原理解读：Spring MVC统一异常处理
Running with Spring Boot v2.5.4, Java 11.0.12
当前，Spring统一异常处理机制是Java开发人员普遍使用的一种技术，在业务校验失败的时候，直接抛出业务异常即可，这明显简化了业务异常的治理流程与复杂度。值得一提的是，统一异常处理机制并不是Spring Boot提供的，而是Spring MVC，前者只是为Spring MVC自动配置了刚好够用的若干组件而已，具体配置了哪些组件，感兴趣的读者可以到spring-boot-autoconfigure模块中找到答案。
1 异常从何而来
DispatcherServlet是Spring MVC的门户，所有Http请求都会通过DispatcherServlet进行路由分发，即使Http请求的处理流程抛出了异常。doDispatch()方法是其核心逻辑，主要内容如下：
public class DispatcherServlet {
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HandlerExecutionChain mappedHandler = null;
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
// Determine handler for the current request.
mappedHandler = getHandler(request);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Invoke all HandlerInterceptor preHandle() in HandlerExecutionChain.
if (!mappedHandler.applyPreHandle(request, response)) {
return;
}
// Actually invoke the handler.
mv = ha.handle(request, response, mappedHandler.getHandler());
// Invoke all HandlerInterceptor postHandle() in HandlerExecutionChain.
mappedHandler.applyPostHandle(request, response, mv);
} catch (Exception ex) {
dispatchException = ex;
}
// Handle the result of handler invocation, which is either a ModelAndView or an Exception to be resolved to a ModelAndView.
processDispatchResult(request, response, mappedHandler, mv, dispatchException);
} catch (Exception ex) {
// Invoke all HandlerInterceptor afterCompletion() in HandlerExecutionChain.
triggerAfterCompletion(request, response, mappedHandler, ex);
}
}
}
阅读上述源码可以看出如果出现了异常，会先将该异常实例赋予dispatchException这一局部变量，然后由processDispatchResult()方法负责异常处理。很明显，在doDispatch()方法内有两处容易抛出异常，第一处在为Http请求寻找相匹配的Handler过程中，Handler是什么东东？一般就是那些由@Controller或@RestController标注的自定义Controller，这些Controller会由HandlerMethod包装起来；另一处就是在执行Handler的过程中。
1.1 获取Handler过程中抛出异常
获取Handler离不开HandlerMapping，由于@RequestMapping注解的广泛应用，使得RequestMappingHandlerMapping成为了一等宠臣，其继承关系如下图所示：
继承关系图清晰交代了HandlerMapping的子类AbstractHandlerMethodMapping实现了InitializingBean接口这一事实。众所周知：Spring IoC容器在构建Bean的过程中，如果当前Bean实现了InitializingBean接口，那么就会通过后者的afterPropertiesSet()方法来进行初始化操作，具体初始化逻辑如下：
public class RequestMappingHandlerMapping implements RequestMappingInfoHandlerMapping {
@Override
public void afterPropertiesSet() {
super.afterPropertiesSet();
}
}
public class AbstractHandlerMethodMapping extends AbstractHandlerMapping implements InitializingBean{
@Override
public void afterPropertiesSet() {
initHandlerMethods();
}
protected void initHandlerMethods() {
// obtainApplicationContext().getBeanNamesForType(Object.class))
for (String beanName : getCandidateBeanNames()) {
if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
processCandidateBean(beanName);
}
}
}
protected void processCandidateBean(String beanName) {
Class<?> beanType = beanType = obtainApplicationContext().getType(beanName);
// AnnotatedElementUtils.hasAnnotation(beanType, Controller.class)
// || AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class)
if (beanType != null && isHandler(beanType)) {
detectHandlerMethods(beanName);
}
}
protected void detectHandlerMethods(Object handler) {
Class<?> handlerType = (handler instanceof String ? obtainApplicationContext().getType((String) handler) : handler.getClass());
if (handlerType != null) {
Class<?> userType = ClassUtils.getUserClass(handlerType);
// private RequestMappingInfo createRequestMappingInfo(AnnotatedElement element) {
// RequestMapping requestMapping = AnnotatedElementUtils.findMergedAnnotation(element, RequestMapping.class);
