遇到400错误不要慌

很多人都会在平时开发过程中遇到400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：

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 if   instanceof 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 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 {
 @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。