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

1. @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
2. @ConditionalOnXXX在之前的文章中已经无数次提到了，就不再阐述了
3. @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

1. @Import引入了4个类，看都是什么吧
2. 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

1. 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

1. 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);
 }
 }
 }