// RequestCondition<?> condition = (element instanceof Class ? getCustomTypeCondition((Class<?>) element) : getCustomMethodCondition((Method) element));
// return createRequestMappingInfo(requestMapping, condition);
// }
Map<Method, RequestMappingInfo> methods = MethodIntrospector.selectMethods(userType,
(MethodIntrospector.MetadataLookup) method -> getMappingForMethod(method, userType));
methods.forEach((method, mapping) -> {
Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
registerHandlerMethod(handler, invocableMethod, mapping);
});
}
}
protected void registerHandlerMethod(Object handler, Method method, RequestMappingInfo mapping) {
this.mappingRegistry.register(mapping, handler, method);
}
}
public class AbstractHandlerMethodMapping.MappingRegistry {
private final Map<RequestMappingInfo, MappingRegistration> registry = new HashMap<>();
private final ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
public void register(RequestMappingInfo mapping, Object handler, Method method) {
this.readWriteLock.writeLock().lock();
try {
HandlerMethod handlerMethod = createHandlerMethod(handler, method);
Set<String> directPaths = getDirectPaths(mapping);
for (String path : directPaths) {
this.pathLookup.add(path, mapping);
}
this.registry.put(mapping, new MappingRegistration<>(mapping, handlerMethod, directPaths));
} finally {
this.readWriteLock.writeLock().unlock();
}
}
}
上述初始化逻辑主要为：
- 从ApplicationContext中获取所有Bean，遍历每一个Bean；
- 判断当前Bean是否含有
@Controller注解(注意：@RestController依然由@Controller标注)，若无则遍历下一个Bean，若有则意味着这是一个Handler； - 从当前Handler中探测出所有由
@RequestMapping标注的方法，然后构建出一个以Method实例为key、RequestMappingInfo实例为value的Map(注意：@GetMapping、@PostMapping等也由@RequestMapping标注)； - 遍历该Map，填充
MappingRegistry中Map<RequestMappingInfo, MappingRegistration>类型的成员变量registry。registry中所填充的内容示例如下：
{
"registry": [
{
"key": {
"RequestMappingInfo": {
"patternsCondition": "/crimson_typhoon/v1/fire",
"methodsCondition": "POST"
}
},
"value": {
"MappingRegistration": {
"HandlerMethod": {
"bean": "customExceptionHandler",
"beanType": "com.example.crimson_typhoon.controller.CrimsonTyphoonController",
"method": "com.example.crimson_typhoon.controller.CrimsonTyphoonController.v1Fire(com.example.crimson_typhoon.dto.UserDto,java.lang.Boolean)"
}
}
}
}
]
}
贴了这么一大段源码，只是想说明一个事实：DispatcherServlet可以快速根据Http请求解析出Handler，因为Http请求与Handler的映射关系被预先缓存在MappingRegistry中了。
下面步入正题：在获取Handler过程中究竟是否会抛出异常？又是哪些异常呢？
根据上图，我们直接去看AbstractHandlerMethodMapping中lookupHandlerMethod()方法的逻辑，如下：
protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
List<AbstractHandlerMethodMapping.Match> matches = new ArrayList<>();
List<RequestMappingInfo> directPathMatches = this.mappingRegistry.getMappingsByDirectPath(lookupPath);
if (directPathMatches != null) {
for (T mapping : mappings) {
T match = getMatchingMapping(mapping, request);
if (match != null) {
matches.add(new AbstractHandlerMethodMapping.Match(match, this.mappingRegistry.getRegistrations().get(mapping)));
}
}
}
if (!matches.isEmpty()) {
// 详细决策逻辑跳过
return 最匹配的HandlerMethod;
} else {
// 没找到匹配的HandlerMethod
return handleNoMatch(this.mappingRegistry.getRegistrations().keySet(), lookupPath, request);
}
}
顺藤摸瓜，继续：
protected HandlerMethod handleNoMatch(Set<RequestMappingInfo> infos, String lookupPath, HttpServletRequest request) throws ServletException {
RequestMappingInfoHandlerMapping.PartialMatchHelper helper = new RequestMappingInfoHandlerMapping.PartialMatchHelper(infos, request);
