SpringBoot源码系列（8）：拦截器执行顺序
前言
在SpringBoot/SpringMVC中，拦截器有三个方法
applyPreHandle（前置操作，目标方法执行前）applyPostHandle（后置操作，目标方法执行后）triggerAfterCompletion（最终操作，渲染结束后）
正常情况下，会顺序执行所有前置操作，逆序执行所有后置操作，逆序执行所有最终操作。
如果中间被拦截，会顺序执行所有前置操作，直至被拦截，逆序执行所有放行的后置操作。
我们根据源码来看一看其中的原理
原文地址：https://xuedongyun.cn/post/5855/
拦截器原理
DispatcherServlet
我知道，请求的核心函数是DispatcherServlet中的doDispatch方法
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| protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
try {
mappedHandler = getHandler(processedRequest);
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
mappedHandler.applyPostHandle(processedRequest, response, mv);
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
} catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
}
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目标方法出错，也会执行后置操作
HandlerExecutionChain
mappedHandler中包含：handler和拦截器链
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| mappedHandler = {HandlerExecutionChain}
handler = {HandlerMethod}
interceptorList = {ArrayList} size = 3
0={Onelnterceptor}
1={Twolnterceptor}
2={Threelnterceptor}
3={ConversionServiceExposinglnterceptor}
4={ResourceUrlProviderExposinglnterceptor}
|
applyPreHandle（前置处理）：可以看到，顺序执行了所有拦截器的前置操作。每次都记录了this.interceptorIndex。如果中间有返回false的，则立刻执行triggerAfterCompletion方法。
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| boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
for (int i = 0; i < this.interceptorList.size(); i++) {
HandlerInterceptor interceptor = this.interceptorList.get(i);
if (!interceptor.preHandle(request, response, this.handler)) {
triggerAfterCompletion(request, response, null);
return false;
}
this.interceptorIndex = i;
}
return true;
}
|
而triggerAfterCompletion则是从this.interceptorIndex开始逆序执行了所有的后置操作
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| void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex) {
for (int i = this.interceptorIndex; i >= 0; i--) {
HandlerInterceptor interceptor = this.interceptorList.get(i);
interceptor.afterCompletion(request, response, this.handler, ex);
}
}
|
applyPostHandle（后置处理）：可以看到，逆序执行了所有后置操作
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| void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)
throws Exception {
for (int i = this.interceptorList.size() - 1; i >= 0; i--) {
HandlerInterceptor interceptor = this.interceptorList.get(i);
interceptor.postHandle(request, response, this.handler, mv);
}
}
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processDispatchResult（渲染页面）：可以看到，渲染完成后，逆序执行了所有的最终操作
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| private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler,
@Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}
}
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| void triggerAfterCompletion(HttpServletRequest request,
HttpServletResponse response,
@Nullable Exception ex) {
for (int i = this.interceptorIndex; i >= 0; i--) {
HandlerInterceptor interceptor = this.interceptorList.get(i);
interceptor.afterCompletion(request, response, this.handler, ex);
}
}
|
