Spring Boot优雅地处理404异常

背景

在使用SpringBoot的过程中，你肯定遇到过404错误。比如下面的代码：

@RestController@RequestMapping(value = "/hello")public class HelloWorldController {@RequestMapping("/test")public Object getObject1(HttpServletRequest request){Response response = new Response();response.success("请求成功...");response.setResponseTime();retu response;}}

当我们使用错误的请求地址（POST http://127.0.0.1:8888/hello/test1?id=98）进行请求时，会报下面的错误：

{  "timestamp": "2020-11-19T08:30:48.844+0000",  "status": 404,  "error": "Not Found",  "message": "No message available",  "path": "/hello/test1"}

虽然上面的返回很清楚，但是我们的接口需要返回统一的格式，比如：

{"rtnCode":"9999","rtnMsg":"404 /hello/test1 Not Found"}

这时候你可能会想有Spring的统一异常处理，在Controller类上加@RestControllerAdvice注解。但是这种做法并不能统一处理404错误。

404错误产生的原因

产生404的原因是我们调了一个不存在的接口，但是为什么会返回下面的json报错呢？我们先从Spring的源代码分析下。

{  "timestamp": "2020-11-19T08:30:48.844+0000",  "status": 404,  "error": "Not Found",  "message": "No message available",  "path": "/hello/test1"}

为了代码简单起见，这边直接从DispatcherServlet的doDispatch方法开始分析。（如果不知道为什么要从这边开始，你还要熟悉下SpringMVC的源代码）。

... 省略部分代码....// Actually invoke the handler.mv = ha.handle(processedRequest, response, mappedHandler.getHandler());... 省略部分代码

Spring MVC会根据请求URL的不同，配置的RequestMapping的不同，为请求匹配不同的HandlerAdapter。

对于上面的请求地址：http://127.0.0.1:8888/hello/test1?id=98匹配到的HandlerAdapter是HttpRequestHandlerAdapter。

我们直接进入到HttpRequestHandlerAdapter中看下这个类的handle方法。

@Override@Nullablepublic ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)throws Exception {((HttpRequestHandler) handler).handleRequest(request, response);retu null;}

这个方法没什么内容，直接是调用了HttpRequestHandler类的handleRequest(request, response)方法。所以直接进入这个方法看下吧。

@Overridepublic void handleRequest(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {// For very general mappings (e.g. "/") we need to check 404 firstResource resource = getResource(request);if (resource == null) {logger.trace("No matching resource found - retuing 404");// 这个方法很简单，就是设置404响应码，然后将Response的errorState状态从0设置成1response.sendError(HttpServletResponse.SC_NOT_FOUND);// 直接返回retu;}... 省略部分方法}

这个方法很简单，就是设置404响应码，将Response的errorState状态从0设置成1，然后就返回响应了。整个过程并没有发生任何异常，所以不能触发Spring的全局异常处理机制。

到这边还有一个问题没有解决：就是下面的404提示信息是怎么返回的。

{  "timestamp": "2020-11-19T08:30:48.844+0000",  "status": 404,  "error": "Not Found",  "message": "No message available",  "path": "/hello/test1"}

我们继续往下看。Response响应被返回，进入org.apache.catalina.core.StandardHostValve类的invoke方法进行处理。（不要问我为什么知道是在这里？Debug的能力是需要自己摸索出来的，自己调试多了，你也就会了）

