环境：Springboot2.4.12
环境配置
接下来的演示都是基于如下接口进行。
@RestController
@RequestMapping("/exceptions")
public class ExceptionsController {
@GetMapping("/index")
public Object index(int a) {
if (a == 0) {
throw new BusinessException() ;
}
return "exception" ;
}
}默认错误输出
默认情况下，当请求一个接口发生异常时会有如下两种情况的错误信息提示
- 基于HTML
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- 基于JSON
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上面两个示例通过请求的Accept请求头设置希望接受的数据类型，得到不同的响应数据类型。
标准web错误页配置
在标准的java web项目中我们一般是在web.xml文件中进行错误页的配置，如下：
<error-page>
<location>/error</location>
</error-page>如上配置后，如发生了异常以后容器会自动地跳转到错误页面。
Spring实现原理
在Springboot中没有web.xml，并且Servlet API也没有提供相应的API进行错误页的配置。那么在Springboot中又是如何实现错误页的配置呢？
Springboot内置了应用服务，如Tomcat，Undertow，Jetty，默认是Tomcat。那接下来看下基于默认的Tomcat容器错误页是如何进行配置的。
- Servlet Web服务自动配置
@EnableConfigurationProperties(ServerProperties.class)
@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,...})
public class ServletWebServerFactoryAutoConfiguration {
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
static class EmbeddedTomcat {
// 这里主要就是配置Web 容器服务，如这里使用的Tomcat
// 注意该类实现了ErrorPageRegistry ，那么也就是说该类可以用来注册错误页的
@Bean
TomcatServletWebServerFactory tomcatServletWebServerFactory(
ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
ObjectProvider<TomcatContextCustomizer> contextCustomizers,
ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
factory.getTomcatConnectorCustomizers().addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatContextCustomizers().addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatProtocolHandlerCustomizers().addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
return factory;
}
}
}在@Import中只列出了两个比较重要的BeanPostProcessorsRegistrar与EmbeddedTomcat
BeanPostProcessorsRegistrar注册了两个BeanPostProcessor处理器
public static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
if (this.beanFactory == null) {
return;
}
registerSyntheticBeanIfMissing(registry, "webServerFactoryCustomizerBeanPostProcessor", WebServerFactoryCustomizerBeanPostProcessor.class, WebServerFactoryCustomizerBeanPostProcessor::new);
registerSyntheticBeanIfMissing(registry, "errorPageRegistrarBeanPostProcessor", ErrorPageRegistrarBeanPostProcessor.class, ErrorPageRegistrarBeanPostProcessor::new);
}
}通过名称也能知道WebServerFactoryCustomizerBeanPostProcessor用来处理Tomcat相关的自定义信息；ErrorPageRegistrarBeanPostProcessor 这个就是重点了，这个就是用来配置我们的自定义错误页面的。
public class ErrorPageRegistrarBeanPostProcessor implements BeanPostProcessor, BeanFactoryAware {
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
// 这里判断了当前的bean对象是否是ErrorPageRegistry的实例
// 当前类既然是BeanPostProcessor实例，同时上面注册了一个TomcatServletWebServerFactory Bean实例
// 那么在实例化TomcatServletWebServerFactory时一定是会调用该BeanPostProcessor处理器的
if (bean instanceof ErrorPageRegistry) {
postProcessBeforeInitialization((ErrorPageRegistry) bean);
}
return bean;
}
// 注册错误页面
private void postProcessBeforeInitialization(ErrorPageRegistry registry) {
for (ErrorPageRegistrar registrar : getRegistrars()) {
registrar.registerErrorPages(registry);
}
}
private Collection<ErrorPageRegistrar> getRegistrars() {
if (this.registrars == null) {
// Look up does not include the parent context
// 从当前上下文中（比包括父上下文）查找ErrorPageRegistrar Bean对象
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;
}
}注册错误页面
在上一步中知道了错误页的注册入口是在一个ErrorPageRegistrarBeanPostProcessor Bean后处理器中进行注册的，接下来继续深入查看这个错误页是如何被注册的。
接着上一步在ErrorPageRegistrarBeanPostProcessor中查找ErrorPageRegistrar类型的Bean对象。在另外一个自动配置中（ErrorMvcAutoConfiguration）有注册ErrorPageRegistrar Bean对象
@AutoConfigureBefore(WebMvcAutoConfiguration.class)
@EnableConfigurationProperties({ ServerProperties.class, WebMvcProperties.class })
public class ErrorMvcAutoConfiguration {
// 该类是ErrorPageRegistrar子类，那么在注册错误页的时候注册的就是该类中生成的错误页信息
static class ErrorPageCustomizer implements ErrorPageRegistrar, Ordered {
private final ServerProperties properties;
