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作者：fredalxin\
地址：https://fredal.xin/400-error-…
很多人都会在平时开发过程中遇到 400 或 500 异样，并且也没有走到服务端 controller 中，就变得有些手足无措。
咱们晓得 SpringMVC 从 DispatchServlet 开始接管与散发申请，从入口开始 debug，还能找不到问题所在么？
从 DispatchServlet 的 doDispatch()办法开始解决申请：
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; // 这里捕捉了异样 TypeMismatchException}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) { }
finally {// 删除一些代码}
}其实在这儿咱们就能看到 exception 的具体异样栈，有趣味的能够持续看 springMVC 的解决办法 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);// 执行这个办法
errorView = (mv != null);
}
}
// 不便浏览，删除了其余代码
}这个办法中对异样进行判断，发现不是“ModelAndViewDefiningException”就交给 processHandlerException 办法持续解决。
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 来解决这个异样。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 接口约定的办法的所实现代码如下：
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;}
}首先判断以后异样处理器是否能够解决以后的指标 handler。例如通过 for 循环顺次发现轮到 DefaultHandlerExceptionResolver 能力解决，那么最终会执行该 handlerExceptionResolver 的 doResolveException 办法。
protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
try {if (ex instanceof NoSuchRequestHandlingMethodException) {return handleNoSuchRequestHandlingMethod(...);
}
// 删除局部 else if instanceof 判断
else if (ex instanceof TypeMismatchException) {
// 执行到了这里
return handleTypeMismatch((TypeMismatchException) ex, request, response, handler);
}
// 删除局部 else if instanceof 判断
else if (ex instanceof BindException) {return handleBindException((BindException) ex, request, response, handler);
}
}
catch (Exception handlerException) { }
return null;
}通过对异样类型的判断，来执行相应 handleXXXException 办法。而 handleXXXException 办法中，有很多是会抛出 400 谬误的！
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 谬误的时候该怎么解决呢？
从服务端角度登程，能够定义欠缺的全局异样处理器 exceptionHandler，把易抛出 400 的谬误例如 TypeMismatchException、BindException 都给解决掉，返回能看得懂的信息。
从客户端申请过程中来看，能够自定义 handlerExceptionResolver，只需实现 HandlerExceptionResolver 接口即可，例如：
public class ApiHandlerExceptionResolver implements HandlerExceptionResolver {
@Override
public ModelAndView resolveException(HttpServletRequest request,
HttpServletResponse response, Object handler, Exception exception) {ModelAndView model = new ModelAndView();
// do something ...
return model;
}
} 所以遇到 400 谬误的时候不要慌，毕竟 400 它是个规范的错误码，好好 debug 或者查阅一下相干材料便能迎刃而解。
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