[Java] 代理模式 Proxy Mode

文章目录

[Java] 代理模式 Proxy Mode
- 1. 代理思想
- 2.java.lang.reflect.Proxy类
- - 2.1 利用反射创建proxy类
  - - 2.1.1 动态代理实现一
    - 2.1.2 动态代理实现二
    - 2.1.3 动态代理实现三

1. 代理思想

场景：假设我们有一个interface Calculator, Class CalculatorImpl和Proxy CalculatorProxy为接口的实现类，Class CalculatorImpl为目标类，是我们需要实现的业务逻辑，Proxy CalculatorProxy为代理类。实例化Proxy CalculatorProxy时，可以将Class B的实例传入，通过该实例可以调用目标类的目标方法。

举例：

public interface Calculator {int add(int i, int j);
}

public class CalculatorImpl implements Calculator {@Overridepublic int add(int i, int j) {return i + j;}
}

public class CalculatorProxy implements Calculator {private Calculator target;public CalculatorProxy (Calculator target){this.target=target;}@Overridepublic int add(int i, int j) {System.out.println("preCalculator\n");//目标方法int result = target.add(i, j);System.out.println("postCalculator\n");return result;}
}

主程序调用时：

public class mainTest {public static void main(String[] avrg){//目标类实例化CalculatorImpl calculator = new CalculatorImpl();//代理类实例化CalculatorProxy proxy = new CalculatorProxy(calculator);//通过代理类的实例，传递到目标类的目标方法，且实现了执行目标方法外围包了一层增强代码int result = proxy.add(2,3);}
}

output:

preCalculator
result is 5
postCalculatorProcess finished with exit code 0

这样实现的好处，是可以在执行目标方法的前后，自由包裹增强代码，此类实现方式也就是静态代理。

缺点是：每个类都需要创建一个代理类，还得给代理类实例化，too annoying. 如果有方法可以自动创建代理类，甚至直接给代理类实例化就好啦！

2.java.lang.reflect.Proxy类

2.1 利用反射创建proxy类

jdk提供的java.lang.reflect.Proxy中getProxyClass方法可以实现自动创建代理类。

public static Class<?> getProxyClass(ClassLoader loader,Class<?>... interfaces)throws IllegalArgumentException

Returns the java.lang.Class object for a proxy class given a class loader and an array of interfaces. The proxy class will be defined by the specified class loader and will implement all of the supplied interfaces. If any of the given interfaces is non-public, the proxy class will be non-public. If a proxy class for the same permutation of interfaces has already been defined by the class loader, then the existing proxy class will be returned; otherwise, a proxy class for those interfaces will be generated dynamically and defined by the class loader.

说人话：

方法实现了通过ClassLoader定义一个proxy类，实现给所有interface。¹
如果给的interface不是public的，则生成的proxy类也不是public的。
如果给的interfaces的proxy类已经有了，直接拎出来返回
如果对应的proxy类还没有，则动态生成
返回结果是一个Object类型的proxy class

实现的思路是：
(1) 用getProxyClass获取/生成Proxy代理类
(2) 用Class类中getConstructor()方法获取代理类的构造函数，以便实例化代理类，需要提供入参类型
(3) 实例化代理类，将目标类的实例或者是可以实现目标类的目标方法的东东作为参数，传给代理类的实例。

Proxy类中唯一一个非static field，为InvocationHandler h，用作使用指定的代理指定的参数调用实例对象正在调用的方法，只要重写为InvocationHandler接口中invoke方法，即可代替

来吧，理论结合实践吧.

public interface Calculator {int add(int i, int j);
}

//目标类
public class CalculatorImpl implements Calculator {@Overridepublic int add(int i, int j) {return i + j;}
}

2.1.1 动态代理实现一

定义InvocationHandlerImpl类实现InvocationHandler接口，重写invoke方法。在invoke方法中实例化代理目标类CalculatorImpl

Object result = method.invoke(calculator1, args); 完成目标对象的方法的有参调用

import java.lang.reflect.*;public class mainTest {private static class InvocationHandlerImpl implements InvocationHandler {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws InvocationTargetException, IllegalAccessException {///return 666;CalculatorImpl calculator1 = new CalculatorImpl();System.out.println("pre calculator");Object result = method.invoke(calculator1, args);System.out.println(result);System.out.println("post calculator");return result;}}public static void main(String[] avrg) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {/*1. 静态代理实现CalculatorImpl calculator = new CalculatorImpl();System.out.println(calculator.getClass());System.out.println(calculator.getClass().getClassLoader());CalculatorProxy proxy = new CalculatorProxy(calculator);int result = proxy.add(2,3);*///2. getProxyClass方法获取代理类,入参为目标类的ClassLoader,和需要实现的interfaceClass CalculatorProxyClass = Proxy.getProxyClass(CalculatorImpl.class.getClassLoader(), Calculator.class);//获取以InvocationHandler类型为入参的代理类的构造函数，以便后面为代理类实例化Constructor CalculatorProxyClassCons = CalculatorProxyClass.getConstructor(InvocationHandler.class);InvocationHandlerImpl handler = new InvocationHandlerImpl();Calculator calculator = (Calculator) CalculatorProxyClassCons.newInstance(handler);calculator.add(1, 2);}}