@Overridepublic final void invoke(Request request, Response response)throws IOException, ServletException {Context context = request.getContext();if (context == null) {response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR,sm.getString("standardHost.noContext"));retu;}if (request.isAsyncSupported()) {request.setAsyncSupported(context.getPipeline().isAsyncSupported());}boolean asyncAtStart = request.isAsync();boolean asyncDispatching = request.isAsyncDispatching();try {context.bind(Globals.IS_SECURITY_ENABLED, MY_CLASSLOADER);if (!asyncAtStart && !context.fireRequestInitEvent(request.getRequest())) {retu;}try {if (!asyncAtStart || asyncDispatching) {context.getPipeline().getFirst().invoke(request, response);} else {if (!response.isErrorReportRequired()) {throw new IllegalStateException(sm.getString("standardHost.asyncStateError"));}}} catch (Throwable t) {ExceptionUtils.handleThrowable(t);container.getLogger().error("Exception Processing " + request.getRequestURI(), t);if (!response.isErrorReportRequired()) {request.setAttribute(RequestDispatcher.ERROR_EXCEPTION, t);throwable(request, response, t);}}response.setSuspended(false);Throwable t = (Throwable) request.getAttribute(RequestDispatcher.ERROR_EXCEPTION);if (!context.getState().isAvailable()) {retu;}// 在这里判断请求是不是发生了错误，错误的话就进入StandardHostValve的status(Request request, Response response)方法。// Look for (and render if found) an application level error pageif (response.isErrorReportRequired()) {if (t != null) {throwable(request, response, t);} else {status(request, response);}}if (!request.isAsync() && !asyncAtStart) {context.fireRequestDestroyEvent(request.getRequest());}} finally {// Access a session (if present) to update last accessed time, based// on a strict interpretation of the specificationif (ACCESS_SESSION) {request.getSession(false);}context.unbind(Globals.IS_SECURITY_ENABLED, MY_CLASSLOADER);}  }

这个方法会根据返回的响应判断是不是发生了错了，如果发生了error，则进入StandardHostValve的status(Request request, Response response)方法。这个方法“兜兜转转”又进入了StandardHostValve的custom(Request request, Response response,ErrorPage errorPage)方法。这个方法中将请求重新forward到了"/error"接口。

 private boolean custom(Request request, Response response, ErrorPage errorPage) {if (container.getLogger().isDebugEnabled()) {container.getLogger().debug("Processing " + errorPage);}try {// Forward control to the specified locationServletContext servletContext =request.getContext().getServletContext();RequestDispatcher rd =servletContext.getRequestDispatcher(errorPage.getLocation());if (rd == null) {container.getLogger().error(sm.getString("standardHostValue.customStatusFailed", errorPage.getLocation()));retu false;}if (response.isCommitted()) {rd.include(request.getRequest(), response.getResponse());} else {// Reset the response (keeping the real error code and message)response.resetBuffer(true);response.setContentLength(-1);// 1: 重新forward请求到/error接口rd.forward(request.getRequest(), response.getResponse());response.setSuspended(false);}retu true;} catch (Throwable t) {ExceptionUtils.handleThrowable(t);container.getLogger().error("Exception Processing " + errorPage, t);retu false;}}

上面标号1处的代码重新将请求forward到了/error接口。所以如果我们开着Debug日志的话，你会在后台看到下面的日志。

[http-nio-8888-exec-7] DEBUG org.springframework.web.servlet.DispatcherServlet:891 - DispatcherServlet with name 'dispatcherServlet' processing POST request for [/error]2020-11-19 19:04:04.280 [http-nio-8888-exec-7] DEBUG org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping:313 - Looking up handler method for path /error2020-11-19 19:04:04.281 [http-nio-8888-exec-7] DEBUG org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping:320 - Retuing handler method [public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)]2020-11-19 19:04:04.281 [http-nio-8888-exec-7] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory:255 - Retuing cached instance of singleton bean 'basicErrorController'

上面是/error的请求日志。到这边还是没说明为什么能返回json格式的404返回格式。我们继续往下看。

到这边为止，我们好像没有任何线索了。但是如果仔细看上面日志的话，你会发现这个接口的处理方法是：

org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)]

