首先异常处理属于SpringMVC模块，而不属于spring core，有这种意识能帮助我们更好的理解spring framework。SpringMVC统一异常处理机制，一共有两种，第一种实现HandlerExceptionResolver接口、第二种在异常处理类上添加@ControllerAdvice注解，在方法上加@ExceptionHandler处理指定异常类型。
实现HandlerExceptionResolver
HandlerExceptionResolver只有一个resolveException方法，返回参数ModelAndView，前后端不分离时通常使用这种方式，大体逻辑是判断是否是ajax请求，如果是ajax请求，则返回null，否则返回非null的ModelAndView。
public interface HandlerExceptionResolver {
@Nullable
ModelAndView resolveException(
HttpServletRequest request, HttpServletResponse response,
@Nullable Object handler, Exception ex);
}DefaultHandlerExceptionResolver是此接口的默认实现，看一下DefaultHandlerExceptionResolver#doResolveException实现。
protected ModelAndView doResolveException(
HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) {
try {
//这就是常见的，状态码405，请求方法不正确导致
if (ex instanceof HttpRequestMethodNotSupportedException) {
return handleHttpRequestMethodNotSupported(
(HttpRequestMethodNotSupportedException) ex, request, response, handler);
}
//这就是常见的，状态码415，媒体类型不正确导致
else if (ex instanceof HttpMediaTypeNotSupportedException) {
return handleHttpMediaTypeNotSupported(
(HttpMediaTypeNotSupportedException) ex, request, response, handler);
}
//其他逻辑
}
catch (Exception handlerEx) {
if (logger.isWarnEnabled()) {
logger.warn("Failure while trying to resolve exception [" + ex.getClass().getName() + "]", handlerEx);
}
}
//本类处理不了，返回null，以让后边的ExceptionResolver处理
return null;
}这是第一种处理异常的方法。
@ControllerAdvice、@ExceptionHandler
写一个类，添加@ControllerAdvice(更通常的使用@RestControllerAdvice)，在类的方法上添加@ExceptionHandler，并指明要处理的Exception类型。
@ControllerAdvice
public class MyExceptionHanlder{
//注解可以不指定异常，方法参数上指定
@ExceptionHandler
public ModelAndView handleException1(Exception1 excep){
//只处理Exception1类型的异常
}
//也可以配置在注解上，优先级大于方法参数，一般用来缩小可处理的异常范围
@ExceptionHandler({Exception2.class})
public ModelAndView handleException1(Exception2Parent excep){
//只处理Exception1类型的异常
}
@ExceptionHandler
public ModelAndView handleException(Exception excep){
//兜底，处理其他未处理的异常
}-.
}@RestControllerAdvice适合前后端分离情况，它等于@ControllerAdvice+@ResponseBody
@RestControllerAdvice
public class MyExceptionHanlder{
@ExceptionHandler
public MyVO handleException1(Exception1 excep){
}
}这是第二种处理异常的方法。
SpringMVC中返回值统一处理
有时需要对返回值做统一处理，比如对返回值统一包装为{code:200,msg:"",data:object}，controller方法返回参数可以是POJO,而不必是Result<T>的样子，那么就需要实现ResponseBodyAdvice接口，它在@ResponseBody或者ResponseEntity之后，但在HttpMessageConverter之前执行，通常与异常处理类一起使用。
@RestControllerAdvice
public class MyExceptionHanlder implements ResponseBodyAdvice<Object>{
@ExceptionHandler
public MyVO handleException1(Exception1 excep){
}
boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType){
return true;
}
//在这里将返回值包装为统一数据类型
Object beforeBodyWrite(@Nullable Object body, MethodParameter returnType, MediaType selectedContentType,
Class<? extends HttpMessageConverter<?>> selectedConverterType,
ServerHttpRequest request, ServerHttpResponse response){
if(body.getClass != Result.class){
return Result<>(body);
}
return body;
}
}springmvc 异常处理机制原理
MvcNameSpaceHandler解析<mvc:annotation-config/>时依次注入ExceptionHandlerExceptionResolver、ResponseStatusExceptionHandler(用来处理ResponseStatusException)、DefaultHandlerExceptionResolver
public BeanDefinition parse(Element element, ParserContext context) {
// 先注册ExceptionHandlerExceptionResolver，用来处理加了@ExceptionHandler方法抛出的异常
RootBeanDefinition methodExceptionResolver = new RootBeanDefinition(ExceptionHandlerExceptionResolver.class);
methodExceptionResolver.getPropertyValues().add("order", 0);//优先级最高
String methodExResolverName = readerContext.registerWithGeneratedName(methodExceptionResolver);
// 再注册ResponseStatusExceptionResolver
RootBeanDefinition statusExceptionResolver = new RootBeanDefinition(ResponseStatusExceptionResolver.class);
statusExceptionResolver.getPropertyValues().add("order", 1);//优先级其次
String statusExResolverName = readerContext.registerWithGeneratedName(statusExceptionResolver);
//然后注册DefaultHandlerExceptionResolver
RootBeanDefinition defaultExceptionResolver = new RootBeanDefinition(DefaultHandlerExceptionResolver.class);
defaultExceptionResolver.getPropertyValues().add("order", 2);//优先级再次
String defaultExResolverName = readerContext.registerWithGeneratedName(defaultExceptionResolver);
}然后 DispatcherServlet#initStrategies初始化异常解析器类。
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
//初始化处理异常解析器
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}这一步中除了系统内部定义的ExceptionResolver，还会将自定义HandlerExceptionResolver加入到处理链中。
private void initHandlerExceptionResolvers(ApplicationContext context) {
this.handlerExceptionResolvers = null;
if (this.detectAllHandlerExceptionResolvers) {
// Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
//初始化ExceptionResolver
Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
// We keep HandlerExceptionResolvers in sorted order.
AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
}
}
}遇到异常时会依次执行上边注册的ExceptionHandlerExceptionResolver、ResponseStatusExceptionHandler、DefaultHandlerExceptionResolver，
可以看到这三个类都是HandlerExceptionResovler的子类。
springBoot的初始化ExceptionResolver过程
在
org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#handlerExceptionResolver 中，将ExceptionHandlerExceptionResolver 、ResponseStatusExceptionResolver、DefaultHandlerExceptionResolver封装成HandlerExceptionResolverComposite
@Bean
public HandlerExceptionResolver handlerExceptionResolver(
@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager) {
List<HandlerExceptionResolver> exceptionResolvers = new ArrayList<>();
configureHandlerExceptionResolvers(exceptionResolvers);
if (exceptionResolvers.isEmpty()) {
//获取默认HandlerExceptionResolver，其实就是
addDefaultHandlerExceptionResolvers(exceptionResolvers, contentNegotiationManager);
}
extendHandlerExceptionResolvers(exceptionResolvers);
//
HandlerExceptionResolverComposite composite = new HandlerExceptionResolverComposite();
composite.setOrder(0);
composite.setExceptionResolvers(exceptionResolvers);
return composite;
}在org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#addDefaultHandlerExceptionResolvers中，将3种ExceptionResolver封装成HandlerExceptionResolverComposite 的实现原理。
protected final void addDefaultHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers,
ContentNegotiationManager mvcContentNegotiationManager) {
ExceptionHandlerExceptionResolver exceptionHandlerResolver = createExceptionHandlerExceptionResolver();
exceptionHandlerResolver.setContentNegotiationManager(mvcContentNegotiationManager);
exceptionHandlerResolver.setMessageConverters(getMessageConverters());
exceptionHandlerResolver.setCustomArgumentResolvers(getArgumentResolvers());
exceptionHandlerResolver.setCustomReturnValueHandlers(getReturnValueHandlers());
if (jackson2Present) {
exceptionHandlerResolver.setResponseBodyAdvice(
Collections.singletonList(new JsonViewResponseBodyAdvice()));
}
if (this.applicationContext != null) {
exceptionHandlerResolver.setApplicationContext(this.applicationContext);
}
//解析含有@ControllerAdvice的类
exceptionHandlerResolver.afterPropertiesSet();
//先添加ExceptionHandlerExceptionResolver
exceptionResolvers.add(exceptionHandlerResolver);
//再添加ResponseStatusExceptionResolver
ResponseStatusExceptionResolver responseStatusResolver = new ResponseStatusExceptionResolver();
responseStatusResolver.setMessageSource(this.applicationContext);
exceptionResolvers.add(responseStatusResolver);
//最后添加DefaultHandlerExceptionResolver
exceptionResolvers.add(new DefaultHandlerExceptionResolver());
}@ExceptionHandler的实现原理
org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver#initExceptionHandlerAdviceCache中搜索@ControllerAdvice的bean
private void initExceptionHandlerAdviceCache() {
if (getApplicationContext() == null) {
return;
}
List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
for (ControllerAdviceBean adviceBean : adviceBeans) {
Class<?> beanType = adviceBean.getBeanType();
if (beanType == null) {
throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
}
ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
if (resolver.hasExceptionMappings()) {
this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
}
if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
this.responseBodyAdvice.add(adviceBean);
}
}
}然后在org.springframework.web.method.annotation.ExceptionHandlerMethodResolver#ExceptionHandlerMethodResolver中查找本类及父类含有@ExceptionHandler的方法
public ExceptionHandlerMethodResolver(Class<?> handlerType) {
for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
addExceptionMapping(exceptionType, method);
}
}
}HandlerExceptionResolverComposite中ExceptionResolver的执行顺序
顺序执行，直到返回非null的ModelAndView，这一点与org.springframework.web.servlet.DispatcherServlet#processHandlerException是一样的。
org.springframework.web.servlet.handler.HandlerExceptionResolverComposite#resolveException
@Override
@Nullable
public ModelAndView resolveException(
HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) {
if (this.resolvers != null) {
for (HandlerExceptionResolver handlerExceptionResolver : this.resolvers) {
ModelAndView mav = handlerExceptionResolver.resolveException(request, response, handler, ex);
if (mav != null) {
return mav;
}
}
}
return null;
}初次之外，dispatchservlet还注册了DefaultErrorAttributes、
@Bean
@ConditionalOnMissingBean(value = ErrorAttributes.class, search = SearchStrategy.CURRENT)
public DefaultErrorAttributes errorAttributes() {
return new DefaultErrorAttributes();
}总结
从上可以看出，springmvc与springboot 的异常处理内部实现原理有点差异，springboot兼容了springmvc。
- springmvc
- DispatcherServlet中注册了
ExceptionHandlerExceptionResolver、ResponseStatusExceptionHandler(用来处理ResponseStatusException)、DefaultHandlerExceptionResolver及自定义HandlerExceptionResolver
- springboot
- DispatcherServlet中注册了，
DefaultErrorAttributes、HandlerExceptionResolverComposite及自定义HandlerExceptionResolver，而HandlerExceptionResolverComposite内又依次执行ExceptionHandlerExceptionResolver、ResponseStatusExceptionResolver、DefaultHandlerExceptionResolver，只要ExceptionHandlerExceptionResolver匹配了异常则不再执行后续HandlerExceptionResolver。
