[TOC]
所谓代理代理模式可以在不修改被代理对象的基础上，**通过扩展代理类，进行一些功能的附加与增强**。值得注意的是，代理类和被代理类应该共同实现一个接口，或者是共同继承某个类。
比如，代理类可以补充一些打印日志，统计时长的方法。
## Java 静态代理
~~~
//通用的接口
public interface Movie {
void play();
}
//真正的实现这个 Movie 接口的类
public class RealMovie implements Movie {
@Override
public void play() {
System.out.println("您正在观看电影 《肖申克的救赎》");
}
}
//代理类
public class Cinema implements Movie {
RealMovie movie;
public Cinema(RealMovie movie) {
super();
this.movie = movie;
}
@Override
public void play() {
guanggao(true);
movie.play();
guanggao(false);
}
public void guanggao(boolean isStart){
if ( isStart ) {
System.out.println("电影马上开始了");
} else {
System.out.println("电影马上结束了");
}
}
}
// 方法
public class ProxyTest {
public static void main(String[] args) {
RealMovie realmovie = new RealMovie();
Movie movie = new Cinema(realmovie);
movie.play();
~~~
## Java 动态代理
### Demo
~~~
//通用的接口
public interface SellWine {
void mainJiu();
}
//真正的实现类
public class MaotaiJiu implements SellWine {
@Override
public void mainJiu() {
// TODO Auto-generated method stub
System.out.println("我卖得是茅台酒。");
}
}
//代理类
public class GuitaiA implements InvocationHandler {
private Object pingpai;
public GuitaiA(Object pingpai) {
this.pingpai = pingpai;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable {
System.out.println("销售开始："+this.getClass().getSimpleName());
method.invoke(pingpai, args);
System.out.println("销售结束");
return null;
}
}
public class Test {
public static void main(String[] args) {
MaotaiJiu maotaijiu = new MaotaiJiu();
InvocationHandler jingxiao1 = new GuitaiA(maotaijiu);
SellWine dynamicProxy = (SellWine) Proxy.newProxyInstance(MaotaiJiu.class.getClassLoader(),
MaotaiJiu.class.getInterfaces(), jingxiao1);
dynamicProxy.mainJiu();
}
}
结果；
销售开始 GuitaiA
我卖得是茅
~~~
### 语法
#### Proxy
动态代码涉及了一个非常重要的类 Proxy。正是通过 Proxy 的静态方法 newProxyInstance才会动态创建代理的实例对象。
~~~
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
~~~
下面讲解它的 3 个参数意义。
* loader 自然是类加载器
* interfaces 代码要用来代理的接口
* h 一个 InvocationHandler 对象
#### InvocationHandler
InvocationHandler 是一个接口，官方文档解释说，每个代理的实例都有一个与之关联的 InvocationHandler 实现类，如果代理的方法被调用，那么代理便会通知和转发给内部的 InvocationHandler 实现类，由它决定处理
~~~
public interface InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;
}
~~~
* proxy 代理对象
* method 代理对象调用的方法
* args 调用的方法中的参数
### 原理
#### newProxyInstance
newProxyInstance 实际上就是动态生成的代理类，类名是**包名+$Proxy+id序号**。
~~~
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
//核心方法 创建类定义
Class<?> cl = getProxyClass0(loader, intfs);
try {
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
...。
//反射初始化
return cons.newInstance(new Object[]{h});
} catch (IllegalAccessException|InstantiationException e) {
..
}
~~~
#### getProxyClass0
~~~
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
...
。
// 直接通过缓存获取，如果获取不到，注释说会通过 ProxyClassFactory 生成。
return proxyClassCache.get(loader, interfaces);
}
~~~
#### ProxyClassFactory
~~~
/**
* A factory function that generates, defines and returns the proxy class given
* the ClassLoader and array of interfaces.
