在读本篇文章之前如果你读过这篇文章SpringBoot自动装配原理解析应该会更加轻松

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);

	}

	/**

	 * Registers a {@link WebServerFactoryCustomizerBeanPostProcessor}. Registered via

	 * {@link ImportBeanDefinitionRegistrar} for early registration.

	 */

	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

4. @Import引入了4个类，看都是什么吧

  1. 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

2. 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);

			}

		}

	}

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