运行到invoke()中method.invoke()时，会调用CalculatorImpl.add()

上述写法可改进的空间：

InvocationHandlerImpl只有在实例化代理类的时候使用，可以作为匿名类实现，更简洁；
InvocationHandle 写死了是代理CalculatorImpl类的，不够灵活。可以通过将需要代理的实例传参进invoke，而不是写死代理CalculatorImpl类。

2.1.2 动态代理实现二

import java.lang.reflect.*;public class mainTest {/*  private static class InvocationHandlerImpl implements InvocationHandler {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws InvocationTargetException, IllegalAccessException {///return 666;CalculatorImpl calculator1 = new CalculatorImpl();System.out.println("pre calculator");Object result = method.invoke(calculator1, args);System.out.println(result);System.out.println("post calculator");return result;}}*/public static void main(String[] avrg) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {/*1. 静态代理实现CalculatorImpl calculator = new CalculatorImpl();System.out.println(calculator.getClass());System.out.println(calculator.getClass().getClassLoader());CalculatorProxy proxy = new CalculatorProxy(calculator);int result = proxy.add(2,3);*///getProxyClass方法获取代理类,入参为目标类的ClassLoader,和需要实现的interfaceClass CalculatorProxyClass = Proxy.getProxyClass(CalculatorImpl.class.getClassLoader(), Calculator.class);//获取以InvocationHandler类型为入参的代理类CalculatorProxyClass的构造函数，以便后面为代理类实例化Constructor CalculatorProxyClassCons = CalculatorProxyClass.getConstructor(InvocationHandler.class);//InvocationHandlerImpl handler = new InvocationHandlerImpl();CalculatorImpl target = new CalculatorImpl();//通过代理类的构造函数实例化代理类Calculator calculator = (Calculator) CalculatorProxyClassCons.newInstance(new InvocationHandler() {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {System.out.println("pre calculator");Object result = method.invoke(target,args);System.out.println(result);System.out.println("post calculator");return result;}});calculator.add(1, 2);}}

当调用calculator.add(1,2)时，JVM会自动导向InvocationHandler.invoke(), Object proxy为调用的proxy实例

2.1.3 动态代理实现三

除了使用getProxyClass获取/生成Proxy Class，再获取构造函数，在newInstance外，Proxy还提供了newProxyInstance方法，可直接生产代理类的实例。

import java.lang.reflect.*;public class mainTest {/*  private static class InvocationHandlerImpl implements InvocationHandler {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws InvocationTargetException, IllegalAccessException {///return 666;CalculatorImpl calculator1 = new CalculatorImpl();System.out.println("pre calculator");Object result = method.invoke(calculator1, args);System.out.println(result);System.out.println("post calculator");return result;}}*/public static void main(String[] avrg) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {/*1. 静态代理实现CalculatorImpl calculator = new CalculatorImpl();System.out.println(calculator.getClass());System.out.println(calculator.getClass().getClassLoader());CalculatorProxy proxy = new CalculatorProxy(calculator);int result = proxy.add(2,3);*///getProxyClass方法获取代理类,入参为目标类的ClassLoader,和需要实现的interface/* Class CalculatorProxyClass = Proxy.getProxyClass(CalculatorImpl.class.getClassLoader(), Calculator.class);//获取以InvocationHandler类型为入参的代理类CalculatorProxyClass的构造函数，以便后面为代理类实例化Constructor CalculatorProxyClassCons = CalculatorProxyClass.getConstructor(InvocationHandler.class);//InvocationHandlerImpl handler = new InvocationHandlerImpl();CalculatorImpl target = new CalculatorImpl();//通过代理类的构造函数实例化代理类Calculator calculator = (Calculator) CalculatorProxyClassCons.newInstance(new InvocationHandler() {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {System.out.println("pre calculator");Object result = method.invoke(target,args);System.out.println(result);System.out.println("post calculator");return result;}});*/CalculatorImpl target = new CalculatorImpl();Calculator calculator = (Calculator) Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), new InvocationHandler() {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {System.out.println("pre calculator");Object result = method.invoke(target,args);System.out.println(result);System.out.println("post calculator");return result;}});calculator.add(1, 2);}}

有关ClassLoader，详见 ↩︎