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:
- 通过实现 InvocationHandler 接口创建自己的调用处理器;
- 通过为 Proxy 类指定 ClassLoader 对象和一组 interface 来创建动态代理类;
- 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;
- 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
- 设置参数-Dsun.misc.ProxyGenerator.saveGeneratedFiles=true
- 程序里调用:System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
- 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
$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