小猿圈Java开发之从代码看spring mvc请求处理过程

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Java作为编程界的常青树，有自己生存的独到之处，小猿圈java讲师今天就分享一个关于从代码看spring mvc请求处理过程，通过示例代码介绍的非常详细，对大家的学习或者工作具有一定的参考学习价值。

请求处理的过程：

1.DispatcherServelt作为前端控制器，拦截request对象。

2.DispatcherServlet接收到request对象之后，查询HandlerMapping,得到一个HandlerExecutionChain对象。

3.DispatcherServlet将Handler对象交由HandlerAdapter(适配器模式的典型应用)，调用相应的controller方法。

4.Controller方法返回ModelAndView对象，DispatcherServlet将view交由ViewResolver进行解析，得到相应的视图。用model渲染视图。

5.返回响应结果。

整个过程的流程其实就是DispatcherServelt中doDispatch()方法的调用过程。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {

//第一步拦截request对象，doDispatch()方法在doService()方法中被调用，request对象是经过处理的。

HttpServletRequest processedRequest = request;

HandlerExecutionChain mappedHandler = null;

boolean multipartRequestParsed = false;

WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

try {

ModelAndView mv = null;

Exception dispatchException = null;

try {

//和文件的上传和下载有关系，判断请求的类型是否是multipart类型

processedRequest = checkMultipart(request);

multipartRequestParsed = (processedRequest != request);

// Determine handler for the current request.

//主要看这里，这里是得到HandlerExecutionChain的方法。关于Handler()方法向下看

mappedHandler = getHandler(processedRequest);

if (mappedHandler == null || mappedHandler.getHandler() == null) {

noHandlerFound(processedRequest, response);

return;

}

// Determine handler adapter for the current request.

//这里已经获取到HandlerExecutionChain对象，接下来就要获取HandlerAdapter对象，调用Handler对象的方法。

HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

// Process last-modified header, if supported by the handler

//有关浏览器缓存的设定（304）

String method = request.getMethod();

boolean isGet = "GET".equals(method);

if (isGet || "HEAD".equals(method)) {

long lastModified = ha.getLastModified(request, mappedHandler.getHandler());

if (logger.isDebugEnabled()) {

logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);

}

if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {

return;

}

}

//pan'du

if (!mappedHandler.applyPreHandle(processedRequest, response)) {

return;

}

// Actually invoke the handler.

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

if (asyncManager.isConcurrentHandlingStarted()) {

return;

}

//解析视图，数据渲染

applyDefaultViewName(request, mv);

mappedHandler.applyPostHandle(processedRequest, response, mv);

}

catch (Exception ex) {

dispatchException = ex;

}

processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

}

catch (Exception ex) {

triggerAfterCompletion(processedRequest, response, mappedHandler, ex);

}

catch (Error err) {

triggerAfterCompletionWithError(processedRequest, response, mappedHandler, 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);

}

}

}

}

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {

//遍历HandlerMappingList对象（存储若干个HandlerMapping对象），不断调用，直到不为空为止，否则返回null

for (HandlerMapping hm : this.handlerMappings) {

if (logger.isTraceEnabled()) {

logger.trace(

"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");

}

HandlerExecutionChain handler = hm.getHandler(request);

if (handler != null) {

return handler;

}

}

return null;

}

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