一、基本介绍

代理模式 在不改变原始类（被叫代理类）代码的情况下，通过引入代理类给原始类附加功能。

代理模式的应用场景：业务系统中一些非功能性需求：监控、统计、鉴权限流、事务等，将框架代码和业务代码解耦。

二、代理模式类型

分为静态代理（分为基于接口和继承父类两种类型的静态代理），动态代理

2.1 静态代理

（1）基于接口的静态代理
为了将框架代码和业务代码解耦，代理模式就派上用场了。代理类UserControllerProxy 和原始类 UserController 实现相同的接口 IUserController。 UserController类只负责业务功能。代理类 UserControllerProxy负责在业务代码执行前后附加其他逻辑代码，并通过委托的方式调用原始类来执行业务代码。

public interface IUserController {
  UserVo login(String telephone, String password);
  UserVo register(String telephone, String password);
}

public class UserController implements IUserController {
  //...省略其他属性和方法...

  @Override
  public UserVo login(String telephone, String password) {
    //...省略login逻辑...
    //...返回UserVo数据...
  }

  @Override
  public UserVo register(String telephone, String password) {
    //...省略register逻辑...
    //...返回UserVo数据...
  }
}

public class UserControllerProxy implements IUserController {
  private MetricsCollector metricsCollector;
  private UserController userController;

  public UserControllerProxy(UserController userController) {
    this.userController = userController;
    this.metricsCollector = new MetricsCollector();
  }

  @Override
  public UserVo login(String telephone, String password) {
    long startTimestamp = System.currentTimeMillis();

    // 委托
    UserVo userVo = userController.login(telephone, password);

    long endTimeStamp = System.currentTimeMillis();
    long responseTime = endTimeStamp - startTimestamp;
    RequestInfo requestInfo = new RequestInfo("login", responseTime, startTimestamp);
    metricsCollector.recordRequest(requestInfo);

    return userVo;
  }

  @Override
  public UserVo register(String telephone, String password) {
    long startTimestamp = System.currentTimeMillis();

    UserVo userVo = userController.register(telephone, password);

    long endTimeStamp = System.currentTimeMillis();
    long responseTime = endTimeStamp - startTimestamp;
    RequestInfo requestInfo = new RequestInfo("register", responseTime, startTimestamp);
    metricsCollector.recordRequest(requestInfo);

    return userVo;
  }
}

//UserControllerProxy使用举例
//因为原始类和代理类实现相同的接口，是基于接口而非实现编程
//将UserController类对象替换为UserControllerProxy类对象，不需要改动太多代码
IUserController userController = new UserControllerProxy(new UserController());

（2）基于继承父类的静态代理
对于外部类的功能扩展，让代理类继承原始类，让代理类扩展附加功能。

public class UserControllerProxy extends UserController {
  private MetricsCollector metricsCollector;

  public UserControllerProxy() {
    this.metricsCollector = new MetricsCollector();
  }

  public UserVo login(String telephone, String password) {
    long startTimestamp = System.currentTimeMillis();

    UserVo userVo = super.login(telephone, password);

    long endTimeStamp = System.currentTimeMillis();
    long responseTime = endTimeStamp - startTimestamp;
    RequestInfo requestInfo = new RequestInfo("login", responseTime, startTimestamp);
    metricsCollector.recordRequest(requestInfo);

    return userVo;
  }

  public UserVo register(String telephone, String password) {
    long startTimestamp = System.currentTimeMillis();

    UserVo userVo = super.register(telephone, password);

    long endTimeStamp = System.currentTimeMillis();
    long responseTime = endTimeStamp - startTimestamp;
    RequestInfo requestInfo = new RequestInfo("register", responseTime, startTimestamp);
    metricsCollector.recordRequest(requestInfo);

    return userVo;
  }
}
//UserControllerProxy使用举例
UserController userController = new UserControllerProxy();

2.2 动态代理

当对很多原始类增加相同功能时，如果按照静态代理的方法会产生很多代理类时，增加代码维护成本，动态代理就是解决这种问题。
代理类可以理解为：我们不事先为每个原始类编写代理类，而是运行时动态地创建原始类的代理类。

public class MetricsCollectorProxy {
  private MetricsCollector metricsCollector;

  public MetricsCollectorProxy() {
    this.metricsCollector = new MetricsCollector();
  }

  public Object createProxy(Object proxiedObject) {
    Class<?>[] interfaces = proxiedObject.getClass().getInterfaces();
    DynamicProxyHandler handler = new DynamicProxyHandler(proxiedObject);
    return Proxy.newProxyInstance(proxiedObject.getClass().getClassLoader(), interfaces, handler);
  }

  private class DynamicProxyHandler implements InvocationHandler {
    private Object proxiedObject;

    public DynamicProxyHandler(Object proxiedObject) {
      this.proxiedObject = proxiedObject;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      long startTimestamp = System.currentTimeMillis();
      Object result = method.invoke(proxiedObject, args);
      long endTimeStamp = System.currentTimeMillis();
      long responseTime = endTimeStamp - startTimestamp;
      String apiName = proxiedObject.getClass().getName() + ":" + method.getName();
      RequestInfo requestInfo = new RequestInfo(apiName, responseTime, startTimestamp);
      metricsCollector.recordRequest(requestInfo);
      return result;
    }
  }
}

//MetricsCollectorProxy使用举例
MetricsCollectorProxy proxy = new MetricsCollectorProxy();
IUserController userController = (IUserController) proxy.createProxy(new UserController());