SpringBoot自定义错误页面与原理讲解

迷南。 2022-12-17 01:24 152阅读 0赞

SpringBoot请求错误如404可能看到如下页面：

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center]  
有时可能需要自定义错误页面针对不同的http.status,如404/400。

## **【1】解决方法** ##

### ① 注册错误页面 ###

如下所示：

@Component
    public class ErrorPageConfig implements ErrorPageRegistrar {
        @Override
        public void registerErrorPages(ErrorPageRegistry registry) {
            ErrorPage error400Page = new ErrorPage(HttpStatus.BAD_REQUEST, "/error/404");
            ErrorPage error404Page = new ErrorPage(HttpStatus.NOT_FOUND, "/error/404");
            ErrorPage error500Page = new ErrorPage(HttpStatus.INTERNAL_SERVER_ERROR, "/error/500");
            registry.addErrorPages(error400Page,error404Page,error500Page);
        }
    }

### ② controller进行拦截 ###

然后你只需要写个controller拦截不同请求然后跳到不同的自定义错误页面即可，如下所示：

@RequestMapping("/error/{status}")
    public String errorPage(@PathVariable Integer status){
        switch (status){
            case 401:
            case 400:return "/error/404";
            case 500:return "/error/500";
            default:return "/error/default";
        }
    }

那么原理呢？

## 【2】原理讲解 ##

### ① 启动SpringBoot,注册错误页面 ###

如下图所示，启动项目时候再onRefresh方法中会创建一个WebServer，继而获取ServletWebServerFactory。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 1]

#### 1.1AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization ####

在创建bean-tomcatServletWebServerFactory时会调用AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization,如下所示：

@Override
    public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
    	throws BeansException {
    
    Object result = existingBean;
    for (BeanPostProcessor processor : getBeanPostProcessors()) {
    	Object current = processor.postProcessBeforeInitialization(result, beanName);
    	if (current == null) {
    		return result;
    	}
    	result = current;
    }
    return result;
    }

该方法会获取bean后置处理器，然后循环遍历调用每个bean后置处理器的postProcessBeforeInitialization方法。

#### 1.2 ErrorPageRegistrarBeanPostProcessor.postProcessBeforeInitialization ####

当遍历到ErrorPageRegistrarBeanPostProcessor时会调用其postProcessBeforeInitialization方法，方法源码如下所示：

@Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    	if (bean instanceof ErrorPageRegistry) {
    		postProcessBeforeInitialization((ErrorPageRegistry) bean);
    	}
    	return bean;
    }

方法会判断当前bean是否ErrorPageRegistry类型，如果是，则调用postProcessBeforeInitialization方法，源码如下所示：

private void postProcessBeforeInitialization(ErrorPageRegistry registry) {
    	for (ErrorPageRegistrar registrar : getRegistrars()) {
    		registrar.registerErrorPages(registry);
    	}
    }

该方法会获取Registrars，然后循环遍历调用每一个注册器的registerErrorPages方法。获取注册其源码如下所示：

private Collection<ErrorPageRegistrar> getRegistrars() {
    	if (this.registrars == null) {
    		// Look up does not include the parent context
    		this.registrars = new ArrayList<>(
    				this.beanFactory.getBeansOfType(ErrorPageRegistrar.class, false, false).values());
    		this.registrars.sort(AnnotationAwareOrderComparator.INSTANCE);
    		this.registrars = Collections.unmodifiableList(this.registrars);
    	}
    	return this.registrars;
    }

故而，当我们的ErrorPageConfig 实现了ErrorPageRegistrar时，会被检测到并执行registerErrorPages方法。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 2]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 3]

### ② 把错误页面放到StandardContext.errorPageSupport中 ###

StandardContext是什么？我们可以看下如下类继承示意图。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 4]

在①中我们提到会注册错误页面`registrar.registerErrorPages(registry);`，如下图所示此时的registry为TomcatServletWebServerFactory：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 5]

我们再来看下TomcatServletWebServerFactory继承示意图(可以看到其父类AbstractConfigurableWebServerFactory实现了ErrorPageRegistry接口)：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 6]

#### 2.1 AbstractConfigurableWebServerFactory.addErrorPages ####

方法源码如下：

@Override
    public void addErrorPages(ErrorPage... errorPages) {
    	Assert.notNull(errorPages, "ErrorPages must not be null");
    	this.errorPages.addAll(Arrays.asList(errorPages));
    }

也就说错误页面现在被放到了属性`private Set<ErrorPage> errorPages = new LinkedHashSet<>();`中。

#### 2.2 TomcatServletWebServerFactory.configureContext ####

创建完TomcatServletWebServerFactory后会调用configureContext方法，如下图所示：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 7]  
在configureContext方法中会获取错误页面然后逐个调用StandardContext.addErrorPage方法添加到其`ErrorPageSupport errorPageSupport`中。

