SpringBoot源码分析之Tomcat是如何在SpringBoot中启动的？

一.前言

我们知道SpringBoot可以直接把传统的war包打成可执行的jar包，直接启动。这得益于SpringBoot内置了容器可以直接启动。本文将以 Tomcat 为例，来看看 SpringBoot 是如何启动 Tomcat 的。

二.源码分析

一.SpringApplication初始化

public class SpringBootDemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringBootDemoApplication.class, args);

    }

}

调用到最终的run方法我们来看一下

	public static ConfigurableApplicationContext run(Class[] primarySources, String[] args) {
		return new SpringApplication(primarySources).run(args);
	}

这里面首先创建了一个SpringApplication的对象，然后调用了run方法，我们先看在创建对象的死后做了什么

	public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {
		this.resourceLoader = resourceLoader;
		Assert.notNull(primarySources, "PrimarySources must not be null");
		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
		this.webApplicationType = WebApplicationType.deduceFromClasspath();
		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

这里面对于我们有用的是WebApplicationType.deduceFromClasspath()方法，因为他返回了一个叫webApplicationType的东西。这个我们一会能用得到。看一下deduceFromClasspath（)的源码

	static WebApplicationType deduceFromClasspath() {
		if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
				&& !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
			return WebApplicationType.REACTIVE;
		}
		for (String className : SERVLET_INDICATOR_CLASSES) {
			if (!ClassUtils.isPresent(className, null)) {
				return WebApplicationType.NONE;
			}
		}
		return WebApplicationType.SERVLET;
	}

其中最主要的方法是ClassUtils.isPresent方法

	public static boolean isPresent(String className, @Nullable ClassLoader classLoader) {
		try {
			forName(className, classLoader);
			return true;
		}
		catch (IllegalAccessError err) {
			throw new IllegalStateException("Readability mismatch in inheritance hierarchy of class [" +
					className + "]: " + err.getMessage(), err);
		}
		catch (Throwable ex) {
			// Typically ClassNotFoundException or NoClassDefFoundError...
			return false;
		}
	}

可以看到就是利用反射来判断传入的className是否存在，如果不存在返回false.

最后this.webApplicationType= WebApplicationType.SERVLET这个东东。

至于其他忽略的源码，之后会在其他源码分析模块讲解

二.run方法的调用

	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection 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;
	}

这里面对于我们相关的有两个方法一个是context = createApplicationContext();方法，另一个是refreshContext(context)方法，我们分别分析一下源码。

（1）createApplicationContext()

	protected ConfigurableApplicationContext createApplicationContext() {
		Class contextClass = this.applicationContextClass;
		if (contextClass == null) {
			try {
				switch (this.webApplicationType) {
				case SERVLET:
					contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
					break;
				case REACTIVE:
					contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
					break;
				default:
					contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
				}
			}
			catch (ClassNotFoundException ex) {
				throw new IllegalStateException(
						"Unable create a default ApplicationContext, please specify an ApplicationContextClass", ex);
			}
		}
		return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
	}

还记得我们上一部分返回的this.webApplicationType的值是什么么？是Servlet对吧。

那么返回的contextClass就是对应的DEFAULT_SERVLET_WEB_CONTEXT_CLASS值也就是

public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = "org.springframework.boot." + "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";

这里就是根据我们的 webApplicationType 来判断创建哪种类型的 Servlet,代码中分别对应着 Web 类型(SERVLET),响应式 Web 类型（REACTIVE),非 Web 类型（default),我们建立的是 Web 类型，所以肯定实例化 DEFAULT_SERVLET_WEB_CONTEXT_CLASS 指定的类，也就是 AnnotationConfigServletWebServerApplicationContext 类，我们来用图来说明下这个类的关系

我们看一下返回回来的ConfigurableApplicationContext 发现他是一个接口。因为我们是AnnotationConfigServletWebServerApplicationContext。是他的实现类，所以我们自然而然进入到AnnotationConfigServletWebServerApplicationContext中

 这是他的类的关系 可以发现最后继承的是AbstractApplicationContext。至此创建线程上线文完成。回过头我们来看一下refreshContext()方法

（2）refreshContext（）方法

我们跟随代码来到

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

这块还是调用的本方法的refresh方法最后强转成AbstractApplicationContext，调用它的refresh（）方法 

	protected void refresh(ApplicationContext applicationContext) {
		Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
		((AbstractApplicationContext) applicationContext).refresh();
	}

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();
			}
		}
	}

这里我们看到 onRefresh()方法是调用其子类的实现，根据我们上文的分析，我们这里的子类是 ServletWebServerApplicationContext。

调用的是这个ServletWebServerApplicationContext类的onRefresh(）方法（）

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

关键代码在于createWebServer();

	private void createWebServer() {
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		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();
	}

这就是创建WebServer的意思，其中最重要的就是通过getWebServerFactory()方法来获取webserver,我们来看一下。

 看见了么？这个工厂接口有下面这几种实现类，根据上图我们发现，工厂类是一个接口，各个具体服务的实现是由各个子类来实现的，所以我们就去看看 TomcatServletWebServerFactory.getWebServer()的实现。

	@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);
		return getTomcatWebServer(tomcat);
	}

总结

SpringBoot 的启动是通过 new SpringApplication()实例来启动的，启动过程主要做如下几件事情：> 1. 配置属性 > 2. 获取监听器，发布应用开始启动事件 > 3. 初始化输入参数 > 4. 配置环境，输出 banner > 5. 创建上下文 > 6. 预处理上下文 > 7. 刷新上下文 > 8. 再刷新上下文 > 9. 发布应用已经启动事件 > 10. 发布应用启动完成事件

而启动 Tomcat 就是在第 7 步中“刷新上下文”；