文章目录
本系列结合源码（基于springboot2.1.4），分析springmvc的执行流程。
先上一个完整的流程图，借用网络上的：
我们知道，前端发起的请求首先到tomcat，最终执行的是servlet，springmvc实现了一个servlet，叫做DispatcherServlet，其继承关系如下：
假设发起一个get请求，这时，servlet中的doGet方法会执行，最终执行到DispatcherServlet，链路如下：
FrameworkServlet#doGet(HttpServletRequest request, HttpServletResponse response)
--》FrameworkServlet#processRequest(HttpServletRequest request, HttpServletResponse response)
--》DispatcherServlet#doService(HttpServletRequest request, HttpServletResponse response)
--》DispatcherServlet#doDispatch(HttpServletRequest request, HttpServletResponse response)
最终进入DispatcherServlet类的doDispatch方法中。
doDispatch()方法
这个方法是sprngmvc的核心方法，所有扭转乾坤的逻辑都在这里：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
//处理器执行链，包含拦截器和具体的controller
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
//检查是不是有包含文件上传类型的
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
//确定要执行的mappedHandler，也就是要执行的controller
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
//得到handler适配器，总共有3种
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;
}
}
//执行拦截器的preHandle方法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
//执行controller，得到model和view
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//如果没有view名字，设置一个默认的名字
applyDefaultViewName(processedRequest, mv);
//执行拦截器的postHandle方法
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);
}
//渲染视图，并将跳转路径设置到response中
//然后执行拦截器的afterCompletion方法
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
//抛异常，也要执行拦截器的afterCompletion方法
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);
}
}
}
}
主要步骤：
1.确定handlerMapping，并从其中得到HandlerExecutionChain执行链，这个执行链包含了拦截器以及controller方法路径；
2.确定handler适配器，总共有3种：
3.利用适配器执行handler，也就是controller，但是如果有拦截器，那么要先执行拦截器相关方法。
4.渲染视图并返回
关于handlerMapping和适配器在容器中的初始化在《SpringMVC系列（二）HandlerMapping初始化》中分析。
上面是springmvc大致的请求流程，下面针对具体方法详细分析。
getHandler(HttpServletRequest request)
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;
}
方法逻辑比较简单，就是遍历
这里handlerMappings是个list，默认有5个实例对象，共4种类型：
- SimpleUrlHandlerMapping
- RequestMappingHandlerMapping，处理@Controller修饰的类中，@RequestMapping修饰的方法
- BeanNameUrlHandlerMapping
- SimpleUrlHandlerMapping，类型同第一个，但是是不同的实例对象
- WelcomePageHandlerMapping，默认应用欢迎页面
遍历然后调用mapping.getHandler(request)方法获取HandlerExecutionChain。
其中getHandler方法是在父类AbstractHandlerMapping种实现：
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
Object handler = getHandlerInternal(request);
if (handler == null) {
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = obtainApplicationContext().getBean(handlerName);
}
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
if (logger.isTraceEnabled()) {
logger.trace("Mapped to " + handler);
}
else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) {
logger.debug("Mapped to " + executionChain.getHandler());
}
if (CorsUtils.isCorsRequest(request)) {
CorsConfiguration globalConfig = this.corsConfigurationSource.getCorsConfiguration(request);
CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
}
return executionChain;
}
getHandlerAdapter(Object handler)
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");
}
- RequestMappingHandlerAdapter
- HttpRequestHandlerAdapter
- SimpleControllerHandlerAdapter
processDispatchResult()
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
boolean errorView = false;
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}
// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("No view rendering, null ModelAndView returned.");
}
}
if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}
}
来源：CSDN
作者：雨剑yyy
链接：https://blog.csdn.net/csdn_20150804/article/details/103816072