private final DispatcherServletPath dispatcherServletPath;
protected ErrorPageCustomizer(ServerProperties properties, DispatcherServletPath dispatcherServletPath) {
this.properties = properties;
this.dispatcherServletPath = dispatcherServletPath;
}
@Override
public void registerErrorPages(ErrorPageRegistry errorPageRegistry) {
// 错误页的地址可以在配置文件中自定义server.error.path进行配置，默认：/error
ErrorPage errorPage = new ErrorPage(this.dispatcherServletPath.getRelativePath(this.properties.getError().getPath()));
errorPageRegistry.addErrorPages(errorPage);
}
@Override
public int getOrder() {
return 0;
}
}
}关键代码
// errorPageRegistry对象的实例是TomcatServletWebServerFactory
errorPageRegistry.addErrorPages(errorPage);TomcatServletWebServerFactory中注册错误页信息，该类的父类（AbstractConfigurableWebServerFactory）方法中有添加错误也的方法
public abstract class AbstractConfigurableWebServerFactory {
private Set<ErrorPage> errorPages = new LinkedHashSet<>();
public void addErrorPages(ErrorPage... errorPages) {
this.errorPages.addAll(Arrays.asList(errorPages));
}
}这个错误页的注册到Tomcat容器中又是如何实现的呢？
Tomcat中注册错误页
接下来看看这个错误页是如何与Tomcat关联在一起的。
Spring容器最核心的方法是refresh方法
public abstract class AbstractApplicationContext {
public void refresh() {
// ...
// Initialize other special beans in specific context subclasses.
onRefresh();
// ...
}
}执行onRefresh方法
public class ServletWebServerApplicationContext extends GenericWebApplicationContext {
protected void onRefresh() {
super.onRefresh();
try {
// 创建Tomcat服务
createWebServer();
} catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
private void createWebServer() {
// ...
// 返回应用于创建嵌入的Web服务器的ServletWebServerFactory。默认情况下，此方法在上下文本身中搜索合适的bean。
// 在上面ServletWebServerFactoryAutoConfiguration自动配置中，已经自动的根据当前的环境创建了TomcatServletWebServerFactory对象
ServletWebServerFactory factory = getWebServerFactory();
// 获取WebServer实例， factory = TomcatServletWebServerFactory
this.webServer = factory.getWebServer(getSelfInitializer());
// ...
}
}调用TomcatServletWebServerFactory#getWebServer方法
public class TomcatServletWebServerFactory extends AbstractServletWebServerFactory {
public WebServer getWebServer(ServletContextInitializer... initializers) {
// ...
Tomcat tomcat = new Tomcat();
// ...
// 预处理上下文
prepareContext(tomcat.getHost(), initializers);
return getTomcatWebServer(tomcat);
}
protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
// ...
// 配置上下文
configureContext(context, initializersToUse);
}
protected void configureContext(Context context, ServletContextInitializer[] initializers) {
TomcatStarter starter = new TomcatStarter(initializers);
// ...
// 在这里就将错误的页面注册到了tomcat容器中
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);
}
// ...
}
}到此你就知道了一个错误的页是如何在Springboot中被注册的。到目前为止我们看到的注册到tomcat容器中的错误页都是个地址，比如：默认是/error。那这个默认的/error又是怎么提供的接口呢？
默认错误页
在Springboot中默认有个自动配置的错误页，在上面有一个代码片段你应该注意到了
@AutoConfigureBefore(WebMvcAutoConfiguration.class)
@EnableConfigurationProperties({ ServerProperties.class, WebMvcProperties.class })
public class ErrorMvcAutoConfiguration {
@Bean
@ConditionalOnMissingBean(value = ErrorAttributes.class, search = SearchStrategy.CURRENT)
public DefaultErrorAttributes errorAttributes() {
return new DefaultErrorAttributes();
}
@Bean
@ConditionalOnMissingBean(value = ErrorController.class, search = SearchStrategy.CURRENT)
public BasicErrorController basicErrorController(ErrorAttributes errorAttributes, ObjectProvider<ErrorViewResolver> errorViewResolvers) {
return new BasicErrorController(errorAttributes, this.serverProperties.getError(), errorViewResolvers.orderedStream().collect(Collectors.toList()));
}
}查看这个Controller
// 默认的错误页地址是/error
@Controller
@RequestMapping("${server.error.path:${error.path:/error}}")
public class BasicErrorController extends AbstractErrorController {
@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);
}
}这里有两个方法，分别处理了不同的Accept请求头。到此你是否真正地明白了Springboot中的错误处理的工作原理呢？
