Spring Web http request请求流程：
首先介绍这边你需要知道的继承体系，DispacherServlet继承自FrameworkServlet，FrameworkServlet继承自HttpServletBean，HttpServletBean继承自HttpServlet，HttpServlet继承自GenericServlet，GenericServlet实现Servlet、ServletConfig接口，我们的流程从请求进入到GenericServlet的service方法开始说起。
- 拦截器是在DispacherServlet的doDispatch方法中执行的，（url拦截器和用户自定义的其他业务拦截器）
流程总览：
容器监听端口，接收到请求后调用Servlet的service方法，
在service方法中判断用户使用的是get还是post请求，决定后续调用对应的doGet/doPost方法，
在doGet/doPost方法中执行processRequest方法，
processRequest方法执行请求上下文的初始化过程，
然后调用doService()方法，
由该doService方法执行doDispatch方法，
最后由doDispatch方法去获取请求的mappedHandler和HandlerAdapter，处理完用户请求之后通过新建一个ServletWebRequest来渲染返回数据
首先调用GenericServlet的service()方法，源码如下：
public abstract void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;
事实上GenericServlet的service()方法是一个抽象方法，而HttpServlet继承了GenericServlet，所以此处实际调用了HttpServlet的service方法，HttpServlet的service方法源码如下：
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
if (req instanceof HttpServletRequest && res instanceof HttpServletResponse) {
HttpServletRequest request = (HttpServletRequest)req;
HttpServletResponse response = (HttpServletResponse)res;
this.service(request, response);
} else {
throw new ServletException("non-HTTP request or response");
}
}
首先判断ServletRequest、ServletResponse是否是标准的HttpServletRequest、HttpServletResponse，如果不是则抛出ServletException异常，否则继续调用实现类的service()方法
对源码做debug我们知道，this.service(request, response);执行到spring FrameworkServlet的service方法。
在spring框架中，FrameworkServlet继承自HttpServletBean，HttpServletBean是一个抽象类，HttpServletBean继承自HttpServlet。
所以HttpServlet的service方法中调用的this.service(request, response);其实是执行的是FrameworkServlet中的service方法。继续看源码：
protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
if (httpMethod != HttpMethod.PATCH && httpMethod != null) {
super.service(request, response);
} else {
this.processRequest(request, response);
}
}
我们的HttpMethod 应该是POST或者GET方法，所以执行super.service(request, response);回到HttpServlet的service(HttpServletRequest req, HttpServletResponse resp)方法：
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String method = req.getMethod();
long lastModified;
if (method.equals("GET")) {
lastModified = this.getLastModified(req);
if (lastModified == -1L) {
this.doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader("If-Modified-Since");
if (ifModifiedSince < lastModified) {
this.maybeSetLastModified(resp, lastModified);
this.doGet(req, resp);
} else {
resp.setStatus(304);
}
}
} else if (method.equals("HEAD")) {
lastModified = this.getLastModified(req);
this.maybeSetLastModified(resp, lastModified);
this.doHead(req, resp);
} else if (method.equals("POST")) {
this.doPost(req, resp);
} else if (method.equals("PUT")) {
this.doPut(req, resp);
} else if (method.equals("DELETE")) {
this.doDelete(req, resp);
} else if (method.equals("OPTIONS")) {
this.doOptions(req, resp);
} else if (method.equals("TRACE")) {
this.doTrace(req, resp);
} else {
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[]{method};
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(501, errMsg);
}
}
我们一GET请求为例，直接看this.doGet(req, resp);回到FrameworkServlet的doGet方法：
protected final void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.processRequest(request, response);
}
执行processRequest方法：
protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
//
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = this.buildLocaleContext(request);//此处调用其实现类的buildLocaleContext方法，spring中即DispatcherServlet，来创建localeContext 对象
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = this.buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
//注册拦截器registerCallableInterceptor
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new FrameworkServlet.RequestBindingInterceptor());
//初始化应用context
this.initContextHolders(request, localeContext, requestAttributes);
try {
this.doService(request, response);
} catch (ServletException var17) {
failureCause = var17;
throw var17;
} catch (IOException var18) {
failureCause = var18;
throw var18;
} catch (Throwable var19) {
failureCause = var19;
throw new NestedServletException("Request processing failed", var19);
} finally {
this.resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
if (this.logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", (Throwable)failureCause);
} else if (asyncManager.isConcurrentHandlingStarted()) {
this.logger.debug("Leaving response open for concurrent processing");
} else {
this.logger.debug("Successfully completed request");
}
}
this.publishRequestHandledEvent(request, response, startTime, (Throwable)failureCause);
}
}
可以看到，在FrameworkServlet的processRequest方法中执行了一系列的初始化操作，然后调用this.doService(request, response);也就是调用DispatcherServlet的doService方法
来看DispatcherServlet的doService方法的源码：
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
//省略掉一些request属性的代码
try {
this.doDispatch(request, response);
} finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
this.restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
该方法中主要完成一些request的属性设置，然后this.doDispatch(request, response);源码如下：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
try {
ModelAndView mv = null;
Object dispatchException = null;
try {
//处理multipart 请求
processedRequest = this.checkMultipart(request);
multipartRequestParsed = processedRequest != request;
//从容器中获取mappedHandler
mappedHandler = this.getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
this.noHandlerFound(processedRequest, response);
return;
}
// 从容器中获取适配器HandlerAdapter 。这个过程中，会调用MappedInterceptor的matches方法，以此来判断次url是否在拦截范围内
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
String method = request.getMethod();
boolean isGet = "GET".equals(method);
//处理get请求
if (isGet || "HEAD".equals(method)) {
//lastModified最后修改时间 此处返回-1
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (this.logger.isDebugEnabled()) {
this.logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
}
// 检查请求是否使用缓存
if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
return;
}
}
//执行拦截器中的preHandle拦截方法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
//执行用户controller中对应的业务
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//渲染默认的mv
this.applyDefaultViewName(processedRequest, mv);
//执行PostHandle拦截方法
mappedHandler.applyPostHandle(processedRequest, response, mv);
} catch (Exception var20) {
dispatchException = var20;
} catch (Throwable var21) {
dispatchException = new NestedServletException("Handler dispatch failed", var21);
}
//渲染返回值
this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
} catch (Exception var22) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
} catch (Throwable var23) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
}
} finally {
if (asyncManager.isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
} else if (multipartRequestParsed) {
this.cleanupMultipart(processedRequest);
}
}
}