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The role of proxy, application scenarios, and implementation in Java programs.

代理是程序设计和开发时频繁使用的技术，可以提高程序灵活性和可扩展性。

1 代理作用

1. 在不修改原代码的基础上，扩展和增强实现；
2. 代码解耦，在代理中通过参数可以判断真实类，做出不同的响应或调用，灵活方便；
3. 隐藏部分实现过程和细节。

2 应用场景

ICar接口类，有一个drive方法，传入speed(速度)行驶，现在已经有Mitsubishi(三菱车)和Bwm(宝马车)实现了接口，随着业务发展，可能有更多的车加入；程序稳健运行一段时间后，针对不同的车发现了不同的问题需要处理，比如Mitsubishi车drive前必须检查和保证冷却液充足；Bwm车drive前必须检查和保证机油充足；由于改写ICar接口可能影响到其它的车；如果有很多的车要处理，更改实现类可能需要花费大量时间，此时可以通过代理类扩展和增强。

在代理类(CarProxy)中通过carCheck方法实现对不同汽车检查，检查通过后drive汽车，从而实现对原有代码增强。

3 代理的实现

3.1 静态代理

静态代理的思想：代理类和其它汽车类一样实现接口类，在重写方法里实现扩展或增强，并调要汽车类实现。

//宝马汽车实现类
class Bwm implements ICar {
    public void drive(int speed) {
        System.out.println("Bwm drive with speed:"+speed);
    }
//三菱汽车实现类   
class Mitsubishi implements ICar {
    public void drive(int speed) {
        System.out.println("Mitsubishi drive with speed:"+speed);
    }

//汽车代理类
public class CarProxy implements ICar {
    private ICar carTarget;
    public CarProxy(ICar carTarget){
        this.carTarget=carTarget;
    }
    // 车辆检查
    private boolean carCheck() {
        if(this.carTarget instanceof Mitsubishi){ //判断汽车类型
            System.out.println("Mitsubishi 检查冷却液完成。");
            return true;
        }else if(this.carTarget instanceof Bwm){
            System.out.println("Bwm 检查机油完成。");
            return true;
        }else{
            System.out.println("该车不需要检查。");
            return true;
        }
    }

    /** 驾驶
     * @param speed 速度
     */
    public void drive(int speed) {
        if(carCheck()){ //检查通过后drive
            carTarget.drive(speed); //调用汽车类
        }
    }
}

在CarProxy代理类中，增加了对不同汽车的检查方法(carCheck)，当汽车检查通过后调用(drive)方法驾驶汽车。这样代理类便实现了对三菱和宝马车的扩展，调用时传入ICar实现类对象；

new CarProxy(new Mitsubishi()).drive(80); //drive 三菱车
new CarProxy(new Bwm()).drive(100); //drive 宝马车

运行输出；

Mitsubishi 检查冷却液完成。
Mitsubishi drive with speed:80
Bwm 检查机油完成。
Bwm drive with speed:100

3.1.1 缺点

接口类变化会影响实现类和代理类；比如方法修改返回值、参数类型、增加方法，实现类和代理类都需要修改。

3.2 动态代理

Java 提供(java.lang.reflect.InvocationHandler)代理实例接口，代理类实现该接口关联调用处理程序的方法(invoke)，当在代理实例调用方法时，方法调用被编码(encoded)并分配给其调用处理程序的方法。

public class DynamicProxy implements InvocationHandler{
    Object carTarget;

   //车辆检查
   private boolean carCheck() {
       if(this.carTarget instanceof Mitsubishi){
            System.out.println("Mitsubishi 检查冷却液完成。");
            return true;
       }else if(this.carTarget instanceof Bwm){
            System.out.println("Bwm 检查机油完成。");
            return true;
       }else{
           System.out.println("该车不需要检查。");
           return true;
       }
   }

    //创建代理类实例
    Object crateProxyInstance(Object carTarget) {
            this.carTarget = carTarget;
            return Proxy.newProxyInstance(carTarget.getClass().getClassLoader(), 
            carTarget.getClass().getInterfaces(),this);
        }

        @Override
        public Object invoke(Object proxy, Method method, Object[] args)
                throws Throwable {
            if(carCheck()){//汽车检查通过后drive
                return method.invoke(carTarget,args);
            }
            return null;
        }
}

除了crateProxyInstance 和 invoke 方法外，对三菱和宝马车的扩展(车辆检查)与静态类相同。调用时同样传入ICar实现类对象。

((ICar)new DynamicProxy().crateProxyInstance(new Mitsubishi())).drive(80);//drive 三菱车
((ICar)new DynamicProxy().crateProxyInstance(new Bwm())).drive(80);//drive 宝马车

3.3 cglib 代理

除了java自身的代理类，还有第三方代理类，比如(cglib)，通过实现方法拦截器(MethodInterceptor)，在拦截方法中(intercept)调用处理程序的方法。

import java.lang.reflect.Method;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
public class CarInterceptor implements MethodInterceptor{
    Object carTarget;

   //车辆检查
   private boolean carCheck() {
       if(this.carTarget instanceof Mitsubishi){
            System.out.println("Mitsubishi 检查冷却液完成。");
            return true;
       }else if(this.carTarget instanceof Bwm){
            System.out.println("Bwm 检查机油完成。");
            return true;
       }else{
           System.out.println("该车不需要检查。");
           return true;
       }
   }
    //拦截方法 
    @Override
    public Object intercept(Object carTarget, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        this.carTarget=carTarget;
        if(carCheck()){//汽车检查通过后drive
            Object obj=proxy.invokeSuper(carTarget, args);
            return obj;
        }
        return null;
    }
}

最后调用时，首先实例化(Enhancer)对象启用方法拦截器，设置方法拦截类和Superclass；

Enhancer en=new Enhancer(); 
en.setCallback(new CarInterceptor());//设置拦截类
//drive 三菱车
en.setSuperclass(new Mitsubishi().getClass());//设置superClass
((ICar) en.create()).drive(80);
//drive 宝马车
en.setSuperclass(new Bwm().getClass());
((ICar) en.create()).drive(100);

4 总结

静态代理更像是开发层面的实现，每次修改接口类或实现类都可能需要修改代理类，随着程序扩展，代码量和维护量增加；Java 提供的(java.lang.reflect.InvocationHandler)代理实例接口和第三方代理类(cglib)大大弥补了静态类的不足，也应用在许多开源框架上，比如Spring。

参考文献

• https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/InvocationHandler.html – Java Interface InvocationHandler
• https://github.com/cglib/cglib/wiki – cglib