SpringBoot启动Tomcat容器的源码主流程

SpringBoot中内置了Tomcat和Jetty容器，默认会启动Tomcat容器。接下来我们点进源码看一下SpringBoot启动Tomcat容器的主流程。

SpringBoot启动类：

@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

SpringApplication类：

点进SpringApplication的run()方法：

public class SpringApplication {

	public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
		return run(new Class<?>[] { primarySource }, args);
	}

	public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
		return new SpringApplication(primarySources).run(args);
	}
	
	// 点击refreshContext(context)
	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
			configureIgnoreBeanInfo(environment);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			prepareContext(context, environment, listeners, applicationArguments, printedBanner);
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
			}
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, listeners);
			throw new IllegalStateException(ex);
		}

		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

	// 点击refresh((ApplicationContext) context);
	private void refreshContext(ConfigurableApplicationContext context) {
		refresh((ApplicationContext) context);
		if (this.registerShutdownHook) {
			try {
				context.registerShutdownHook();
			}
			catch (AccessControlException ex) {
				// Not allowed in some environments.
			}
		}
	}

	// 点击refresh((ConfigurableApplicationContext) applicationContext);
	@Deprecated
	protected void refresh(ApplicationContext applicationContext) {
		Assert.isInstanceOf(ConfigurableApplicationContext.class, applicationContext);
		refresh((ConfigurableApplicationContext) applicationContext);
	}

	// 进入applicationContext.refresh();
	protected void refresh(ConfigurableApplicationContext applicationContext) {
		applicationContext.refresh();
	}
}

AbstractApplicationContext类：

选择AbstractApplicationContext的实现，方法里做完准备工作后，查看onRefresh()方法，开始具体的容器Bean初始化了。

	// 点击onRefresh();
	@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

ServletWebServerApplicationContext：

SpringBoot在启动时做了switch的判断，如果是web项目，则选择ServletWebServerApplicationContext的实现做初始化：

public class ServletWebServerApplicationContext extends GenericWebApplicationContext
		implements ConfigurableWebServerApplicationContext {

	// 点击createWebServer()查看创建web服务的过程
	@Override
	protected void onRefresh() {
		super.onRefresh();
		try {
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}

	// getWebServerFactory()选择容器创建的工厂类，web项目是TomcatServletWebServerFactory
	// 然后调用factory.getWebServer(getSelfInitializer())
	private void createWebServer() {
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		if (webServer == null && servletContext == null) {
			ServletWebServerFactory factory = getWebServerFactory();
			this.webServer = factory.getWebServer(getSelfInitializer());
			getBeanFactory().registerSingleton("webServerGracefulShutdown",
					new WebServerGracefulShutdownLifecycle(this.webServer));
			getBeanFactory().registerSingleton("webServerStartStop",
					new WebServerStartStopLifecycle(this, this.webServer));
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context", ex);
			}
		}
		initPropertySources();
	}
}

TomcatServletWebServerFactory类：

在这里new出了Tomcat的实例，做一些Tomcat的配置操作，然后把实例交给TomcatWebServer。

	@Override
	public WebServer getWebServer(ServletContextInitializer... initializers) {
		if (this.disableMBeanRegistry) {
			Registry.disableRegistry();
		}
		Tomcat tomcat = new Tomcat();
		File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
		tomcat.setBaseDir(baseDir.getAbsolutePath());
		Connector connector = new Connector(this.protocol);
		connector.setThrowOnFailure(true);
		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);
		// 点击查看怎么获取Tomcat容器的
		return getTomcatWebServer(tomcat);
	}

	// 在这里new出了TomcatWebServer，点击进入构造方法
	protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
		return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());
	}

TomcatWebServer类：

public class TomcatWebServer implements WebServer {

	// 在构造方法里，把前面new出来的Tomcat实例赋值给TomcatWebServer
	public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {
		Assert.notNull(tomcat, "Tomcat Server must not be null");
		this.tomcat = tomcat;
		this.autoStart = autoStart;
		this.gracefulShutdown = (shutdown == Shutdown.GRACEFUL) ? new GracefulShutdown(tomcat) : null;
		// 继续跟进初始化方法
		initialize();
	}

	// 在此方法调用this.tomcat.start()启动Tomcat容器
	// 同时开启一个守护线程startDaemonAwaitThread();
	private void initialize() throws WebServerException {
		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) {
				stopSilently();
				destroySilently();
				throw new WebServerException("Unable to start embedded Tomcat", ex);
			}
		}
	}

	// 开启一个守护线程
	// 并TomcatWebServer.this.tomcat.getServer().await();
	private void startDaemonAwaitThread() {
		Thread awaitThread = new Thread("container-" + (containerCounter.get())) {

			@Override
			public void run() {
				TomcatWebServer.this.tomcat.getServer().await();
			}

		};
		awaitThread.setContextClassLoader(getClass().getClassLoader());
		awaitThread.setDaemon(false);
		awaitThread.start();
	}
}

至此，SpringBoot启动Tomcat容器的主流程完毕。