准备工作

我们知道SpringBoot的自动装配的秘密在
org.springframework.boot.autoconfigure包下的spring.factories文件中，而嵌入Tomcat的原理就在这个文件中加载的一个配置类：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 { @Bean public 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); } public static class BeanPostProcessorsRegistrar implements importBeanDefinitionRegistrar, BeanFactoryAware { private ConfigurableListableBeanFactory beanFactory; @Override public void setBeanFactory(BeanFactory beanFactory) throws BeansException { if (beanFactory instanceof ConfigurableListableBeanFactory) { this.beanFactory = (ConfigurableListableBeanFactory) beanFactory; } } @Override public 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); } } }}

首先看一下上方的几个注解

@AutoConfigureOrder这个注解是决定配置类的加载顺序的，当注解里的值越小越先加载，而Ordered.HIGHEST_PRECEDENCE的值是Integer.MIN_VALUE也就是说这个类肯定是最先加载的那一批 @ConditionalOnXXX在之前的文章中已经无数次提到了，就不再阐述了 @EnableConfigurationProperties开启ServerProperties类的属性值配置。而这个类里面包含的就是Web服务的配置

@ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)public class ServerProperties { private Integer port; private InetAddress address; @NestedConfigurationProperty private final ErrorProperties error = new ErrorProperties(); private Boolean useForwardHeaders; private String serverHeader; private int maxHttpHeaderSize = 0; // bytes private Duration connectionTimeout; @NestedConfigurationProperty private Ssl ssl; @NestedConfigurationProperty private final Compression compression = new Compression(); @NestedConfigurationProperty private 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();}

这个类的代码太多了，这里就不一一贴出来了，我们平常在application.properties中配置的server.xxx就是这个类中属性

@import引入了4个类，看都是什么吧 BeanPostProcessorsRegistrar

public static class BeanPostProcessorsRegistrar implements importBeanDefinitionRegistrar, BeanFactoryAware { @Override public 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); } } }

这个类注册了两个bean：
WebServerFactoryCustomizerBeanPostProcessor和ErrorPageRegistrarBeanPostProcessor关于这两个bean的作用稍后再详细介绍

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(); } }

这个类会在存在Tomcat相关jar包时添加一个
TomcatServletWebServerFactorybean

其他两个相信大家都知道怎么回事了

除了这些这个类还注入了两个类ServletWebServerFactoryCustomizer和TomcatServletWebServerFactoryCustomizer

现在前期准备工作已经做好了，看一下这个Tomcat是如何启动的吧

启动

启动入口在
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的启动就在createWebServer方法里面了

private void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = getServletContext(); //第一次访问的时候两个对象都为空 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() { // 这里获取的beanname就是上方注册的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); }

准备环境里注册的bean现在出来一个了。注意，上方还注册了一个后置处理器
EmbeddedServletContainerCustomizerBeanPostProcessor，获取beantomcatServletWebServerFactory的时候就会执行后置处理器的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(); }

这个处理器的作用是获得所有定制器，然后执行定制器的方法

接着往下看

这个时候就可以启动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); } } }