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：

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。

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