关于java:HandlerInterceptor-源码解析

介绍

HandlerInterceptor 中文名拦截器，其实 Interceptor 也是拦截器的意思，这里加个 Handler 示意这只是一个接口，具体的实现须要自定义。

• 机会：客户端拜访服务端的 controller
• 具体作用点：拜访前，拜访后
• 作用形式：对 request 或 response 进行解决，加日志，包装返回值，增加申请头属性。

应用形式

以通用返回值为例。

1. 自定义拦截器

实现 HandlerInterceptor 并注册为 Bean

// 通用返回值拦截器
@Component
public class CommonResultInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {if (handler instanceof HandlerMethod) {final HandlerMethod handlerMethod = (HandlerMethod) handler;
            final Class<?> clazz = handlerMethod.getBeanType();
            final Method method = handlerMethod.getMethod();
 
            if (clazz.isAnnotationPresent(CommonResult.class)) {request.setAttribute(RESPONSE_RESULT, clazz.getAnnotation(CommonResult.class));
            } else if (method.isAnnotationPresent(CommonResult.class)) {request.setAttribute(RESPONSE_RESULT, method.getAnnotation(CommonResult.class));
            }
        }
        return true;
    }
}

2. 增加拦截器

@Configuration
public class WebAppConfig implements WebMvcConfigurer {
 
    @Override
    public void addInterceptors(InterceptorRegistry registry) {CommonResultInterceptor interceptor = new CommonResultInterceptor();
        registry.addInterceptor(interceptor);
        WebMvcConfigurer.super.addInterceptors(registry);
    }
}

3. 解决返回值

解决的形式有很多种，例如再加拦截器等等，这里我应用 springboot 的 ResponseBodyAdvice 接口

@ControllerAdvice
public class CommonResultHandler implements ResponseBodyAdvice<Object> {
    /**
     * 判断是否要执行 beforeBodyWrite 办法
     *
     * @param methodParameter
     * @param aClass
     * @return true 为执行，false 不执行，有注解标记的时候解决返回值
     */
    @Override
    public boolean supports(MethodParameter methodParameter, Class<? extends HttpMessageConverter<?>> aClass) {ServletRequestAttributes attributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = attributes.getRequest();
 
        CommonResult commonResult = (CommonResult) request.getAttribute(CRM_RESPONSE_RESULT);
        return Objects.nonNull(commonResult);
 
    }
 
    /**
     * 对返回值做包装解决
     *
     * @return ResultBody
     */
    @Override
    public Object beforeBodyWrite(Object body, MethodParameter methodParameter, MediaType mediaType,
                                  Class<? extends HttpMessageConverter<?>> aClass,
                                  ServerHttpRequest request,
                                  ServerHttpResponse response) {if (body instanceof ResultBody) {return body;}
 
        // 当返回类型是 String 时，用的是 StringHttpMessageConverter 转换器，无奈转换为 Json 格局
        // 必须在办法体上标注 RequestMapping(produces = "application/json; charset=UTF-8")
        if (body instanceof String) {String str = JSON.toJSONString(ResultBody.ok(body));
            return str;
        }
 
        return ResultBody.ok(body);
    }
 
}

作用原理

1. 申请首先会达到 org.springframework.web.servlet.DispatcherServlet 下的 doDispatch 办法

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
  HttpServletRequest processedRequest = request;
  HandlerExecutionChain mappedHandler = null;
  boolean multipartRequestParsed = false;
 
  WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
 
  try {
    ModelAndView mv = null;
    Exception dispatchException = null;
 
    try {processedRequest = checkMultipart(request);
      multipartRequestParsed = (processedRequest != request);
 
      // Determine handler for the current request.
      mappedHandler = getHandler(processedRequest);
      if (mappedHandler == null) {noHandlerFound(processedRequest, response);
        return;
      }
 
      // Determine handler adapter for the current request.
      HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
 
      // Process last-modified header, if supported by the handler.
      String method = request.getMethod();
      boolean isGet = "GET".equals(method);
      if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
        if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}
      }
 
      // 调用已注册拦截器的 prehandle 办法
      if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}
 
      // 真正激活 handler，也就是对应 controller 的办法
      mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
 
      if (asyncManager.isConcurrentHandlingStarted()) {return;}
 
      applyDefaultViewName(processedRequest, mv);
      
          // 调用已注册拦截器的 posthandle 办法
      mappedHandler.applyPostHandle(processedRequest, response, mv);
    }
    catch (Exception ex) {dispatchException = ex;}
    catch (Throwable err) {
      // As of 4.3, we're processing Errors thrown from handler methods as well,
      // making them available for @ExceptionHandler methods and other scenarios.
      dispatchException = new NestedServletException("Handler dispatch failed", err);
    }
    processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
  }
  catch (Exception ex) {triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
  }
  catch (Throwable err) {
    triggerAfterCompletion(processedRequest, response, mappedHandler,
                           new NestedServletException("Handler processing failed", err));
  }
  finally {if (asyncManager.isConcurrentHandlingStarted()) {
      // 调用 posthandle 和 aftercompletion 办法
      if (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
      }
    }
    else {
      // Clean up any resources used by a multipart request.
      if (multipartRequestParsed) {cleanupMultipart(processedRequest);
      }
    }
  }
}

2. 接着，咱们来看看 preHandle 办法的实现。

/**
     * Apply preHandle methods of registered interceptors.
     * @return {@code true} if the execution chain should proceed with the
     * next interceptor or the handler itself. Else, DispatcherServlet assumes
     * that this interceptor has already dealt with the response itself.
     */
boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {HandlerInterceptor[] interceptors = getInterceptors();
  if (!ObjectUtils.isEmpty(interceptors)) {for (int i = 0; i < interceptors.length; i++) {HandlerInterceptor interceptor = interceptors[i];
      // 执行 prehandle
      if (!interceptor.preHandle(request, response, this.handler)) {triggerAfterCompletion(request, response, null);
        return false;
      }
      this.interceptorIndex = i;
    }
  }
  return true;
}

3. 逻辑很简略，就是获取拦截器数组，而后调用 preHandle 办法，那么接下来看看 getInterceptors 办法

/**
  * Return the array of interceptors to apply (in the given order).
    */
@Nullable
public HandlerInterceptor[] getInterceptors() {if (this.interceptors == null && this.interceptorList != null) {this.interceptors = this.interceptorList.toArray(new HandlerInterceptor[0]);
  }
  return this.interceptors;
}

4. 这里是间接调用 this.interceptors，那么要害就在于HandlerExecutionChain 的初始化怎么实现的

doDispatch办法中，有以下片段

// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {noHandlerFound(processedRequest, response);
  return;
}

这里 getHandler 正式获取了这个 request 对应的 handler

/**
 * Return the HandlerExecutionChain for this request.
 * <p>Tries all handler mappings in order.
 * @param request current HTTP request
 * @return the HandlerExecutionChain, or {@code null} if no handler could be found
 */
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {if (this.handlerMappings != null) {for (HandlerMapping mapping : this.handlerMappings) {HandlerExecutionChain handler = mapping.getHandler(request);
         if (handler != null) {return handler;}
      }
   }
   return null;
}

显然，在这里获取了 request 对应的所有的 HandlerExecutionChain，察看到这是通过handlerMappings 拿到HandlerMapping，再查问 Handler。

handlerMappings的作用是通过 urlpath 匹配对应的 controller，HandlerMapping 就通过申请失去 HandlerExecutionChain, 至于HandlerMapping 的源码剖析请参考其余文章。