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Decompiling Java

2 replies on 1 page. Most recent reply: Jun 22, 2005 10:14 AM by Christopher Diggins

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Christopher Diggins

Posts: 1215
Nickname: cdiggins
Registered: Feb, 2004

Decompiling Java (View in Weblogs)
Posted: Jun 21, 2005 9:46 PM
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Summary
Here is some Java code which uses the BCEL to convert a class file into java byte code.
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I have been playing with Java byte code lately and I've just discovered the awesome Byte Code Engineering Library (BCEL). I feel like a kitten with a ball of yarn. Here is the source for a simple decompiler:
import java.io.*;

import java.util.Iterator;
import org.apache.bcel.*;
import org.apache.bcel.classfile.*;
import org.apache.bcel.generic.*;
import org.apache.bcel.Repository;

public class Decompiler
{
  public static void main(String[] argv)
  {
    try
    {
      // Load the class from CLASSPATH.
      JavaClass       clazz   = Repository.lookupClass(argv[0]);
      Method[]        methods = clazz.getMethods();
      ConstantPoolGen cp = new ConstantPoolGen(clazz.getConstantPool());
      InstructionFactory f = new InstructionFactory(cp);

      for(int i=0; i < methods.length; i++)
      {
        Method m = methods[i];
        MethodGen mg = new MethodGen(m, clazz.getClassName(), cp);
        decompileMethod(mg, f);
      }
    }
    catch(Exception e)
    {
      e.printStackTrace();
    }
  }

  private static final void decompileMethod(MethodGen mg, InstructionFactory f)
  {
    InstructionList   il    = mg.getInstructionList();
    int               count = 0;

    System.out.println("Method " + mg.getName());
    InstructionHandle ih = il.getStart();
    while (ih != null) {
      System.out.println(ih.toString());
      ih = ih.getNext();
    }

    il.dispose(); // Reuse instruction handles
  }
}
I am planning next on a writing a ClassLoader which hacks code as it loads it. This is so much fun!


Eugene Kuleshov

Posts: 1
Nickname: eu
Registered: Jun, 2005

Re: Decompiling Java Posted: Jun 22, 2005 5:42 AM
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Christopher, it is really time to discover ASM. :-)
http://www.jroller.com/page/eu/20050622#it_is_time_to_discover

Christopher Diggins

Posts: 1215
Nickname: cdiggins
Registered: Feb, 2004

Re: Decompiling Java Posted: Jun 22, 2005 10:14 AM
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> Christopher, it is really time to discover ASM. :-)
> http://www.jroller.com/page/eu/20050622#it_is_time_to_disco
> ver

Very nice! Thanks for bringing ASM to my attention. The direct link to ASM for those following the discussion is:

http://asm.objectweb.org/

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