if (helper.hasMethodsMismatch()) {
throw new HttpRequestMethodNotSupportedException();
}
if (helper.hasConsumesMismatch()) {
throw new HttpMediaTypeNotSupportedException();
}
if (helper.hasProducesMismatch()) {
throw new HttpMediaTypeNotAcceptableException();
}
if (helper.hasParamsMismatch()) {
throw new UnsatisfiedServletRequestParameterException();
}
return null;
}
最终，抛出哪些异常还是让我们定位到了，比如大名鼎鼎的HttpRequestMethodNotSupportedException就是在这里被抛出的。
事实上，如果最终没有为Http请求寻找到相匹配的Handler，也将抛出异常，它就是NoHandlerFoundException，前提是要在application.properties配置文件中添加spring.mvc.throw-exception-if-no-handler-found=true这一项配置！
protected void noHandlerFound(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (this.throwExceptionIfNoHandlerFound) {
throw new NoHandlerFoundException(request.getMethod(), getRequestUri(request), new ServletServerHttpRequest(request).getHeaders());
} else {
response.sendError(HttpServletResponse.SC_NOT_FOUND);
}
}
1.2 Handler执行过程中抛出异常
Handler执行过程中抛出的异常比较宽泛，一般可以归纳为两种：一种是执行Handler后抛出的异常，比如：业务逻辑层中未知的运行时异常和开发人员自定义的异常；另一种是还未开始执行Handler，而是在为其方法参数进行数据绑定时抛出的异常，比如：BindingException及其子类MethodArgumentNotValidException。大家可能对MethodArgumentNotValidException尤为熟悉，常见的异常抛出场景如下所示：
@RestController
@RequestMapping(path = "/crimson_typhoon")
public class CrimsonTyphoonController {
@PostMapping(path = "/v1/fire")
public Map<String, Object> v1Fire(@RequestBody @Valid UserDto userDto, @RequestParam("dryRun") Boolean dryRun) {
return ImmutableMap.of("status", "success", "code", 200, "data", ImmutableList.of(userDto));
}
}
public class UserDto {
@NotBlank
private String name;
@NotNull
private int age;
}
如果调用方传递的请求体参数不符合Bean Validation的约束规则，那么就会抛出MethodArgumentNotValidException异常。
2 异常如何处理
无论是在获取Handler过程中、在为Handler的方法参数进行数据绑定过程中亦或在Handler执行过程中出现了异常，总是会先将该异常实例赋予dispatchException这一局部变量，然后由processDispatchResult()方法负责异常处理。下面来看看DispatcherServlet中processDispatchResult()方法是究竟如何处理异常的，源码逻辑很直白，最终是将异常委派给HandlerExceptionResolver处理的，如下：
public class DispatcherServlet {
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
HandlerExecutionChain mappedHandler,
ModelAndView mv, Exception exception) {
boolean errorView = false;
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
} else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
}
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
ModelAndView exMv = null;
if (this.handlerExceptionResolvers != null) {
for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
exMv = resolver.resolveException(request, response, handler, ex);
if (exMv != null) {
break;
}
}
}
if (exMv != null) {
if (exMv.isEmpty()) {
request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
return null;
}
if (!exMv.hasView()) {
String defaultViewName = getDefaultViewName(request);
if (defaultViewName != null) {
exMv.setViewName(defaultViewName);
}
}
WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
return exMv;
}
throw ex;
}
}
主角登场！HandlerExceptionResolver是一个函数式接口，即有且只有一个resolveException()方法：
public interface HandlerExceptionResolver {
ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);
}
HandlerExceptionResolver与HandlerMapping、HandlerAdapter等类似，一般不需要开发人员自行定义，Spring MVC默认会提供一些不同风格的HandlerExceptionResolver，这些HandlerExceptionResolver会通过initHandlerExceptionResolvers()方法被提前填充到DispatcherServlet中handlerExceptionResolvers这一成员变量中，具体地：
private void initHandlerExceptionResolvers(ApplicationContext context) {
if (this.detectAllHandlerExceptionResolvers) {
Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
}
}
}
HandlerExceptionResolverComposite是DispatcherServlet中handlerExceptionResolvers这一成员变量所持有的最重要的异常解析器，Composite后缀表明这是一个复合类，自然会通过其成员变量持有若干HandlerExceptionResolver类型的苦力小弟，而在这众多苦力小弟中最为重要的非ExceptionHandlerExceptionResolver异常解析器莫属！查阅其源码后发现它也实现了InitializingBean接口：
public class ExceptionHandlerExceptionResolver extends AbstractHandlerMethodExceptionResolver
implements ApplicationContextAware, InitializingBean {
private final Map<Class<?>, ExceptionHandlerMethodResolver> exceptionHandlerCache =
new ConcurrentHashMap<>(64);
private final Map<ControllerAdviceBean, ExceptionHandlerMethodResolver> exceptionHandlerAdviceCache =
new LinkedHashMap<>();
@Override
public void afterPropertiesSet() {
initExceptionHandlerAdviceCache();
}
private void initExceptionHandlerAdviceCache() {
// ControllerAdvice controllerAdvice = beanFactory.findAnnotationOnBean(name, ControllerAdvice.class)
List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
for (ControllerAdviceBean adviceBean : adviceBeans) {
Class<?> beanType = adviceBean.getBeanType();
ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
if (resolver.hasExceptionMappings()) {
this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
}
}
}
}
public class ExceptionHandlerMethodResolver {
public static final ReflectionUtils.MethodFilter EXCEPTION_HANDLER_METHODS = method ->
AnnotatedElementUtils.hasAnnotation(method, ExceptionHandler.class);
private final Map<Class<? extends Throwable>, Method> mappedMethods = new HashMap<>(16);
public ExceptionHandlerMethodResolver(Class<?> handlerType) {
for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
this.mappedMethods.put(exceptionType, method);
}
}
}
private List<Class<? extends Throwable>> detectExceptionMappings(Method method) {
List<Class<? extends Throwable>> result = new ArrayList<>();
ExceptionHandler ann = AnnotatedElementUtils.findMergedAnnotation(method, ExceptionHandler.class);
result.addAll(Arrays.asList(ann.value()));
if (result.isEmpty()) {
for (Class<?> paramType : method.getParameterTypes()) {
if (Throwable.class.isAssignableFrom(paramType)) {
result.add((Class<? extends Throwable>) paramType);
}
}
}
return result;
}
}
上述关于ExceptionHandlerExceptionResolver的初始化逻辑很清晰：首先，从IoC容器中获取所有由@ControllerAdvice注解接口标注的Bean，这个Bean一般就是我们平时自定义的全局异常统一处理器；然后，逐一遍历这些全局异常处理器Bean，将其作为ExceptionHandlerMethodResolver构造方法的参数，后者会解析出含有@ExceptionHandler注解的异常处理方法，按照以Class<? extends Throwable>实例为key、以Method实例为value的映射规则填充其成员变量mappedMethods；最后，ExceptionHandlerExceptionResolver再按照以ControllerAdviceBean实例为key、以ExceptionHandlerMethodResolver实例为value的映射规则填充其成员变量exceptionHandlerAdviceCache。本文为了更直观地展示这种映射关系，笔者这里通过JSON来表达：
{
"exceptionHandlerAdviceCache": {
"key": {
"ControllerAdviceBean": {
"beanName": "customExceptionHandler",
"beanType": "com.example.crimson_typhoon.config.exception.CustomExceptionHandler"
}
},
"value": {
"ExceptionHandlerMethodResolver": {
"mappedMethods": [
{
"key": "class java.lang.Exception",
"value": "public org.springframework.http.ResponseEntity com.example.crimson_typhoon.config.exception.CustomExceptionHandler.handleUnknownException(Exception)"
},
{
"key": "class java.lang.NullPointerException",
"value": "public org.springframework.http.ResponseEntity com.example.crimson_typhoon.config.exception.CustomExceptionHandler.handleNullPointerException(NullPointerException)"
}
]
}
}
}
}
