学步园

1、Person2.java

package com.study.asm;

public class Person2 {

	public String name = "yuanyi";
	public String address = "bj";
}

为上述类添加一个age属性：

package com.study.asm;

import java.io.File;
import java.io.FileOutputStream;

import org.objectweb.asm.ClassAdapter;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.FieldVisitor;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.Type;


public class AddOneField {

	/**
	 * @param args
	 */
	public static void main(String[] args) throws Exception{
		
		ClassReader classReader = new ClassReader("com.study.asm.Person2");
		
		ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_MAXS);
		
		ClassAdapter adapter = new MyClassAdapter2(classWriter);
		classReader.accept(adapter, ClassReader.SKIP_DEBUG);
		
		byte[] buf = classWriter.toByteArray();
		
		GeneratorClassLoader  classLoader = new GeneratorClassLoader ();
		Class clazz = classLoader.defineClassFromClassFile("com.study.asm.Person2", buf);
		
		Object obj = clazz.newInstance();
	//	System.out.println(clazz.getDeclaredField("name").get(obj));
		System.out.println(clazz.getDeclaredField("age").get(obj));
		
		File file = new File("D://Person2.class");
		FileOutputStream fos = new FileOutputStream(file);
		
		fos.write(buf);
		fos.close();
		
	}

}

class MyClassAdapter2 extends ClassAdapter{

	public MyClassAdapter2(ClassVisitor cv) {
		super(cv);
	}
	@Override
	public void visitEnd() {
		cv.visitField(Opcodes.ACC_PUBLIC, "age", Type.getDescriptor(int.class), null, null);
	}
	
/*	
 * 调用此方法会重复添加age属性，会报错
 * 具体原因参看：http://victorzhzh.iteye.com/blog/882699
 * @Override
	public FieldVisitor visitField(int access, String name, String desc,
			String signature, Object value) {
		cv.visitField(Opcodes.ACC_PUBLIC, "age", Type.getDescriptor(int.class), null, null);	
		return super.visitField(access, name, desc, signature, value);
	}*/
}


class GeneratorClassLoader extends ClassLoader {
	@SuppressWarnings("rawtypes")  
	public Class defineClassFromClassFile(String className, byte[] classFile)throws Exception{
		return defineClass(className, classFile, 0, classFile.length);   
	}
}

3、结果

反编译后，得到如下结果：

package com.study.asm;

public class Person2
{
  public String name = "yuanyi";
  public String address = "bj";
  public int age;
}