5代理模式

在不改变原始类（或叫被代理类）的情况下，通过引入代理类来给原始类附加功能
一般情况下，让代理类和原始类实现同样的接口。
如果原始类并没有定义接口，并且原始类代码并不是我们开发维护的。可以让代理类继承原始类的方法来实现代理模式。

原理解析

给controller加入性能计数器功能

普通写法

public class UserController {
  //...省略其他属性和方法...
  private MetricsCollector metricsCollector; // 依赖注入

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

    RequestInfo requestInfo = new RequestInfo("login", endTimeStamp - startTimestamp, startTimestamp);
    metricsCollector.recordRequest(requestInfo);
    return UserVo;
  }
}

代理模式-接口实现

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

  @Override
  public UserVo login(String telephone, String password) {
    //...
  }

}

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();
    UserVo userVo = userController.login(telephone, password);
    long endTimeStamp = System.currentTimeMillis();
    RequestInfo requestInfo = new RequestInfo("login", endTimeStamp - startTimestamp, 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());

代理模式-继承实现

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();
    RequestInfo requestInfo = new RequestInfo("login", endTimeStamp - startTimestamp, startTimestamp);
    metricsCollector.recordRequest(requestInfo);
    return userVo;
  }  
}
//UserControllerProxy使用举例
UserController userController = new UserControllerProxy();

动态代理原理解析

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();

      String apiName = proxiedObject.getClass().getName() + ":" + method.getName();
      RequestInfo requestInfo = new RequestInfo(apiName, endTimeStamp - startTimestamp, startTimestamp);
      metricsCollector.recordRequest(requestInfo);
      return result;
    }
  }
}

//应用
MetricsCollectorProxy proxy = new MetricsCollectorProxy();
IUserController userController = (IUserController) proxy.createProxy(new UserController());

应用

代理模式在RPC、缓存中的应用

其他应用：监控、统计、鉴权、限流、事务、幂等、日志。

缓存

需求：开发一个有缓存功能接口，对于某些接口请求，如果入参相同，在设定的过期时间内，直接返回缓存结果