我们打开BasicErrorController这个类的源代码，一切豁然开朗。

@Controller@RequestMapping("${server.error.path:${error.path:/error}}")public class BasicErrorController extends AbstractErrorController {@RequestMapping(produces = "text/html")public ModelAndView errorHtml(HttpServletRequest request,HttpServletResponse response) {HttpStatus status = getStatus(request);Map<String, Object> model = Collections.unmodifiableMap(getErrorAttributes(request, isIncludeStackTrace(request, MediaType.TEXT_HTML)));response.setStatus(status.value());ModelAndView modelAndView = resolveErrorView(request, response, status, model);retu (modelAndView == null ? new ModelAndView("error", model) : modelAndView);}@RequestMapping@ResponseBodypublic ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {Map<String, Object> body = getErrorAttributes(request,isIncludeStackTrace(request, MediaType.ALL));HttpStatus status = getStatus(request);retu new ResponseEntity<Map<String, Object>>(body, status);}... 省略部分方法}

BasicErrorController是Spring默认配置的一个Controller，默认处理/error请求。BasicErrorController提供两种返回错误一种是页面返回、当你是页面请求的时候就会返回页面，另外一种是json请求的时候就会返回json错误。

自定义404错误处理类

我们先看下BasicErrorController是在哪里进行配置的。

在IDEA中，查看BasicErrorController的usage，我们发现这个类是在ErrorMvcAutoConfiguration中自动配置的。

@Configuration@ConditionalOnWebApplication(type = Type.SERVLET)@ConditionalOnClass({ Servlet.class, DispatcherServlet.class })// Load before the main WebMvcAutoConfiguration so that the error View is available@AutoConfigureBefore(WebMvcAutoConfiguration.class)@EnableConfigurationProperties({ ServerProperties.class, ResourceProperties.class })public class ErrorMvcAutoConfiguration {@Bean	@ConditionalOnMissingBean(value = ErrorController.class, search = SearchStrategy.CURRENT)	public BasicErrorController basicErrorController(ErrorAttributes errorAttributes) {		retu new BasicErrorController(errorAttributes, this.serverProperties.getError(),				this.errorViewResolvers);	}	... 省略部分代码}

从上面的配置中可以看出来，只要我们自己配置一个ErrorController，就可以覆盖掉BasicErrorController的行为。

@Controller@RequestMapping("${server.error.path:${error.path:/error}}")public class CustomErrorController extends BasicErrorController {@Value("${server.error.path:${error.path:/error}}")private String path;public CustomErrorController(ServerProperties serverProperties) {super(new DefaultErrorAttributes(), serverProperties.getError());}/** * 覆盖默认的JSON响应 */@Overridepublic ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {HttpStatus status = getStatus(request);Map<String, Object> map = new HashMap<String, Object>(16);Map<String, Object> originalMsgMap = getErrorAttributes(request, isIncludeStackTrace(request, MediaType.ALL));String path = (String)originalMsgMap.get("path");String error = (String)originalMsgMap.get("error");String message = (String)originalMsgMap.get("message");StringJoiner joiner = new StringJoiner(",","[","]");joiner.add(path).add(error).add(message);map.put("rtnCode", "9999");map.put("rtnMsg", joiner.toString());retu new ResponseEntity<Map<String, Object>>(map, status);}/** * 覆盖默认的HTML响应 */@Overridepublic ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {//请求的状态HttpStatus status = getStatus(request);response.setStatus(getStatus(request).value());Map<String, Object> model = getErrorAttributes(request,isIncludeStackTrace(request, MediaType.TEXT_HTML));ModelAndView modelAndView = resolveErrorView(request, response, status, model);//指定自定义的视图retu (modelAndView == null ? new ModelAndView("error", model) : modelAndView);}}

默认的错误路径是/error，我们可以通过以下配置进行覆盖：

server:  error:path: /xxx

更详细的内容请参考Spring Boot的 章节。

简单总结

• 如果在过滤器（Filter）中发生异常，或者调用的接口不存在，Spring会直接将Response的errorStatus状态设置成1，将http响应码设置为500或者404，Tomcat检测到errorStatus为1时，会将请求重现forward到/error接口；
• 如果请求已经进入了Controller的处理方法，这时发生了异常，如果没有配置Spring的全局异常机制，那么请求还是会被forward到/error接口，如果配置了全局异常处理，Controller中的异常会被捕获；
• 继承BasicErrorController就可以覆盖原有的错误处理方式。