ExceptionHandlerExceptionResolver的初始化用意与RequestMappingHandlerMapping一致，也是为了提前缓存，这样后期可以快速地根据异常获取相匹配的@ExceptionHandler异常处理方法。
通过分析ExceptionHandlerExceptionResolver的初始化逻辑，大家应该明白了为什么它是最为重要的一个异常解析器，因为它与由@ControllerAdvice标注的统一异常处理器息息相关。此外，大家不要把ExceptionHandlerExceptionResolver和ExceptionHandlerMethodResolver搞混淆了，从后者名称来看，它只是一个面向@ExceptionHandler注解的方法解析器，压根不会解析异常哈。
下面回过头来看看HandlerExceptionResolverComposite中的逻辑，核心内容如下：
public class HandlerExceptionResolverComposite implements HandlerExceptionResolver, Ordered {
private List<HandlerExceptionResolver> resolvers;
@Override
public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, 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;
}
}
上述源码说明：HandlerExceptionResolverComposite会让其持有的异常解析器逐一解析异常，如果谁能返回一个非空的ModelAndView实例对象，那么谁就是赢家；绝大多数情况下，都是ExceptionHandlerExceptionResolver获得最后的胜利。ExceptionHandlerExceptionResolver中的异常解析逻辑在doResolveHandlerMethodException()方法中：
public class ExceptionHandlerExceptionResolver extends AbstractHandlerMethodExceptionResolver
implements InitializingBean {
@Override
protected ModelAndView doResolveHandlerMethodException(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod, Exception exception) {
ServletInvocableHandlerMethod exceptionHandlerMethod = getExceptionHandlerMethod(handlerMethod, exception);
if (exceptionHandlerMethod == null) {
return null;
}
if (this.argumentResolvers != null) {
exceptionHandlerMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
}
if (this.returnValueHandlers != null) {
exceptionHandlerMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
}
ServletWebRequest webRequest = new ServletWebRequest(request, response);
ModelAndViewContainer mavContainer = new ModelAndViewContainer();
ArrayList<Throwable> exceptions = new ArrayList<>();
Throwable exToExpose = exception;
while (exToExpose != null) {
exceptions.add(exToExpose);
Throwable cause = exToExpose.getCause();
exToExpose = (cause != exToExpose ? cause : null);
}
Object[] arguments = new Object[exceptions.size() + 1];
exceptions.toArray(arguments);
arguments[arguments.length - 1] = handlerMethod;
exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, arguments);
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();
RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
}
return mav;
}
}
}
从上述ExceptionHandlerExceptionResolver的源码中，最终看到了执行@ExceptionHandler异常处理方法的身影，与执行Handler中目标方法的原理一致，都是通过反射调用的，不再赘述。这里必须要重点看一下getExceptionHandlerMethod()方法的逻辑，如下：
public class ExceptionHandlerExceptionResolver extends AbstractHandlerMethodExceptionResolver
implements InitializingBean {
protected ServletInvocableHandlerMethod getExceptionHandlerMethod(HandlerMethod handlerMethod, Exception exception) {
Class<?> handlerType = null;
if (handlerMethod != null) {
handlerType = handlerMethod.getBeanType();
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);
}
if (Proxy.isProxyClass(handlerType)) {
handlerType = AopUtils.getTargetClass(handlerMethod.getBean());
}
}
for (Map.Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
ControllerAdviceBean advice = entry.getKey();
if (advice.isApplicableToBeanType(handlerType)) {
ExceptionHandlerMethodResolver resolver = entry.getValue();
Method method = resolver.resolveMethod(exception);
if (method != null) {
return new ServletInvocableHandlerMethod(advice.resolveBean(), method);
}
}
}
return null;
}
}
在刚才介绍ExceptionHandlerExceptionResolver的初始化逻辑时已经提到了：其成员变量缓存了ControllerAdviceBean与ExceptionHandlerMethodResolver的映射关系。可是这个成员变量却在最后时刻才被遍历，这是为什么呢？原来，ExceptionHandlerExceptionResolver并不会首先从统一异常处理器中寻找@ExceptionHandler异常处理方法，而是先从当前Handler中查找，找到之后缓存在其另一个成员变量exceptionHandlerCache中。
