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JDK动态代理--实例/原理


简介

说明

本文用示例介绍JDK动态代理的用法

JDK动态代理的原理简述

生成的代理类继承Proxy,且实现了业务的接口,Proxy中有InvocationHandler的实现类的引用。

调用被代理的类的方法时,会调用父类(Proxy)的InvocationHandler成员的invoke方法。

限制

只能代理实现了接口的类。

实例

动态代理具体步骤


  1. 通过实现 InvocationHandler 接口创建自己的调用处理器;
  2. 通过为 Proxy 类指定 ClassLoader 对象和一组 interface 来创建动态代理类;
  3. 通过反射机制获得动态代理类的构造函数,其唯一参数类型是调用处理器接口类型;
  4. 通过构造函数创建动态代理类实例,构造时调用处理器对象作为参数被传入。

实例1:统计执行时间

package com.example.a;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

//代理接口
interface ExecutionTime {
public void dealTask(String taskName);
public void sayHello(String name);
}

//被代理类
class ExecutionTimeImpl implements ExecutionTime {
//这里打印出任务名,并休眠500ms模拟任务执行了很长时间
@Override
public void dealTask(String taskName) {
System.out.println("Task is running: "+taskName);
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}

@Override
public void sayHello(String name) {
System.out.println("hello" + name);
}
}

// 动态代理类对应的调用处理程序类
class ExecutionTimeProxy implements InvocationHandler {
private Object delegate = null;

public ExecutionTimeProxy(ExecutionTime delegate) {
this.delegate = delegate;
}

// proxy:代理类代理的真实代理对象。例如:com.sun.proxy.$Proxy0
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
long startTime = System.currentTimeMillis();
method.invoke(delegate, args);
long endTime = System.currentTimeMillis();
System.out.println("Elapsed time: "+(endTime - startTime)+" ms");

return null;
}
}

//生成动态代理对象的工厂
class DynamicProxyFactory {
public static ExecutionTime getInstance(){
ExecutionTime origin = new ExecutionTimeImpl();
InvocationHandler handler = new ExecutionTimeProxy(origin);
ExecutionTime proxy = (ExecutionTime) Proxy.newProxyInstance(
origin.getClass().getClassLoader(),
origin.getClass().getInterfaces(),
handler);
return proxy;
}
}

public class Demo{
public static void main(String[] args) {
System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

ExecutionTime proxy = DynamicProxyFactory.getInstance();
System.out.println(proxy.getClass());
System.out.println("-----------------------------------");
proxy.dealTask("TestTask");
}
}

运行结果

class org.example.a.$Proxy0
-----------------------------------
Task is running: TestTask
Elapsed time: 500 ms

实例2:ArrayList

package org.example.a;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.List;

public class Demo {
public static void main(String[] args) {
final ArrayList<String> list = new ArrayList<>();
List<String> listProxy=(List<String>) Proxy.newProxyInstance(
list.getClass().getClassLoader(),
list.getClass().getInterfaces(),
new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
return method.invoke(list,args);
}
});
listProxy.add("hello") ;
listProxy.add("world") ;
System.out.println(list);
}
}

运行结果

[hello, world]

原理

相关类

InvocationHandler

package java.lang.reflect;

public interface InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable;
}

Proxy

package java.lang.reflect;

public class Proxy implements java.io.Serializable {

private static final long serialVersionUID = -2222568056686623797L;

/** parameter types of a proxy class constructor */
private static final Class<?>[] constructorParams = { InvocationHandler.class };

/**
* a cache of proxy classes
*/
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

/**
* the invocation handler for this proxy instance.
* @serial
*/
protected InvocationHandler h;

/**
* Prohibits instantiation.
*/
private Proxy() {
}

// 其他代码
}

生成的代理类的源码

查看自动生成的proxy class

默认在运行时程序自动生成proxy class,若想看下自动生成的proxy class有三种方法


  1. 设置参数-Dsun.misc.ProxyGenerator.saveGeneratedFiles=true
  2. 程序里调用:System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
  3. 在程序中直接调用ProxyGenerator.generateProxyClass()来生成动态代理类,如下所示:

String name = "$Proxy00";
byte[] data = ProxyGenerator.generateProxyClass("com.sun.proxy." + name, new Class[] { Hello.class } );
FileOutputStream out = new FileOutputStream("/tmp/" + name + ".class" );
out.write(data);
out.close();

以前边的“时间统计示例”为例,在main方法中最前边添加

System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

执行结果:生成了$Proxy0.class

JDK动态代理--实例/原理_代理模式

$Proxy0.class代码如下

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.example.a;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

final class $Proxy0 extends Proxy implements ExecutionTime {
private static Method m1;
private static Method m4;
private static Method m2;
private static Method m3;
private static Method m0;

static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m4 = Class.forName("org.example.a.ExecutionTime").getMethod("sayHello", Class.forName("java.lang.String"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("org.example.a.ExecutionTime").getMethod("dealTask", Class.forName("java.lang.String"));
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}

public $Proxy0(InvocationHandler var1) throws {
super(var1);
}

public final void dealTask(String var1) throws {
try {
super.h.invoke(this, m3, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}

public final void sayHello(String var1) throws {
try {
super.h.invoke(this, m4, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}

public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}

public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}

public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
}

JDK动态代理只能代理接口

        类Proxy的作用是保存自定义的InvocationHandler,便于在方法代理时执行自定义InvocationHandler的逻辑。由于$Proxy0已经继承了类Proxy,所以不能再extends一个类了,所以只能implements一个接口了。

执行流程

创建代理类

Proxy.newProxyInstance时,入参:代理类的类加载器和接口、InvocationHandler接口的实现类。返回:proxy(Proxy$0类型)

执行流程

proxy.dealTask(Proxy$0的dealTask方法)
// Proxy类的h(前边保存的InvocationHandler接口对象)的invoke方法
// m1是通过反射获得的方法,var1是传进来的String类型的参数
super.h.invoke(this, m1, new Object[]{var1})
ExecutionTimeProxy(InvocationHandler接口对象)的invoke方法
// Method类的invoke方法
method.invoke(delegate, args);

生成Proxy.class的流程

Proxy.newProxyInstance=> ProxyClassFactory.apply=> ProxyGenerator.generateProxyClass=> .class文件=> Class对象=> Proxy实例

关于ProxyGenerator.generateProxyClass(),内部主要是按照固定”模板“按照字节码格式生成对应class字节码,大致是:实现要代理接口的所有public方法及固定的方法(hashCode、equals和toString),然后调用InvocationHandler的invoke方法。(就是走到示例代码中的HelloProxy类(class HelloProxy implements InvocationHandler)。

其他网址

​​JDK动态代理为什么不能代理类 - 知乎​​


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