**configureContext方法中遍历错误页面如下所示：**

for (ErrorPage errorPage : getErrorPages()) {
    	org.apache.tomcat.util.descriptor.web.ErrorPage tomcatErrorPage = new org.apache.tomcat.util.descriptor.web.ErrorPage();
    	tomcatErrorPage.setLocation(errorPage.getPath());
    	tomcatErrorPage.setErrorCode(errorPage.getStatusCode());
    	tomcatErrorPage.setExceptionType(errorPage.getExceptionName());
    	context.addErrorPage(tomcatErrorPage);
    }

**StandardContext.addErrorPage方法源码如下所示：**

@Override
     public void addErrorPage(ErrorPage errorPage) {
         // Validate the input parameters
         if (errorPage == null)
             throw new IllegalArgumentException
                 (sm.getString("standardContext.errorPage.required"));
         String location = errorPage.getLocation();
         if ((location != null) && !location.startsWith("/")) {
             if (isServlet22()) {
                 if(log.isDebugEnabled())
                     log.debug(sm.getString("standardContext.errorPage.warning",
                                  location));
                 errorPage.setLocation("/" + location);
             } else {
                 throw new IllegalArgumentException
                     (sm.getString("standardContext.errorPage.error",
                                   location));
             }
         }
    //调用errorPageSupport.add
         errorPageSupport.add(errorPage);
         fireContainerEvent("addErrorPage", errorPage);
     }

**ErrorPageSupport.add方法如下所示：**

public void add(ErrorPage errorPage) {
         String exceptionType = errorPage.getExceptionType();
         if (exceptionType == null) {
             statusPages.put(Integer.valueOf(errorPage.getErrorCode()), errorPage);
         } else {
             exceptionPages.put(exceptionType, errorPage);
         }
     }

通过该方法可以看到，不止可以通过HTTP状态码定义错误页面，还可以通过异常类型进行定义。

那么ErrorPageSupport、statusPages、exceptionPages分别是什么呢？我们看下图示意：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 8]

### ③ 错误页面如何被用到 ###

在ResourceHttpRequestHandler.handleRequest方法处理请求时，找不到资源会调用`response.sendError`方法：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 9]  
这里只需要关注这一点，无需关注细节，我们继续往下走。。。。一直走到StandardHostValve.status方法。

StandardHostValve.status中会对响应状态码进行处理。

private void status(Request request, Response response) {
    
         int statusCode = response.getStatus();
    
         // Handle a custom error page for this status code
         Context context = request.getContext();
         if (context == null) {
             return;
         }
    
         /* Only look for error pages when isError() is set.
          * isError() is set when response.sendError() is invoked. This
          * allows custom error pages without relying on default from
          * web.xml.
          */
         if (!response.isError()) {
             return;
         }
    //这里会从errorPageSupport.find(errorCode)获取到错误页
    //根据错误码，比如404从statusPages获取对应的ErrorPage对象
         ErrorPage errorPage = context.findErrorPage(statusCode);
         if (errorPage == null) {
             // Look for a default error page
             errorPage = context.findErrorPage(0);
         }
         if (errorPage != null && response.isErrorReportRequired()) {
             response.setAppCommitted(false);
             request.setAttribute(RequestDispatcher.ERROR_STATUS_CODE,
                               Integer.valueOf(statusCode));
    
             String message = response.getMessage();
             if (message == null) {
                 message = "";
             }
             request.setAttribute(RequestDispatcher.ERROR_MESSAGE, message);
             request.setAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR,
                     errorPage.getLocation());
             request.setAttribute(Globals.DISPATCHER_TYPE_ATTR,
                     DispatcherType.ERROR);
    
    
             Wrapper wrapper = request.getWrapper();
             if (wrapper != null) {
                 request.setAttribute(RequestDispatcher.ERROR_SERVLET_NAME,
                                   wrapper.getName());
             }
             request.setAttribute(RequestDispatcher.ERROR_REQUEST_URI,
                                  request.getRequestURI());
                                  
    //这里很重要，将会尝试跳转到我们自定义错误请求页面
             if (custom(request, response, errorPage)) {
                 response.setErrorReported();
                 try {
                     response.finishResponse();
                 } catch (ClientAbortException e) {
                     // Ignore
                 } catch (IOException e) {
                     container.getLogger().warn("Exception Processing " + errorPage, e);
                 }
             }
         }
     }

如下图所示，在StandardHostValve.custom方法中将会调用ApplicationDispatcher.forwar进行请求转发。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 10]

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