目錄

準備工作

我們知道SpringBoot的自動裝配的秘密在 org.springframework.boot.autoconfigure 包下的 spring.factories 文件中，而嵌入Tomcat的原理就在這個文件中加載的一個配置類： org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration

@Configuration@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)@ConditionalOnClass(ServletRequest.class)@ConditionalOnWebApplication(type = Type.SERVLET)@EnableConfigurationProperties(ServerProperties.class)@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,ServletWebServerFactoryConfiguration.EmbeddedJetty.class,ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })public class ServletWebServerFactoryAutoConfiguration {@Beanpublic ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties) {return new ServletWebServerFactoryCustomizer(serverProperties);}@Bean@ConditionalOnClass(name = 'org.apache.catalina.startup.Tomcat')public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(ServerProperties serverProperties) {return new TomcatServletWebServerFactoryCustomizer(serverProperties);}/** * Registers a {@link WebServerFactoryCustomizerBeanPostProcessor}. Registered via * {@link ImportBeanDefinitionRegistrar} for early registration. */public static class BeanPostProcessorsRegistrarimplements ImportBeanDefinitionRegistrar, BeanFactoryAware {private ConfigurableListableBeanFactory beanFactory;@Overridepublic void setBeanFactory(BeanFactory beanFactory) throws BeansException {if (beanFactory instanceof ConfigurableListableBeanFactory) {this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;}}@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,BeanDefinitionRegistry registry) {if (this.beanFactory == null) {return;}registerSyntheticBeanIfMissing(registry,'webServerFactoryCustomizerBeanPostProcessor',WebServerFactoryCustomizerBeanPostProcessor.class);registerSyntheticBeanIfMissing(registry,'errorPageRegistrarBeanPostProcessor',ErrorPageRegistrarBeanPostProcessor.class);}private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,String name, Class<?> beanClass) {if (ObjectUtils.isEmpty(this.beanFactory.getBeanNamesForType(beanClass, true, false))) {RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);beanDefinition.setSynthetic(true);registry.registerBeanDefinition(name, beanDefinition);}}}}

首先看一下上方的幾個注解

@AutoConfigureOrder 這個注解是決定配置類的加載順序的，當注解里的值越小越先加載，而 Ordered.HIGHEST_PRECEDENCE 的值是 Integer.MIN_VALUE 也就是說這個類肯定是最先加載的那一批 @ConditionalOnXXX 在之前的文章中已經無數次提到了，就不再闡述了 @EnableConfigurationProperties 開啟 ServerProperties 類的屬性值配置。而這個類里面包含的就是Web服務的配置

@ConfigurationProperties(prefix = 'server', ignoreUnknownFields = true)public class ServerProperties {private Integer port;private InetAddress address;@NestedConfigurationPropertyprivate final ErrorProperties error = new ErrorProperties();private Boolean useForwardHeaders;private String serverHeader;private int maxHttpHeaderSize = 0; // bytesprivate Duration connectionTimeout;@NestedConfigurationPropertyprivate Ssl ssl;@NestedConfigurationPropertyprivate final Compression compression = new Compression();@NestedConfigurationPropertyprivate final Http2 http2 = new Http2();private final Servlet servlet = new Servlet();private final Tomcat tomcat = new Tomcat();private final Jetty jetty = new Jetty();private final Undertow undertow = new Undertow();}

這個類的代碼太多了，這里就不一一貼出來了，我們平常在 application.properties 中配置的server.xxx就是這個類中屬性

@ImportBeanPostProcessorsRegistrarpublic static class BeanPostProcessorsRegistrarimplements ImportBeanDefinitionRegistrar, BeanFactoryAware {@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,BeanDefinitionRegistry registry) {if (this.beanFactory == null) {return;}registerSyntheticBeanIfMissing(registry,'webServerFactoryCustomizerBeanPostProcessor',WebServerFactoryCustomizerBeanPostProcessor.class);registerSyntheticBeanIfMissing(registry,'errorPageRegistrarBeanPostProcessor',ErrorPageRegistrarBeanPostProcessor.class);}private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,String name, Class<?> beanClass) {if (ObjectUtils.isEmpty(this.beanFactory.getBeanNamesForType(beanClass, true, false))) {RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);beanDefinition.setSynthetic(true);registry.registerBeanDefinition(name, beanDefinition);}}}

這個類注冊了兩個bean： WebServerFactoryCustomizerBeanPostProcessor 和 ErrorPageRegistrarBeanPostProcessor 關于這兩個bean的作用稍后再詳細介紹

EmbeddedTomcat

@Configuration@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)public static class EmbeddedTomcat { @Bean public TomcatServletWebServerFactory tomcatServletWebServerFactory() { return new TomcatServletWebServerFactory(); }}

這個類會在存在Tomcat相關jar包時添加一個 TomcatServletWebServerFactory bean

其他兩個相信大家都知道怎么回事了

除了這些這個類還注入了兩個類 ServletWebServerFactoryCustomizer 和 TomcatServletWebServerFactoryCustomizer現在前期準備工作已經做好了，看一下這個Tomcat是如何啟動的吧

