web九大组件之---HandlerAdapter适配器模式实践源码分析【享学Spring MVC】

public interface HandlerAdapter {
  // 判断当前的这个HandlerAdapter  是否 支持给与的handler
  // 因为一般来说：每个适配器只能作用于一种处理器（你总不能把手机适配器拿去用于电脑吧）
  boolean supports(Object handler);
  // 核心方法：利用 Handler 处理请求，然后返回一个ModelAndView 
  // DispatcherServlet最终就是调用此方法，来返回一个ModelAndView的~
  @Nullable
  ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;
  // 同HttpServlet 的 getLastModified方法
  // Can simply return -1 if there's no support in the handler class.
  long getLastModified(HttpServletRequest request, Object handler);
}

// 适配`org.springframework.web.servlet.mvc.Controller`这种Handler
public class SimpleControllerHandlerAdapter implements HandlerAdapter {
  @Override
  public boolean supports(Object handler) {
    return (handler instanceof Controller);
  }
  // 最终执行逻辑的还是Handler啊~~~~
  @Override
  @Nullable
  public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
      throws Exception {
    return ((Controller) handler).handleRequest(request, response);
  }
  // 此处注意：若处理器实现了`LastModified`接口，那就委托给它了
  // 否则返回-1  表示不要缓存~
  @Override
  public long getLastModified(HttpServletRequest request, Object handler) {
    if (handler instanceof LastModified) {
      return ((LastModified) handler).getLastModified(request);
    }
    return -1L;
  }
}

// @since 2.0
public class HttpRequestHandlerAdapter implements HandlerAdapter {
  @Override
  public boolean supports(Object handler) {
    return (handler instanceof HttpRequestHandler);
  }
  @Override
  public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {\
    ((HttpRequestHandler) handler).handleRequest(request, response);
    return null;
  }
  @Override
  public long getLastModified(HttpServletRequest request, Object handler) {
    if (handler instanceof LastModified) {
      return ((LastModified) handler).getLastModified(request);
    }
    return -1L;
  }
}

public class SimpleServletHandlerAdapter implements HandlerAdapter {
  @Override
  public boolean supports(Object handler) {
    return (handler instanceof Servlet);
  }
  @Override
  @Nullable
  public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
    ((Servlet) handler).service(request, response);
    return null;
  }
  @Override
  public long getLastModified(HttpServletRequest request, Object handler) {
    return -1;
  }
}

// @since 3.1 @RequestMapping注解是Spring2.5出现的
// 注意：它实现了Ordered接口
public abstract class AbstractHandlerMethodAdapter extends WebContentGenerator implements HandlerAdapter, Ordered {
  // 唯一构造函数。传的false表示：忽略掉supportedMethods这个属性
  // 默认它的值是GET、POST、HEAD（见WebContentGenerator）
  public AbstractHandlerMethodAdapter() {
    // no restriction of HTTP methods by default
    super(false);
  }
  // 只处理HandlerMethod 类型的处理器。抽象方法supportsInternal默认返回true
  // 是流出的钩子可以给你自己扩展的
  @Override
  public final boolean supports(Object handler) {
    return (handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler));
  }
  // 抽象方法交给子类handleInternal去实现
  @Override
  @Nullable
  public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
    return handleInternal(request, response, (HandlerMethod) handler);
  }
  ...
}

DispatcherServlet：
  protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    ...
    //1、根据URL（当然不一定非得是URL）匹配到一个处理器
    mappedHandler = getHandler(processedRequest);
    if (mappedHandler == null) {
      // 若匹配不到Handler处理器，就404了
      noHandlerFound(processedRequest, response);
      return;
    }
    //2、从Chain里拿出Handler（注意是Object类型哦~ ）然后找到属于它的适配器
    HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
    ...
    //3、执行作用在此Handler上的所有拦截器的Pre方法
    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
      return;
    }
    //4、真正执行handle方法（也就是你自己书写的逻辑方法），得到一个ModelAndView
    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    //5、视图渲染
    applyDefaultViewName(processedRequest, mv);
    //6、执行拦截器的post方法（可见它是视图渲染完成了才会执行的哦~）
    mappedHandler.applyPostHandle(processedRequest, response, mv);
    ...
    //7、执行拦截器的afterCompletion方法（不管抛出与否）
  }