SpringMVC 执行流程（深入源码）

曾经终败给现在 2021-09-01 08:30 387阅读 0赞

### SpringMVC 执行流程 ###

*  1. 简介
 *  2. 深入源码
 *   *  2.1 Debug
     *  2.2 doDispatch
     *   *  2.2.1 getHandler(processedRequest)
         *  2.2.2 getHandlerAdapter(mappedHandler.getHandler())
         *  2.2.3 handle(HttpServletRequest request, HttpServletResponse response, Object handler)
         *  2.2.4 applyPostHandle(processedRequest, response, mv)
 *  3. 总结

# 1. 简介 #

相信`SpringMVC`处理请求的流程各位滚瓜烂熟了都，如下图所示。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center]  
       但是如果没有读过代码，还是有很多问题，如`Request`如何被接收到，`ModelAndView`是否可以为空，项目中我们通常返回一串`json`给前端，是怎么处理的？

# 2. 深入源码 #

## 2.1 Debug ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center 1]

/** * @author wangzhao * @date 2020/8/12 23:07 */
    @RestController
    public class HelloController { 
    
        @RequestMapping("/hello")
        public String sayHello(HttpServletRequest req, HttpServletResponse resp) { 
            return "Hello World";
        }
    
    }

首先的话，我们打断点，分析其调用栈。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center 2]

直接进入到`service`方法，之前的代码并不需要关心。

public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { 
            HttpServletRequest request;
            HttpServletResponse response;
            try { 
                request = (HttpServletRequest)req;
                response = (HttpServletResponse)res;
            } catch (ClassCastException var6) { 
                throw new ServletException(lStrings.getString("http.non_http"));
            }
    
            this.service(request, response);
        }

该方法位于`HttpServlet`中，这不就和我们一开始用`HttpServlet`写的代码一样吗，请求直接进入到了`HttpServlet#service`方法中。

一直向下进行调用，最后到了`org.springframework.web.servlet.DispatcherServlet#doDispatch`。

![在这里插入图片描述][20200812231627509.png_pic_center]

至此，请求才算正式进入到`MVC` 请求的处理流程中，相信你也一定知道请求是如何被`DispatcherServlet`所接受到。

## 2.2 doDispatch ##

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { 
    		HttpServletRequest processedRequest = request;
    		HandlerExecutionChain mappedHandler = null;
    		boolean multipartRequestParsed = false;
    
    		// 获取异步请求管理器
    		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    
    		try { 
    			ModelAndView mv = null;
    			Exception dispatchException = null;
    
    			try { 
    				// 是否是上传文件请求
    				processedRequest = checkMultipart(request);
    				multipartRequestParsed = (processedRequest != request);
    
    				// 获取请求的handler
    				mappedHandler = getHandler(processedRequest);
    				if (mappedHandler == null) { 
    					// 找不到handler报错 
    					noHandlerFound(processedRequest, response);
    					return;
    				}
    
    				// 获取处理器适配器
    				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;
    					}
    				}
    
    				// 执行前置拦截器
    				if (!mappedHandler.applyPreHandle(processedRequest, response)) { 
    					return;
    				}
    
    				// 执行handler
    				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    
    				if (asyncManager.isConcurrentHandlingStarted()) { 
    					return;
    				}
    
    				// 设置视图名称
    				applyDefaultViewName(processedRequest, mv);
    				// 执行后置拦截器，这里面进行了视图的解析
    				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()) { 
    				// Instead of postHandle and afterCompletion
    				if (mappedHandler != null) { 
    					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
    				}
    			}
    			else { 
    				// Clean up any resources used by a multipart request.
    				if (multipartRequestParsed) { 
    					cleanupMultipart(processedRequest);
    				}
    			}
    		}
    	}

### 2.2.1 getHandler(processedRequest) ###

获取请求对应的`Handler`，可以看到，已经成功获取到目标`Handler`。

![在这里插入图片描述][20200812232111754.png_pic_center]

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;
    	}

### 2.2.2 getHandlerAdapter(mappedHandler.getHandler()) ###

获取`Handler`对应的适配器。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center 3]

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException { 
    		if (this.handlerAdapters != null) { 
    			for (HandlerAdapter adapter : this.handlerAdapters) { 
    				if (adapter.supports(handler)) { 
    					return adapter;
    				}
    			}
    		}
    		throw new ServletException("No adapter for handler [" + handler +
    				"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    	}

### 2.2.3 handle(HttpServletRequest request, HttpServletResponse response, Object handler) ###

执行`handle`，返回`ModelAndView`。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center 4]  
       可以看到，返回的`ModelAndView`是`null`，这样的逻辑也是符合我们的代码的，我们并没有在`sayHello`中返回`ModelAndView`。

那如果我们直接返回`json`格式的数据，那么该数据哪里去了？答案是已经被封装到了`response`中，查找了一番没有找到，该对象属性太多，最后只找到了`json`的长度为`11`字节，所以可知其也一定被封装到里卖弄。

这一下是不是已经解决了我们开始提问的两个问题？

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center 5]

### 2.2.4 applyPostHandle(processedRequest, response, mv) ###

视图解析。

进入该方法后，直接跟踪到如下方法。

// org.springframework.web.servlet.handler.WebRequestHandlerInterceptorAdapter#postHandle
    	public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
    			@Nullable ModelAndView modelAndView) throws Exception { 
    
    		this.requestInterceptor.postHandle(new DispatcherServletWebRequest(request, response),
    				(modelAndView != null && !modelAndView.wasCleared() ? modelAndView.getModelMap() : null));
    	}

在这里做的就是将`ModelAndView`中的数据填充到`View`中，最后通过`Response`展示给用户，如下便完成了一个`MVC`的请求流程。

# 3. 总结 #

通过此次代码分析，相信之前很多不懂或模糊的地方可以豁然开朗。

`Talk is cheap. Show me the code.`

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20210813/6f5f4abbe4ee4dd6b45bd418009b4176.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center 2]: /images/20210813/0c440805bc5f434585ee2672b2d7c55e.png
[20200812231627509.png_pic_center]: /images/20210813/d8073dfc033b4fb09f31f3eab53fb262.png
[20200812232111754.png_pic_center]: /images/20210813/c185abf5c949484cb6036fde2b374d46.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdfemhhb18_size_16_color_FFFFFF_t_70_pic_center 3]: /images/20210813/a5ef097ee63148b38c5a44f89ef6909c.png
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