啟動

啟動入口在 ServletWebServerApplicationContext 中的 onRefresh 方法

protected void onRefresh() {super.onRefresh(); try { createWebServer(); }catch (Throwable ex) {throw new ApplicationContextException('Unable to start web server', ex);}}

Tomcat的啟動就在 createWebServer 方法里面了

private void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = getServletContext(); //第一次訪問的時候兩個對象都為空 if (webServer == null && servletContext == null) { ServletWebServerFactory factory = getWebServerFactory(); this.webServer = factory.getWebServer(getSelfInitializer()); } else if (servletContext != null) { try { getSelfInitializer().onStartup(servletContext); } catch (ServletException ex) { throw new ApplicationContextException('Cannot initialize servlet context', ex); } } initPropertySources();}

首先看一下 getWebServerFactory

protected ServletWebServerFactory getWebServerFactory() { // 這里獲取的beanname就是上方注冊的tomcatServletWebServerFactory了 String[] beanNames = getBeanFactory() .getBeanNamesForType(ServletWebServerFactory.class); if (beanNames.length == 0) { throw new ApplicationContextException( 'Unable to start ServletWebServerApplicationContext due to missing ' + 'ServletWebServerFactory bean.'); } if (beanNames.length > 1) { throw new ApplicationContextException( 'Unable to start ServletWebServerApplicationContext due to multiple ' + 'ServletWebServerFactory beans : ' + StringUtils.arrayToCommaDelimitedString(beanNames)); } return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);}

準備環境里注冊的bean現在出來一個了。注意，上方還注冊了一個后置處理器 EmbeddedServletContainerCustomizerBeanPostProcessor ，獲取bean tomcatServletWebServerFactory 的時候就會執行后置處理器的 postProcessBeforeInitialization 方法

public Object postProcessBeforeInitialization(Object bean, String beanName)throws BeansException {if (bean instanceof WebServerFactory) {postProcessBeforeInitialization((WebServerFactory) bean);}return bean;}private void postProcessBeforeInitialization(WebServerFactory webServerFactory) { LambdaSafe .callbacks(WebServerFactoryCustomizer.class, getCustomizers(), webServerFactory) .withLogger(WebServerFactoryCustomizerBeanPostProcessor.class) .invoke((customizer) -> customizer.customize(webServerFactory));}private Collection<WebServerFactoryCustomizer<?>> getCustomizers() { if (this.customizers == null) { // Look up does not include the parent context this.customizers = new ArrayList<>(getWebServerFactoryCustomizerBeans()); this.customizers.sort(AnnotationAwareOrderComparator.INSTANCE); this.customizers = Collections.unmodifiableList(this.customizers); } return this.customizers;}@SuppressWarnings({ 'unchecked', 'rawtypes' })private Collection<WebServerFactoryCustomizer<?>> getWebServerFactoryCustomizerBeans() { return (Collection) this.beanFactory .getBeansOfType(WebServerFactoryCustomizer.class, false, false).values();}

這個處理器的作用是獲得所有定制器，然后執行定制器的方法

接著往下看

這個時候就可以啟動Tomcat了

public WebServer getWebServer(ServletContextInitializer... initializers) { Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null ? this.baseDirectory : createTempDir('tomcat')); tomcat.setBaseDir(baseDir.getAbsolutePath()); Connector connector = new Connector(this.protocol); tomcat.getService().addConnector(connector); customizeConnector(connector); tomcat.setConnector(connector); tomcat.getHost().setAutoDeploy(false); configureEngine(tomcat.getEngine()); for (Connector additionalConnector : this.additionalTomcatConnectors) { tomcat.getService().addConnector(additionalConnector); } prepareContext(tomcat.getHost(), initializers); return getTomcatWebServer(tomcat);}protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) { return new TomcatWebServer(tomcat, getPort() >= 0);}public TomcatWebServer(Tomcat tomcat, boolean autoStart) { Assert.notNull(tomcat, 'Tomcat Server must not be null'); this.tomcat = tomcat; this.autoStart = autoStart; initialize();}private void initialize() throws WebServerException { TomcatWebServer.logger.info('Tomcat initialized with port(s): ' + getPortsDescription(false)); synchronized (this.monitor) { try { addInstanceIdToEngineName(); Context context = findContext(); context.addLifecycleListener((event) -> { if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) { // Remove service connectors so that protocol binding doesn’t // happen when the service is started. removeServiceConnectors(); } }); // Start the server to trigger initialization listeners this.tomcat.start(); // We can re-throw failure exception directly in the main thread rethrowDeferredStartupExceptions(); try { ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader()); } catch (NamingException ex) { // Naming is not enabled. Continue } // Unlike Jetty, all Tomcat threads are daemon threads. We create a // blocking non-daemon to stop immediate shutdown startDaemonAwaitThread(); } catch (Exception ex) { throw new WebServerException('Unable to start embedded Tomcat', ex); } }}

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