本文详细介绍了SpringMVC中的异常处理机制，包括SimpleMappingExceptionResolver的配置与使用，@ExceptionHandler注解的灵活配置，以及异常处理的具体实现原理。
SpringMVC除了对请求URL的路由处理特别方便外，还支持对异常的统一处理机制，可以对业务操作时抛出的异常，unchecked异常以及状态码的异常进行统一处理。SpringMVC既提供简单的配置类，也提供了细粒度的异常控制机制。
SpringMVC中所有的异常处理通过接口HandlerExceptionResolver来实现，接口中只定义了一个方法
public interface HandlerExceptionResolver {
ModelAndView resolveException(
HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);
}
方法中接受request和response信息，以及当前的处理Handler，和抛出的异常对象。并且提供抽象类AbstractHandlerExceptionResolver，实现resolveException方法，支持前置判断和处理，将实际处理抽象出doResolveException方法由子类来实现。
1.SimpleMappingExceptionResolver
SimpleMappingExceptionResolver是SpringMVC提供的一个非常便捷的简易异常处理方式，在XML中进行配置即可使用。
<bean class="org.springframework.web.servlet.handler.SimpleMappingExceptionResolver">
<!-- 默认异常视图 -->
<property name="defaultErrorView" value="error"/>
<!-- 视图中获取exception信息变量名 -->
<property name="exceptionAttribute" value="ex"></property>
<!-- 异常同视图映射关系 -->
<property name="exceptionMappings">
<props>
<prop key="com.lcifn.springmvc.exception.BusinessException">businessEx</prop>
</props>
</property>
</bean>
这是极简的一种配置，exceptionMappings配置的是异常同视图之间的映射关系，它是一个Properties对象，key-value分别是异常的类路径和视图名称。defaultErrorView表示默认异常视图，如果抛出的异常没有匹配到任何视图，即会走默认异常视图。exceptionAttribute表示在视图中获取exception信息变量名，默认为exception。还有一些其他配置可以查看SimpleMappingExceptionResolver的源码来使用。
2.@ExceptionHandler
SpringMVC提供了一种注解方式来灵活地配置异常处理，@ExceptionHandler中可以配置要处理的异常类型，然后定义在处理此种异常的方法上，方法只要写在Controller中，即可对Controller中所有请求方法有效。
我们定义一个BaseController，并且将需要处理的异常通过@ExceptionHandler定义好处理方法，这样业务Controller只需要继承这个基类就可以了。处理方法中支持Request/Response/Sessioin等相关的参数绑定。
[@Controller](https://my.oschina.net/u/1774615)
public class BaseController {
@ExceptionHandler(RuntimeException.class)
public ModelAndView handleRuntimeException(HttpServletRequest req, HttpServletResponse resp, RuntimeException ex){
return new ModelAndView("error");
}
}
但是继承的方式还是对业务代码造成侵入，Spring非常重要的特性就是非侵入性，因而SpringMVC提供了@ControllerAdvice，简单来说就是Controller的切面，支持对可选择的Controller进行统一配置，用于异常处理简直再合适不过了，我们只需要将BaseController稍稍改一下。
@ControllerAdvice
public class AdviceController {
@ExceptionHandler(RuntimeException.class)
public ModelAndView handleRuntimeException(HttpServletRequest req, HttpServletResponse resp, RuntimeException ex){
return new ModelAndView("error");
}
}
只需要在统一配置类上加上@ControllerAdvice注解，支持包路径，注解等过滤方式，即可完成对所有业务Controller进行控制，而业务Controller不用做anything。
如果请求为ajax方式，需要其他格式返回异常，在方法上加上@ResponseBody即可。
3.异常处理原理
上面介绍了常用的两种异常处理的配置方式，所谓知其然要知其所以然，SpringMVC怎么在请求处理的过程中完成对异常的统一处理的呢？我们从源码来深度解读。
回到DispatcherServlet的doDispatcher方法
try {
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
catch (Throwable err) {
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
可以看到对请求处理的核心处理使用一个大的try/catch，如果出现异常，统一封装成dispatchException交给processDispatchResult方法进行处理。我们知道processDispatchResult方法用来对返回视图进行操作，而同时也对异常进行统一处理。
在processDispatchResult中，首先对异常进行判断。
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);
errorView = (mv != null);
}
}
如果不是特殊的ModelAndViewDefiningException，则由processHandlerException来操作。
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
Object handler, Exception ex) throws Exception {
// Check registered HandlerExceptionResolvers...
ModelAndView exMv = null;
// 遍历所有注册的异常处理器，由异常处理器进行处理
for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
if (exMv != null) {
break;
}
}
// 如果异常视图存在，则转向异常视图
if (exMv != null) {
if (exMv.isEmpty()) {
request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
return null;
}
// We might still need view name translation for a plain error model...
if (!exMv.hasView()) {
exMv.setViewName(getDefaultViewName(request));
}
if (logger.isDebugEnabled()) {
logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " + exMv, ex);
}
WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
return exMv;
}
throw ex;
}
我们主要关注异常处理器对异常的处理，SpringMVC通过HandlerExceptionResolver的resolveException调用实现类的实际实现方法doResolveException。
SimpleMappingExceptionResolver
来看SimpleMappingExceptionResolver的实现：
protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response,
Object handler, Exception ex) {
// Expose ModelAndView for chosen error view.
