遇到 400、500 錯誤千萬不要慌！
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作者：fredalxin
地址：https://fredal.xin/400-error-deal
很多人都會在平時開發(fā)過程中遇到400或500異常，并且也沒有走到服務(wù)端controller中，就變得有些不知所措。
我們知道SpringMVC從DispatchServlet開始接收與分發(fā)請求，從入口開始debug，還能找不到問題所在么？
從DispatchServlet的doDispatch()方法開始處理請求：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
//刪除一些代碼
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
// 刪除一些代碼
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; // 這里捕獲了異常TypeMismatchException
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
}
finally {
// 刪除一些代碼
}
}
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);// 執(zhí)行這個方法
errorView = (mv != null);
}
}
// 方便閱讀，刪除了其他代碼
}
這個方法中對異常進(jìn)行判斷，發(fā)現(xiàn)不是“ModelAndViewDefiningException”就交給processHandlerException方法繼續(xù)處理。
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;
}
}
// 去掉了一些代碼
throw ex;
}
Spring Boot 系列最新教程推薦看下：https://github.com/javastacks/spring-boot-best-practice
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;
}
}
protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
try {
if (ex instanceof NoSuchRequestHandlingMethodException) {
return handleNoSuchRequestHandlingMethod(...);
}
// 刪除部分else if instanceof 判斷
else if (ex instanceof TypeMismatchException) {
// 執(zhí)行到了這里
return handleTypeMismatch((TypeMismatchException) ex, request, response, handler);
}
// 刪除部分else if instanceof 判斷
else if (ex instanceof BindException) {
return handleBindException((BindException) ex, request, response, handler);
}
}
catch (Exception handlerException) {
}
return null;
}
通過對異常類型的判斷，來執(zhí)行相應(yīng)handleXXXException方法。而handleXXXException方法中，有很多是會拋出400錯誤的！
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();
}
從服務(wù)端角度出發(fā)，可以定義完善的全局異常處理器exceptionHandler，把易拋出400的錯誤例如TypeMismatchException、BindException都給處理掉，返回能看得懂的信息。
從客戶端請求過程中來看，可以自定義handlerExceptionResolver，只需實現(xiàn)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錯誤的時候不要慌，畢竟400它是個標(biāo)準(zhǔn)的錯誤碼，好好debug或者查閱一下相關(guān)資料便能迎刃而解。
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