36.拦截器的执行时机和原理
1.DispatcherServlet的doDispatch方法
DispatcherServlet的doDispatch方法是请求的入口。
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.
// 拿到请求处理器，里面有拦截器的数组HandlerInterceptor[]
// 根据当前请求，找到HandlerExecutionChain【可以处理请求的handler以及handler的所有 拦截器】
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;
}
}
// 目标方法执行之前，会执行该方法
// 遍历所有拦截器，执行拦截器的preHandle方法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
// 如果任何一个拦截器返回false。直接跳出不执行目标方法。
return;
}
// 执行目标方法
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
// 倒叙执行所有拦截器的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);
}
// 该方法是页面渲染逻辑，页面成功渲染完成以后，也会倒序触发 afterCompletion
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
// 前面的步骤有任何异常也都会直接倒序触发 afterCompletion
// 一切正常也会触发afterCompletion
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
// 前面的步骤有任何异常也都会直接倒序触发 afterCompletion
// 一切正常也会触发afterCompletion
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);
}
}
}
}请求处理器，里面有拦截器的数组HandlerInterceptor[]
2.执行拦截器的preHandle方法
先来顺序执行 所有拦截器的 preHandle方法
① 如果当前拦截器prehandler返回为true。则执行下一个拦截器的preHandle方法。
② 如果当前拦截器返回为false。直接 倒序执行所有已经执行了的拦截器的 afterCompletion方法。
如果任何一个拦截器返回false。直接跳出不执行目标方法。
// org.springframework.web.servlet.HandlerExecutionChain#applyPreHandle
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];
if (!interceptor.preHandle(request, response, this.handler)) {
triggerAfterCompletion(request, response, null);
return false;
}
this.interceptorIndex = i;
}
}
return true;
}// org.springframework.web.servlet.HandlerExecutionChain#triggerAfterCompletion
void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex)
throws Exception {
HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = this.interceptorIndex; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
try {
interceptor.afterCompletion(request, response, this.handler, ex);
}
catch (Throwable ex2) {
logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
}
}
}
}3.倒叙执行所有拦截器的postHandle方法
// org.springframework.web.servlet.HandlerExecutionChain#applyPostHandle
void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)
throws Exception {
HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = interceptors.length - 1; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
interceptor.postHandle(request, response, this.handler, mv);
}
}
}4.页面成功渲染完成以后触发 afterCompletion
// org.springframework.web.servlet.DispatcherServlet#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) {
// Exception (if any) is already handled..
// 页面成功渲染完成以后触发 afterCompletion
mappedHandler.triggerAfterCompletion(request, response, null);
}
}页面成功渲染完成以后，也会倒序触发 afterCompletion
// org.springframework.web.servlet.HandlerExecutionChain#triggerAfterCompletion
void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex)
throws Exception {
HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = this.interceptorIndex; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
try {
interceptor.afterCompletion(request, response, this.handler, ex);
}
catch (Throwable ex2) {
logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
}
}
}
}6.总结
1、根据当前请求，找到HandlerExecutionChain【可以处理请求的handler以及handler的所有 拦截器】
2、先来顺序执行 所有拦截器的 preHandle方法
（1）、如果当前拦截器prehandler返回为true。则执行下一个拦截器的preHandle
（2）、如果当前拦截器返回为false。直接 倒序执行所有已经执行了的拦截器的 afterCompletion；
3、如果任何一个拦截器返回false。直接跳出不执行目标方法
4、所有拦截器都返回True。则继续执行目标方法
5、目标方法执行完成之后，倒序执行所有拦截器的postHandle方法。
6、前面的步骤有任何异常都会直接倒序触发 afterCompletion
7、页面成功渲染完成以后，也会倒序触发 afterCompletion
