_1)
    11:	(0xb6 invokevirtual) 0 1b -> Method getLocation:()Ljava/awt/Point; in class org.byteslayer.test.Shape
    14:	(0xb5 putfield) 0 d -> Method location:Ljava/awt/Point; in class org.byteslayer.test.Shape
    17:	(0xb1 return)
  }

  public abstract  void paint();

  public native  void renderOpenGL();

  public  java.awt.Point  getLocation() {
    0:	(0x2a aload_0)
    1:	(0xb4 getfield) 0 d -> Method location:Ljava/awt/Point; in class org.byteslayer.test.Shape
    4:	(0xb0 areturn)
  }

}


Get the everything for ByteSlayer.zip (630 Kb),
or just the byteslayer-src.zip (40 Kb),
or just the executable byteslayer-0.2.jar (64 Kb), to be used like:

[Usage] java -cp %CP% org.byteslayer.main.Javap 

Also, you might like this list of opcodes-with-semicolon.txt (4 Kb),
or more, this XL spreadsheet opcodes.xls (96 Kb) that I used to generate some code.

The ByteSlayer javadocs are also available.

What's next?
Well, I have some refactoring to do here because it was my first time ;-) I am not pleased with the way
I handled the operands. This version does not handle the wide and lookupswitch instructions.
What I would like to do is to come up with my own decompiler tool, and also my own simplistic AOP framework.

thierry at janaudy dot com