Programmatische Bytecodeerzeugung mit BCEL

[Thomas Darimont]

Hallo,

hier mal ein Beispiel wie man mit der im JDK integrierten BCEL Bibliothek Bytecode Programmatisch generieren kann - genauer gesagt lassen wir uns Code für eine beliebige Java-Klasse generieren der Bytecode-Instruktionen erzeugt welcher den ursprünglichen Bytecode der Klasse generiert. Sehr interessant für kleine Bytecodeexperimente

package de.tutorials;

import com.sun.org.apache.bcel.internal.classfile.JavaClass;
import com.sun.org.apache.bcel.internal.util.BCELifier;
import com.sun.org.apache.bcel.internal.util.ClassLoaderRepository;

public class BytecodeFun {

	public static void main(String[] args) throws Exception{
		ClassLoaderRepository clRepo = new ClassLoaderRepository(X.class.getClassLoader());
		JavaClass clazz = clRepo.loadClass(X.class.getName());
		
		BCELifier bcelifier = new BCELifier(clazz,System.out);
		bcelifier.start();
		
	}
	
	public static class X{
		private int a;
		protected String b;
		public int op(String arg){
			return Integer.parseInt(arg);
		}
	}
}

Ausgabe:

package de.tutorials;

import com.sun.org.apache.bcel.internal.generic.*;
import com.sun.org.apache.bcel.internal.classfile.*;
import com.sun.org.apache.bcel.internal.*;
import java.io.*;

public class BytecodeFun$XCreator implements Constants {
  private InstructionFactory _factory;
  private ConstantPoolGen    _cp;
  private ClassGen           _cg;

  public BytecodeFun$XCreator() {
    _cg = new ClassGen("de.tutorials.BytecodeFun$X", "java.lang.Object", "BytecodeFun.java", ACC_PUBLIC | ACC_SUPER, new String[] {  });

    _cp = _cg.getConstantPool();
    _factory = new InstructionFactory(_cg, _cp);
  }

  public void create(OutputStream out) throws IOException {
    createFields();
    createMethod_0();
    createMethod_1();
    _cg.getJavaClass().dump(out);
  }

  private void createFields() {
    FieldGen field;

    field = new FieldGen(ACC_PRIVATE, Type.INT, "a", _cp);
    _cg.addField(field.getField());

    field = new FieldGen(ACC_PROTECTED, Type.STRING, "b", _cp);
    _cg.addField(field.getField());
  }

  private void createMethod_0() {
    InstructionList il = new InstructionList();
    MethodGen method = new MethodGen(ACC_PUBLIC, Type.VOID, Type.NO_ARGS, new String[] {  }, "<init>", "de.tutorials.BytecodeFun$X", il, _cp);

    InstructionHandle ih_0 = il.append(_factory.createLoad(Type.OBJECT, 0));
    il.append(_factory.createInvoke("java.lang.Object", "<init>", Type.VOID, Type.NO_ARGS, Constants.INVOKESPECIAL));
    InstructionHandle ih_4 = il.append(_factory.createReturn(Type.VOID));
    method.setMaxStack();
    method.setMaxLocals();
    _cg.addMethod(method.getMethod());
    il.dispose();
  }

  private void createMethod_1() {
    InstructionList il = new InstructionList();
    MethodGen method = new MethodGen(ACC_PUBLIC, Type.INT, new Type[] { Type.STRING }, new String[] { "arg0" }, "op", "de.tutorials.BytecodeFun$X", il, _cp);

    InstructionHandle ih_0 = il.append(_factory.createLoad(Type.OBJECT, 1));
    il.append(_factory.createInvoke("java.lang.Integer", "parseInt", Type.INT, new Type[] { Type.STRING }, Constants.INVOKESTATIC));
    InstructionHandle ih_4 = il.append(_factory.createReturn(Type.INT));
    method.setMaxStack();
    method.setMaxLocals();
    _cg.addMethod(method.getMethod());
    il.dispose();
  }

  public static void _main(String[] args) throws Exception {
    de.tutorials.BytecodeFun$XCreator creator = new de.tutorials.BytecodeFun$XCreator();
    creator.create(new FileOutputStream("de.tutorials.BytecodeFun$X.class"));
  }
}

Gruß Tom