*/
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// Proxy class 的前缀是 “$Proxy”，
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class<?> interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null; // package to define proxy class in
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
/*
* Record the package of a non-public proxy interface so that the
* proxy class will be defined in the same package. Verify that
* all non-public proxy interfaces are in the same package.
*/
for (Class<?> intf : interfaces) {
int flags = intf.getModifiers();
if (!Modifier.isPublic(flags)) {
accessFlags = Modifier.FINAL;
String name = intf.getName();
int n = name.lastIndexOf('.');
String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
if (proxyPkg == null) {
proxyPkg = pkg;
} else if (!pkg.equals(proxyPkg)) {
throw new IllegalArgumentException(
"non-public interfaces from different packages");
}
}
}
if (proxyPkg == null) {
// if no non-public proxy interfaces, use com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
/*
* A ClassFormatError here means that (barring bugs in the
* proxy class generation code) there was some other
* invalid aspect of the arguments supplied to the proxy
* class creation (such as virtual machine limitations
* exceeded).
*/
throw new IllegalArgumentException(e.toString());
}
}
~~~
defineClass0() 甚至是一个 native 方法。我们只要知道，动态创建代理这回事就好了。
## Android 通过动态代理设置事件监听
~~~
public class MainActivity extends AppCompatActivity {
private Button bt;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
bt = (Button) findViewById(R.id.bt);
proxySetOnclick();
}
//通过代理为button设置监听
private void proxySetOnclick() {
try {
//获得真实回调的方法(后期可以用注解获得这里hardCode了)
Method realClick = this.getClass().getMethod("realClick",View.class);
//创建一个代理
ProxyHandle handle = new ProxyHandle(this, realClick);
//设置代理 返回的是对应的接口 (OnClickListener实例对象)
View.OnClickListener proxyObj = (View.OnClickListener)Proxy.newProxyInstance(View.OnClickListener.class.getClassLoader(), new Class[]{View.OnClickListener.class}, handle);
//调用对应View的设置监听方法 之后会到代理handle里去(对象可以用注解获得这里暂不分析)
bt.setOnClickListener(proxyObj);
} catch (Exception e) {
e.printStackTrace();
}
}
//真实回调方法
public void realClick(View v) {
Toast.makeText(this,"点击了",Toast.LENGTH_SHORT).show();
}
}
public class ProxyHandle implements InvocationHandler {
private Object realObj;
private Method realMethod;
public ProxyHandle(Object realObj,Method realMethod){
this.realObj=realObj;
this.realMethod=realMethod;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//如果真实对象跟方法都不为null 则直接返回真是方法(这里真实方法必须跟接口方法参数保存一致)
if (realObj!=null&&realMethod!=null){
return realMethod.invoke(realObj,args);
}else {
return method.invoke(proxy,args);
}
~~~
## 参考资料
[轻松学，Java 中的代理模式及动态代理](https://blog.csdn.net/briblue/article/details/73928350)
- Java
- Object
- 内部类
- 异常
- 注解
- 反射
- 静态代理与动态代理
- 泛型
- 继承
- JVM
- ClassLoader
- String
- 数据结构
- Java集合类
- ArrayList
- LinkedList
- HashSet
- TreeSet
- HashMap
- TreeMap
- HashTable
- 并发集合类
- Collections
- CopyOnWriteArrayList
- ConcurrentHashMap
- Android集合类
- SparseArray
- ArrayMap
- 算法
- 排序
- 常用算法
- LeetCode
- 二叉树遍历
- 剑指
- 数据结构、算法和数据操作
- 高质量的代码
- 解决问题的思路
- 优化时间和空间效率
- 面试中的各项能力
- 算法心得
- 并发
- Thread
- 锁
- java内存模型
- CAS
- 原子类Atomic
- volatile
- synchronized
- Object.wait-notify
- Lock
- Lock之AQS
- Lock子类
- 锁小结
- 堵塞队列
- 生产者消费者模型
- 线程池