最后，再介绍一个容易被忽略的知识点。回忆一下，当我们访问服务中不存在的API时，往往会响应一种奇怪的格式；之所以奇怪，是因为咱们平时都会定制化API的响应格式，而此时的响应格式与咱们定制化的格式不匹配，这是咋回事呢？如下所示：
{
"timestamp": "2021-12-06T13:51:34.063+00:00",
"status": 404,
"error": "Not Found",
"path": "/crimson_typhoon/v4/fire"
}
这是因为在根据Http请求获取Handler时，常规的Handler是不可能匹配到了，只能由ResourceHttpRequestHandler这一个HttpRequestHandler来兜底，它在通过handleRequest()方法处理该Http请求时发现自己也搞不定，于是就只能将其转发给Servlet容器中默认的Error Page处理了。只需通过response.sendError(HttpServletResponse.SC_NOT_FOUND)将Response中的errorState这一成员变量的值置为1，那么Servlet容器就会乖乖地进行服务端转发操作。Error Page会由Spring Boot注册到Servlet容器中，它就是BasicErrorController，具体内容如下：
package org.springframework.boot.autoconfigure.web.servlet.error;
@Controller
@RequestMapping("${server.error.path:${error.path:/error}}")
public class BasicErrorController extends AbstractErrorController {
private final ErrorProperties errorProperties;
@RequestMapping(produces = MediaType.TEXT_HTML_VALUE)
public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {
HttpStatus status = getStatus(request);
Map<String, Object> model = Collections
.unmodifiableMap(getErrorAttributes(request, getErrorAttributeOptions(request, MediaType.TEXT_HTML)));
response.setStatus(status.value());
ModelAndView modelAndView = resolveErrorView(request, response, status, model);
return (modelAndView != null) ? modelAndView : new ModelAndView("error", model);
}
@RequestMapping
public ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {
HttpStatus status = getStatus(request);
if (status == HttpStatus.NO_CONTENT) {
return new ResponseEntity<>(status);
}
Map<String, Object> body = getErrorAttributes(request, getErrorAttributeOptions(request, MediaType.ALL));
return new ResponseEntity<>(body, status);
}
}
如果你有强迫症，就是忍不了响应格式不统一的现象，那你可以像下面这样做：
@Configuration
public class CustomErrorHandlerConfig {
@Resource
private ServerProperties serverProperties;
@Bean
public BasicErrorController basicErrorController(ErrorAttributes errorAttributes, ServerProperties serverProperties) {
return new BasicErrorController(errorAttributes, serverProperties.getError()) {
@Override
public ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {
HttpStatus status = getStatus(request);
Map<String, Object> body = getErrorAttributes(request, getErrorAttributeOptions(request, MediaType.ALL));
Map<String, Object> finalBody = ImmutableMap.of("status", body.get("error"), "code", status.value(), "data", List.of());
return ResponseEntity.ok(finalBody);
}
};
}
}
然后响应内容就变了，具体响应格式参照各自项目规范修改即可：
{
"status": "Not Found",
"code": 404,
"data": []
}
3 总结
聊到统一异常治理，自然要对治理对象的分类有一个清晰的认知。异常也就两类：未知异常和已知异常。未知异常多半是由隐藏的BUG造成的，笔者认为统一异常处理层一定要有针对未知异常的处理逻辑，直白点说就是在由@RestControllerAdvice标注的统一异常处理类中要有一个由@ExceptionHandler(value = Exception.class)标注的方法，但千万不要通过getMessage()将异常信息反馈给调用方，因为异常是未知的，可能会将很长串的异常堆栈信息暴漏出来，这样既不友好也不安全，建议反馈简短的信息即可，比如：Internal Server Error，但要在日志中完整地记录异常堆栈信息，方便后期排查。已知异常的范围比较宽泛，针对已知异常，向调用方暴漏的错误信息一定要简洁清晰，这也是完全可以做到的，尤其是开发人员主动抛出的自定义异常，这类异常在统一异常处理层中可以放心大胆地通过getMessage()方式将异常信息反馈给调用方或前台用户，因为开发人员在抛出异常的时候会填充简短精炼的提示信息。
关于最佳实践思路，建议大家自定义的统一异常处理器能够继承ResponseEntityExceptionHandler，大家可以去看看它的源码就知道为什么这么建议了！
4 参考文章
- https://docs.spring.io/spring-framework/docs/5.3.9/reference/html/web.html#mvc-exceptionhandlers
- https://docs.spring.io/spring-boot/docs/2.5.4/reference/html/features.html#features.developing-web-applications.spring-mvc.error-handling