在目前的Java开发包中包含了对动态代理的支持，但是其实现只支持对接口的的实现。
其实现主要通过是java.lang.reflect.Proxy类和java.lang.reflect.InvocationHandler接口。
Proxy类主要用来获取动态代理对象，InvocationHandler接口用来约束调用者实现，如 下，HelloWorld接口定义的业务方法，HelloWorldImpl是HelloWorld接口的实现，HelloWorldHandler是 InvocationHandler接口实现。代码如下：
业务接口:
public interface HelloWorld {
        void sayHelloWorld() ;
}
业务接口实现：
public class HelloWorldImpl implements HelloWorld {
        public void sayHelloWorld() {
               System.out.println("Hello World!");             
        }
}
InvocationHandler实现，需要在接口方法调用前后加入一部份处理工作，这里仅仅在方法调用前后向后台输出两句字符串，其代码如下：
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class HelloWorldHandler implements InvocationHandler {
        //要代理的原始对象
        private Object objOriginal;
        /**
         * 构造函数。
         * @param obj 要代理的原始对象。
         */
        public HelloWorldHandler(Object obj) {
               this.objOriginal = obj ;
        }
        public Object invoke(Object proxy, Method method, Object[] args)
                      throws Throwable {
              
               Object result ;
              
         //方法调用之前
               doBefore();
              
         //调用原始对象的方法
               result = method.invoke(this.objOriginal ,args);
              
         //方法调用之后
               doAfter();
              
               return result ;
        }
       
        private void doBefore() {
               System.out.println("before method invoke!");
        }
       
        private void doAfter() {
               System.out.println("after method invoke!");
        }
}
测试代码：
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Proxy;
public class Test {
        public static void main(String[] args) {
               HelloWorld hw = new HelloWorldImpl();
              
               InvocationHandler handler = new HelloWorldHandler(hw);
              
               HelloWorld proxy = (HelloWorld) Proxy.newProxyInstance(
                             hw.getClass().getClassLoader(),
                             hw.getClass().getInterfaces(),
                             handler);
               proxy.sayHelloWorld();
        }
}
Ø         首先获取一个业务接口的实现对象；
Ø         获取一个InvocationHandler实现，此处是HelloWorldHandler对象；
Ø         创建动态代理对象；
Ø         通过动态代理对象调用sayHelloWorld()方法，此时会在原始对象HelloWorldImpl. sayHelloWorld()方法前后输出两句字符串。
运行测试类输出如下:
before method invoke!
Hello World!
after method invoke!
此处Test类中的方法调用代码比较多，在我们的实际应用中可以通过配置文件来来简化客户端的调用实现。另外也可以通过动态代理来实现简单的AOP。
============================================================
动态字节码生成(asm)

导读：
用ASM写的Hello World。在网上搜索ASM有关的文章，最后居然又找回Matrix。。汗

ASM2.0字节码框架介绍
http://www.matrix.org.cn/resource/article/2006-02-20/ASM+Bytecode+Framework_44220.html

package my;

import java.lang.reflect.Method;

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class Hello {
@SuppressWarnings("unchecked")
public static void main(String[] args) throws Exception {

        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
        // 类访问开始：必须
        cw.visit(Opcodes.V1_5, Opcodes.ACC_PUBLIC, "my/Foo", null, "java/lang/Object", null);

        // 至少提供一个构造函数
        MethodVisitor mv = cw.visitMethod(Opcodes.ACC_PUBLIC, "", "()V", null, null);
        // 代码开始：必须
        mv.visitCode();
        mv.visitVarInsn(Opcodes.ALOAD, 0);
        mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "", "()V");
        mv.visitInsn(Opcodes.RETURN);
        // 计算栈和局部变量最大空间：必须
        mv.visitMaxs(0, 0);
        // 代码结束：必须
        mv.visitEnd();

        mv = cw.visitMethod(Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC, "main",
                        "([Ljava/lang/String;)V", null, null);
        mv.visitCode();
        mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        mv.visitLdcInsn("Hello World!");
        mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
                        "(Ljava/lang/String;)V");
        mv.visitInsn(Opcodes.RETURN);
        mv.visitMaxs(0, 0);
        mv.visitEnd();

        // 类结束：必须
        cw.visitEnd();

        final byte[] bs = cw.toByteArray();
        Class clazz = new ClassLoader() {
                protected Class findClass(String name) throws ClassNotFoundException {
                        return defineClass(name, bs, 0, bs.length);
                }
        }.loadClass("my.Foo");
        Method method = clazz.getMethod("main", new Class[] { String[].class });
        // 数组参数的方法，反射调用方式看起来比较古怪
        method.invoke(null, (Object) new String[0]);

        for (int i = 0; i < bs.length; i++)
                System.out.printf("%d:\t%02X\t%c\n", i, bs[i], (char) bs[i]);

