MakeMeAClass.java

package makemeaclass;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import org.objectweb.asm.*;
import org.objectweb.asm.util.*;
import org.objectweb.asm.tree.*;

public class MakeMeAClass implements Opcodes {

	@SuppressWarnings("unchecked")
	static void createClass() throws IOException {
		ClassNode cnode = new ClassNode();
		cnode.version = V1_5;
		cnode.sourceFile = "Grappa.java";
		cnode.access = ACC_PUBLIC;
		cnode.name = "grappa/Grappa";
		cnode.superName = "java/lang/Object";
		
		cnode.fields.add(new FieldNode(ACC_PRIVATE + ACC_FINAL, "grappa", "Ljava/lang/String;", null, "Grappa!"));
		
		MethodNode ctNode = new MethodNode(ACC_PUBLIC, "<init>", "()V", null, null);
		ctNode.instructions.add(new VarInsnNode(ALOAD, 0));
		ctNode.instructions.add(new MethodInsnNode(INVOKESPECIAL, "java/lang/Object", "<init>", "()V"));		
		ctNode.instructions.add(new VarInsnNode(ALOAD, 0));
		ctNode.instructions.add(new LdcInsnNode("Grappa!"));
		ctNode.instructions.add(new FieldInsnNode(PUTFIELD, "grappa/Grappa", "grappa", "Ljava/lang/String;"));
		ctNode.instructions.add(new InsnNode(RETURN));		
		
		MethodNode mnode = new MethodNode(ACC_PUBLIC, "grappaMe", "()Ljava/lang/String;", null, null);
		mnode.instructions.add(new VarInsnNode(ALOAD, 0));
		mnode.instructions.add(new FieldInsnNode(GETFIELD, "grappa/Grappa", "grappa", "Ljava/lang/String;"));
		mnode.instructions.add(new InsnNode(ARETURN));
		
		cnode.methods.add(ctNode);
		cnode.methods.add(mnode);
		
		ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS);
		cnode.accept(writer);
		
		StringWriter sw = new StringWriter();
		PrintWriter pw = new PrintWriter(sw);
		CheckClassAdapter.verify(new ClassReader(writer.toByteArray()), false, pw);
		
		FileOutputStream out = new FileOutputStream("bin/grappa/Grappa.class");
		out.write(writer.toByteArray());
		out.close();
	}
	
	static void useClass() throws InstantiationException, IllegalAccessException, ClassNotFoundException, SecurityException, NoSuchMethodException, IllegalArgumentException, InvocationTargetException {
		Class<?> clazz = Class.forName("grappa.Grappa");
		Object instance = clazz.newInstance();
		
		Method grappa = clazz.getMethod("grappaMe");
		String result = (String) grappa.invoke(instance);
		System.out.println(result);
	}
	
	public static void main(String[] args) throws Throwable {
		createClass();
		useClass();
	}

}