BCEL in Java 6

Java 6 have bcel packages, so I made interesting code to print "Hello World" every first-experience java developers have to meet.

There are no library have to include. That's my favorite!! 

BCEL tutorial is below web site.

http://jakarta.apache.org/bcel/manual.html

 

import com.sun.org.apache.bcel.internal.Constants;

import com.sun.org.apache.bcel.internal.generic.ArrayType;

import com.sun.org.apache.bcel.internal.generic.ClassGen;

import com.sun.org.apache.bcel.internal.generic.ConstantPoolGen;

import com.sun.org.apache.bcel.internal.generic.InstructionFactory;

import com.sun.org.apache.bcel.internal.generic.InstructionList;

import com.sun.org.apache.bcel.internal.generic.MethodGen;

import com.sun.org.apache.bcel.internal.generic.ObjectType;

import com.sun.org.apache.bcel.internal.generic.PUSH;

import com.sun.org.apache.bcel.internal.generic.Type;

public class SimpleHelloWorldBuilder {

public static void main(String[] args) throws Exception {

// public HellowWorld extends java.lang.Object

ClassGen classGen = new ClassGen("HelloWorld", "java.lang.Object", "", Constants.ACC_PUBLIC

| Constants.ACC_SUPER, null);

ConstantPoolGen constantPoolGen = classGen.getConstantPool();

InstructionList instructionList = new InstructionList();

// public static void main

MethodGen methodGen = new MethodGen(Constants.ACC_PUBLIC | Constants.ACC_STATIC, Type.VOID,

// (String[] args)

new Type[] {new ArrayType(Type.STRING, 1)}, new String[] {"args"}, "main", "HelloWorld", instructionList, constantPoolGen);

InstructionFactory instructionFactory = new InstructionFactory(classGen);

classGen.addEmptyConstructor(Constants.ACC_PUBLIC);

// System.out.println

ObjectType printStream = new ObjectType("java.io.PrintStream");

instructionList.append(instructionFactory.createFieldAccess("java.lang.System", "out", printStream, Constants.GETSTATIC));

// "Hello world!!"

instructionList.append(new PUSH(constantPoolGen, "Hello World!!"));

instructionList.append(instructionFactory.createInvoke("java.io.PrintStream", "println", Type.VOID,

new Type[] {Type.STRING}, Constants.INVOKEVIRTUAL));

classGen.addMethod(methodGen.getMethod());

instructionList.dispose();

classGen.getJavaClass().dump("build/HelloWorld.class");

}

}

After running this class, I can find "build/HelloWorld.class". 

By using jad, I can see the decompiled class.

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.

// Jad home page: http://www.kpdus.com/jad.html

// Decompiler options: packimports(3) 

// Source File Name:   

import java.io.PrintStream;

public class HelloWorld

{

    public HelloWorld()

    {

    }

    public static void main(String args[])

    {

        System.out.println("Hello World!!");

    }

}