        // OutputStream out = new FileOutputStream("d:/my/Foo.class");
        // out.write(bs);
        // out.close();
        }
}
借助ASM写了一个Aqua Data Studio 6.0的破解：

做法很简单：
1、找到判断license的方法，修改代码使总返回true
2、将1个license线程kill掉。

将jar拷到安装目录，修改datastudio.bat文件的最后一行为：
java -javaagent:ads.crack.jar -cp ".\lib\ads.jar;%ADS_PATH%" com.aquafold.datastudio.DataStudio

由于论坛不支持jar文件上传，将文件扩展名改为ads.crack.jar即可。
有时候，如果想要得到程序中某个Class的所有实例，也可以用asm修改代码得到：

package my;

import java.io.FileOutputStream;
import java.io.OutputStream;
import java.lang.reflect.Field;
import java.net.URL;
import java.util.List;

import org.objectweb.asm.ClassAdapter;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodAdapter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class HelloModifyASM {
        public static void main(String[] args) throws Exception {
        URL url = HelloModifyASM.class.getResource("Foo.class");
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
        ClassVisitor cv = new ClassAdapter(cw) {

        public void visit(int version, int access, String name, String signature,
                        String superName, String[] interfaces) {
                super.visit(version, access, name, signature, superName, interfaces);

                // 添加字段：public static List _my_instances;
                super.visitField(Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC, "_my_instances",
                                "Ljava/util/List;", null, null);

                // 添加静态的初始化块
                MethodVisitor mv = super.visitMethod(Opcodes.ACC_STATIC, "", "()V", null,null);
                mv.visitCode();
                mv.visitTypeInsn(Opcodes.NEW, "java/util/ArrayList");
                mv.visitInsn(Opcodes.DUP);
                mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/util/ArrayList", "", "()V");
                mv.visitFieldInsn(Opcodes.PUTSTATIC, "my/Foo", "_my_instances", "Ljava/util/List;");
                mv.visitInsn(Opcodes.RETURN);
                mv.visitMaxs(0, 0);
                mv.visitEnd();
        }

        public MethodVisitor visitMethod(int access, String name, String desc,
                        String signature, String[] exceptions) {
                MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);

                // 修改无参的构造函数：
                if (!"".equals(name) || !"()V".equals(desc))
                        return mv;
                return new MethodAdapter(mv) {

                        public void visitInsn(int opcode) {
                                if (opcode == Opcodes.RETURN) {
                                        visitFieldInsn(Opcodes.GETSTATIC, "my/Foo", "_my_instances",
                                                        "Ljava/util/List;");
                                        visitVarInsn(Opcodes.ALOAD, 0);
                                        visitMethodInsn(Opcodes.INVOKEINTERFACE, "java/util/List", "add",
                                                        "(Ljava/lang/Object;)Z");
                                }
                                super.visitInsn(opcode);
                        }
                };
        }
        };

        ClassReader cr = new ClassReader(url.openStream());
        cr.accept(cv, ClassReader.SKIP_DEBUG);
        final byte[] bs = cw.toByteArray();

        OutputStream out = new FileOutputStream("d:/my/Foo.class");
        out.write(bs);
        out.close();

        Class clazz = new ClassLoader(null) {
                public Class findClass(String name) throws ClassNotFoundException {
                        if (!"my.Foo".equals(name))
                                return ClassLoader.getSystemClassLoader().loadClass(name);
                        return defineClass(name, bs, 0, bs.length);
                }
        }.loadClass("my.Foo");
        clazz.newInstance();
        clazz.newInstance();

        Field field = clazz.getField("_my_instances");
                List instances = (List) field.get(null);
                System.out.println(instances.size());
                for (Object obj : instances) {
                        System.out.println(obj);
                }
        }
}

问题：如果原始类有、或者没有静态初始化块，处理方法就是不同的。
想了一下，也许可以在visitMethod方法中设置标志，再在visitEnd方法中进行补充处理（针对没有的情况）。
另外，如果有多个构造函数，怎样保证插入的代码不会重复执行呢？
甚至，想要在原代码中插入语句，插入位置的寻找也比较费尽，（需要找到不同的RETURN语句的字节码）
本文转自
http://www.matrix.org.cn/thread.shtml?topicId=edd2d10c-a79a-11db-8440-755941c7293d&forumId=1