// 根据request和异常对象获取异常视图名称
String viewName = determineViewName(ex, request);
if (viewName != null) {
// Apply HTTP status code for error views, if specified.
// Only apply it if we're processing a top-level request.
Integer statusCode = determineStatusCode(request, viewName);
if (statusCode != null) {
applyStatusCodeIfPossible(request, response, statusCode);
}
// 组装异常视图模型ModelAndView
return getModelAndView(viewName, ex, request);
}
else {
return null;
}
}
determineViewName方法决定异常视图名称，getModelAndView方法返回ModelAndView对象
protected String determineViewName(Exception ex, HttpServletRequest request) {
String viewName = null;
if (this.excludedExceptions != null) {
for (Class<?> excludedEx : this.excludedExceptions) {
if (excludedEx.equals(ex.getClass())) {
return null;
}
}
}
// Check for specific exception mappings.
if (this.exceptionMappings != null) {
viewName = findMatchingViewName(this.exceptionMappings, ex);
}
// Return default error view else, if defined.
if (viewName == null && this.defaultErrorView != null) {
if (logger.isDebugEnabled()) {
logger.debug("Resolving to default view '" + this.defaultErrorView + "' for exception of type [" +
ex.getClass().getName() + "]");
}
viewName = this.defaultErrorView;
}
return viewName;
}
在determineViewName方法中，我们配置的defaultErrorView和exceptionMappings都起了作用。更细节的就不深入了，有兴趣可以自己去看。
ExceptionHandlerExceptionResolver
ExceptionHandlerExceptionResolver支持了@ExceptionHandler注解的实现。它的抽象基类AbstractHandlerMethodExceptionResolver继承了AbstractHandlerExceptionResolver，doResolveException方法实际调用ExceptionHandlerExceptionResolver的doResolveHandlerMethodException方法。
protected ModelAndView doResolveHandlerMethodException(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod, Exception exception) {
// 根据HandlerMethod和exception获取异常处理的Method
ServletInvocableHandlerMethod exceptionHandlerMethod = getExceptionHandlerMethod(handlerMethod, exception);
if (exceptionHandlerMethod == null) {
return null;
}
// 设置异常处理方法的参数解析器和返回值解析器
exceptionHandlerMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
exceptionHandlerMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
ServletWebRequest webRequest = new ServletWebRequest(request, response);
ModelAndViewContainer mavContainer = new ModelAndViewContainer();
// 执行异常处理方法
try {
if (logger.isDebugEnabled()) {
logger.debug("Invoking @ExceptionHandler method: " + exceptionHandlerMethod);
}
Throwable cause = exception.getCause();
if (cause != null) {
// Expose cause as provided argument as well
exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, cause, handlerMethod);
}
else {
// Otherwise, just the given exception as-is
exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, handlerMethod);
}
}
catch (Throwable invocationEx) {
// Any other than the original exception is unintended here,
// probably an accident (e.g. failed assertion or the like).
if (invocationEx != exception && logger.isWarnEnabled()) {
logger.warn("Failed to invoke @ExceptionHandler method: " + exceptionHandlerMethod, invocationEx);
}
// Continue with default processing of the original exception...
