遇到 400、500 错误千万不要慌！

作者：fredalxin
地址：https://fredal.xin/400-error-deal
很多人都会在平时开发过程中遇到400或500异常，并且也没有走到服务端controller中，就变得有些不知所措。
我们知道SpringMVC从DispatchServlet开始接收与分发请求，从入口开始debug ，还能找不到问题所在么？
从DispatchServlet的doDispatch()方法开始处理请求：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {//删除一些代码try {ModelAndView mv = null;Exception dispatchException = null;try {// 删除一些代码try {// Actually invoke the handler.mv = ha.handle(processedRequest, response, mappedHandler.getHandler());}finally {if (asyncManager.isConcurrentHandlingStarted()) {return;}}applyDefaultViewName(request, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;// 这里捕获了异常TypeMismatchException}processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {}finally {// 删除一些代码}}其实在这儿我们就能看到exception的具体异常栈，有兴趣的可以继续看springMVC的处理方法processDispatchResult 。
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,　HandlerExecutionChain mappedHandler, ModelAndView mv, 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);// 执行这个方法errorView = (mv != null);}}// 方便阅读，删除了其他代码}这个方法中对异常进行判断，发现不是“ModelAndViewDefiningException”就交给processHandlerException方法继续处理。
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,　Object handler, Exception ex) throws Exception {// Check registered HandlerExceptionResolvers...ModelAndView exMv = null;for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);if (exMv != null) {break;}}// 去掉了一些代码throw ex;}这里看到for循环来找一个handlerExceptionResolver来处理这个异常。handler列表有spring自带的ExceptionHandlerExceptionResolver、ResponseStatusExceptionResolver、DefaultHandlerExceptionResolver以及自定义的exceptionResolver 。
这些都继承自AbstractHandlerExceptionResolver类，这个类是一个抽象类，它实现了HandlerExceptionResolver接口，它对HandlerExceptionResolver接口约定的方法的所实现代码如下：
【遇到 400、500 错误千万不要慌！】public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response,　Object handler, Exception ex) {if (shouldApplyTo(request, handler)) {logException(ex, request);prepareResponse(ex, response);return doResolveException(request, response, handler, ex);}else {return null;}}首先判断当前异常处理器是否可以处理当前的目标handler 。例如通过for循环依次发现轮到DefaultHandlerExceptionResolver才能处理，那么最终会执行该handlerExceptionResolver的doResolveException方法。
protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response,　Object handler, Exception ex) {try {if (ex instanceof NoSuchRequestHandlingMethodException) {return handleNoSuchRequestHandlingMethod(...);}// 删除部分else ifinstanceof 判断else if (ex instanceof TypeMismatchException) {// 执行到了这里return handleTypeMismatch((TypeMismatchException) ex, request, response, handler);}// 删除部分else ifinstanceof 判断else if (ex instanceof BindException) {return handleBindException((BindException) ex, request, response, handler);}}catch (Exception handlerException) {}return null;}通过对异常类型的判断，来执行相应handleXXXException方法。而handleXXXException方法中，有很多是会抛出400错误的！
举个几个栗子：
protected ModelAndView handleMissingServletRequestParameter(MissingServletRequestParameterException ex, HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {response.sendError(400, ex.getMessage());return new ModelAndView();}protected ModelAndView handleServletRequestBindingException(ServletRequestBindingException ex, HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {response.sendError(400, ex.getMessage());return new ModelAndView();}protected ModelAndView handleTypeMismatch(TypeMismatchException ex, HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {response.sendError(400);return new ModelAndView();}protected ModelAndView handleHttpMessageNotReadable(HttpMessageNotReadableException ex, HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {response.sendError(400);return new ModelAndView();}protected ModelAndView handleMethodArgumentNotValidException(MethodArgumentNotValidException ex, HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {response.sendError(400);return new ModelAndView();}protected ModelAndView handleMissingServletRequestPartException(MissingServletRequestPartException ex, HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {response.sendError(400, ex.getMessage());return new ModelAndView();}protected ModelAndView handleBindException(BindException ex, HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {response.sendError(400);return new ModelAndView();}那么抛出400错误的时候该怎么解决呢？
从服务端角度出发，可以定义完善的全局异常处理器exceptionHandler ，把易抛出400的错误例如TypeMismatchException、BindException都给处理掉，返回能看得懂的信息。
从客户端请求过程中来看，可以自定义handlerExceptionResolver ，只需实现HandlerExceptionResolver接口即可，例如：
public class ApiHandlerExceptionResolver implements HandlerExceptionResolver { @Overridepublic ModelAndView resolveException(HttpServletRequest request,HttpServletResponse response, Object handler, Exception exception) {ModelAndView model = new ModelAndView();// do something ...return model;} } 所以遇到400错误的时候不要慌，毕竟400它是个标准的错误码，好好debug或者查阅一下相关资料便能迎刃而解。
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