package com.krb;


import java.io.PrintStream;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.Type;
import org.objectweb.asm.commons.GeneratorAdapter;
import org.objectweb.asm.commons.Method;

public class Helloworld extends ClassLoader implements Opcodes {

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

        // Generates the bytecode corresponding to the following Java class:
        //
        // public class Example {
        // public static void main (String[] args) {
        // System.out.println("Hello world!");
        // }
        // }

        // creates a ClassWriter for the Example public class,
        // which inherits from Object
        ClassWriter cw = new ClassWriter(0);

        cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
        cw.visit(V1_1, ACC_PUBLIC, "Example", null, "java/lang/Object", null);

        // creates a GeneratorAdapter for the (implicit) constructor
        Method m = Method.getMethod("void <init> ()");
        GeneratorAdapter mg = new GeneratorAdapter(ACC_PUBLIC,
            m,
            null,
            null,
            cw);
        mg.loadThis();
        mg.invokeConstructor(Type.getType(Object.class), m);
        mg.returnValue();
        mg.endMethod();

        // creates a GeneratorAdapter for the 'main' method
        m = Method.getMethod("void main (String[])");
        mg = new GeneratorAdapter(ACC_PUBLIC + ACC_STATIC, m, null, null, cw);
        mg.getStatic(Type.getType(System.class),
            "out",
            Type.getType(PrintStream.class));
        mg.push("Hello world!");
        mg.invokeVirtual(Type.getType(PrintStream.class),
            Method.getMethod("void println (String)"));
        mg.returnValue();
        mg.endMethod();

        cw.visitEnd();

        byte[] code = cw.toByteArray();
        Helloworld loader = new Helloworld();
        Class exampleClass = loader.defineClass("Example", code, 0, code.length);

        // uses the dynamically generated class to print 'Helloworld'
        exampleClass.getMethods()[0].invoke(null, new Object[] { null });
    }
}