return null;
}
// 对返回的视图模型进行处理
if (mavContainer.isRequestHandled()) {
return new ModelAndView();
}
else {
ModelMap model = mavContainer.getModel();
HttpStatus status = mavContainer.getStatus();
ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model, status);
mav.setViewName(mavContainer.getViewName());
if (!mavContainer.isViewReference()) {
mav.setView((View) mavContainer.getView());
}
if (model instanceof RedirectAttributes) {
Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
request = webRequest.getNativeRequest(HttpServletRequest.class);
RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
}
return mav;
}
}
我们主要关注的是如何匹配到异常处理方法的
protected ServletInvocableHandlerMethod getExceptionHandlerMethod(HandlerMethod handlerMethod, Exception exception) {
Class<?> handlerType = (handlerMethod != null ? handlerMethod.getBeanType() : null);
// 从当前Controller中匹配异常处理Method
if (handlerMethod != null) {
ExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);
if (resolver == null) {
resolver = new ExceptionHandlerMethodResolver(handlerType);
this.exceptionHandlerCache.put(handlerType, resolver);
}
Method method = resolver.resolveMethod(exception);
if (method != null) {
return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);
}
}
// 从ControllerAdvice中匹配异常处理Method
for (Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
if (entry.getKey().isApplicableToBeanType(handlerType)) {
ExceptionHandlerMethodResolver resolver = entry.getValue();
Method method = resolver.resolveMethod(exception);
if (method != null) {
return new ServletInvocableHandlerMethod(entry.getKey().resolveBean(), method);
}
}
}
return null;
}
匹配异常处理方法的来源有两个，一个是当前Controller，一个是所有@ControllerAdvice类。可以看到两种方式都使用了cache的方式，那么ExceptionHandlerMethod的信息怎么初始化的呢？
当前Controller
对每个请求HandlerMethod的Controller类型，都实例化一个ExceptionHandlerMethodResolver来处理异常。ExceptionHandlerMethodResolver的构造函数中初始化了当前Controller中的异常处理配置。
public ExceptionHandlerMethodResolver(Class<?> handlerType) {
for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
// detectExceptionMappings方法执行探查
for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
addExceptionMapping(exceptionType, method);
}
}
}
private List<Class<? extends Throwable>> detectExceptionMappings(Method method) {
List<Class<? extends Throwable>> result = new ArrayList<Class<? extends Throwable>>();
// 探查所有ExceptionHandler注解的方法
detectAnnotationExceptionMappings(method, result);
if (result.isEmpty()) {
for (Class<?> paramType : method.getParameterTypes()) {
if (Throwable.class.isAssignableFrom(paramType)) {
result.add((Class<? extends Throwable>) paramType);
}
}
}
if (result.isEmpty()) {
throw new IllegalStateException("No exception types mapped to " + method);
}
return result;
}
protected void detectAnnotationExceptionMappings(Method method, List<Class<? extends Throwable>> result) {
ExceptionHandler ann = AnnotationUtils.findAnnotation(method, ExceptionHandler.class);
result.addAll(Arrays.asList(ann.value()));
}
@ControllerAdvice类
对@ControllerAdvice统一切面类的处理，则是在ExceptionHandlerExceptionResolver的初始化方法afterPropertiesSet中进行处理。
public void afterPropertiesSet() {
// Do this first, it may add ResponseBodyAdvice beans
// 初始化@ControllerAdvice中的@ExceptionHandler
initExceptionHandlerAdviceCache();
if (this.argumentResolvers == null) {
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
if (this.returnValueHandlers == null) {
List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
}
}
initExceptionHandlerAdviceCache方法遍历上下文中所有有@ControllerAdvice注解的Bean，然后实例化成ExceptionHandlerMethodResolver类，在构造函数中初始化所有@ExceptionHandler。
private void initExceptionHandlerAdviceCache() {
if (getApplicationContext() == null) {
return;
}
if (logger.isDebugEnabled()) {
logger.debug("Looking for exception mappings: " + getApplicationContext());
}
// 查询所有@ControllerAdvice的Bean
List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
AnnotationAwareOrderComparator.sort(adviceBeans);
// 遍历，实例化ExceptionHandlerMethodResolver
for (ControllerAdviceBean adviceBean : adviceBeans) {
ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(adviceBean.getBeanType());
if (resolver.hasExceptionMappings()) {
this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
if (logger.isInfoEnabled()) {
logger.info("Detected @ExceptionHandler methods in " + adviceBean);
}
}
if (ResponseBodyAdvice.class.isAssignableFrom(adviceBean.getBeanType())) {
this.responseBodyAdvice.add(adviceBean);
if (logger.isInfoEnabled()) {
logger.info("Detected ResponseBodyAdvice implementation in " + adviceBean);
}
}
}
}
匹配到exceptionHandlerMethod后，设置一些方法执行的环境，然后调用ServletInvocableHandlerMethod中的invokeAndHandle去执行，这个调用过程和正常请求的调用就是一致了。这里也不向下扩展了，可以参看SpringMVC源码(四)-请求处理。
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 执行请求方法
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
setResponseStatus(webRequest);
if (returnValue == null) {
if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
mavContainer.setRequestHandled(true);
return;
}
}
else if (StringUtils.hasText(getResponseStatusReason())) {
mavContainer.setRequestHandled(true);
return;
}
mavContainer.setRequestHandled(false);
try {
this.returnValueHandlers.handleReturnValue(
returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
}
catch (Exception ex) {
if (logger.isTraceEnabled()) {
logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
}
throw ex;
}
}
至此ExceptionHandlerExceptionResolver的异常处理已经基本完成。SpringMVC还内置了ResponseStatusExceptionResolver和DefaultHandlerExceptionResolver来对状态码异常和常见的请求响应异常进行统一处理。
4.web.xml的配置
某些情况下，SpringMVC的处理并没有异常出现，但在最终的视图输出时找不到视图文件，就会显示404错误页面，非常影响用户体验。我们可以在web.xml中对未捕获的异常以及此种4xx或5xx的异常通过<error-page>进行处理。
<error-page>
<error-code>404</error-code>
<location>/404</location>
</error-page>
<error-page>
<exception-type>java.lang.Throwable</exception-type>
<location>/500</location>
</error-page>
通常对于系统中的异常，业务相关的尽量自定义异常处理方式，而一些系统异常通过统一错误页面进行处理。
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