SpringBoot内置tomcat启动原理

前言

不得不说SpringBoot的开发者是在为大众程序猿谋福利，把大家都惯成了懒汉，xml不配置了，连tomcat也懒的配置了，典型的一键启动系统，那么tomcat在springboot是怎么启动的呢？

内置tomcat

开发阶段对我们来说使用内置的tomcat是非常够用了，当然也可以使用jetty。

<dependency>

   <groupId>org.springframework.boot</groupId>

   <artifactId>spring-boot-starter-web</artifactId>

   <version>2.1.6.RELEASE</version>

</dependency>

@SpringBootApplication

public class MySpringbootTomcatStarter{

    public static void main(String[] args) {

        Long time=System.currentTimeMillis();

        SpringApplication.run(MySpringbootTomcatStarter.class);

        System.out.println("===应用启动耗时："+(System.currentTimeMillis()-time)+"===");

    }

}

这里是main函数入口，两句代码最耀眼，分别是SpringBootApplication注解和SpringApplication.run()方法。

发布生产

发布的时候，目前大多数的做法还是排除内置的tomcat，打瓦包（war）然后部署在生产的tomcat中，好吧，那打包的时候应该怎么处理？

<dependency>

    <groupId>org.springframework.boot</groupId>

    <artifactId>spring-boot-starter-web</artifactId>

    <!-- 移除嵌入式tomcat插件 -->

    <exclusions>

        <exclusion>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter-tomcat</artifactId>

        </exclusion>

    </exclusions>

</dependency>

<!--添加servlet-api依赖--->

<dependency>

    <groupId>javax.servlet</groupId>

    <artifactId>javax.servlet-api</artifactId>

    <version>3.1.0</version>

    <scope>provided</scope>

</dependency>

更新main函数，主要是继承SpringBootServletInitializer，并重写configure()方法。

@SpringBootApplication

public class MySpringbootTomcatStarter extends SpringBootServletInitializer {

    public static void main(String[] args) {

        Long time=System.currentTimeMillis();

        SpringApplication.run(MySpringbootTomcatStarter.class);

        System.out.println("===应用启动耗时："+(System.currentTimeMillis()-time)+"===");

    }

    @Override

    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {

        return builder.sources(this.getClass());

    }

}

从main函数说起

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {

    return run(new Class[]{primarySource}, args);

}

--这里run方法返回的是ConfigurableApplicationContext

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {

	return (new SpringApplication(primarySources)).run(args);

}

public ConfigurableApplicationContext run(String... args) {

	ConfigurableApplicationContext context = null;

	Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();

	this.configureHeadlessProperty();

	SpringApplicationRunListeners listeners = this.getRunListeners(args);

	listeners.starting();

	Collection exceptionReporters;

	try {

		ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);

		ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);

		this.configureIgnoreBeanInfo(environment);

		//打印banner，这里你可以自己涂鸦一下，换成自己项目的logo

		Banner printedBanner = this.printBanner(environment);

		//创建应用上下文

		context = this.createApplicationContext();

		exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);

		//预处理上下文

		this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);

		//刷新上下文

		this.refreshContext(context);

		//再刷新上下文

		this.afterRefresh(context, applicationArguments);

		listeners.started(context);

		this.callRunners(context, applicationArguments);

	} catch (Throwable var10) {

	}

	try {

		listeners.running(context);

		return context;

	} catch (Throwable var9) {

	}

}

既然我们想知道tomcat在SpringBoot中是怎么启动的，那么run方法中，重点关注创建应用上下文（createApplicationContext）和刷新上下文（refreshContext）。

创建上下文

//创建上下文

protected ConfigurableApplicationContext createApplicationContext() {

	Class<?> contextClass = this.applicationContextClass;

	if (contextClass == null) {

		try {

			switch(this.webApplicationType) {

				case SERVLET:

                    //创建AnnotationConfigServletWebServerApplicationContext

				    contextClass = Class.forName("org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext");

					break;

				case REACTIVE:

					contextClass = Class.forName("org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext");

					break;

				default:

					contextClass = Class.forName("org.springframework.context.annotation.AnnotationConfigApplicationContext");

			}

		} catch (ClassNotFoundException var3) {

			throw new IllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", var3);

		}

	}

	return (ConfigurableApplicationContext)BeanUtils.instantiateClass(contextClass);

}

这里会创建AnnotationConfigServletWebServerApplicationContext类。

而AnnotationConfigServletWebServerApplicationContext类继承了ServletWebServerApplicationContext，而这个类是最终集成了AbstractApplicationContext。

刷新上下文

//SpringApplication.java

//刷新上下文

private void refreshContext(ConfigurableApplicationContext context) {

	this.refresh(context);

	if (this.registerShutdownHook) {

		try {

			context.registerShutdownHook();

		} catch (AccessControlException var3) {

		}

	}

}

//这里直接调用最终父类AbstractApplicationContext.refresh()方法

protected void refresh(ApplicationContext applicationContext) {

	((AbstractApplicationContext)applicationContext).refresh();

}

//AbstractApplicationContext.java

public void refresh() throws BeansException, IllegalStateException {

	synchronized(this.startupShutdownMonitor) {

		this.prepareRefresh();

		ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();

		this.prepareBeanFactory(beanFactory);

		try {

			this.postProcessBeanFactory(beanFactory);

			this.invokeBeanFactoryPostProcessors(beanFactory);

			this.registerBeanPostProcessors(beanFactory);

			this.initMessageSource();

			this.initApplicationEventMulticaster();

			//调用各个子类的onRefresh()方法，也就说这里要回到子类：ServletWebServerApplicationContext，调用该类的onRefresh()方法

			this.onRefresh();

			this.registerListeners();

			this.finishBeanFactoryInitialization(beanFactory);

			this.finishRefresh();

		} catch (BeansException var9) {

			this.destroyBeans();

			this.cancelRefresh(var9);

			throw var9;

		} finally {

			this.resetCommonCaches();

		}

	}

}

//ServletWebServerApplicationContext.java

//在这个方法里看到了熟悉的面孔，this.createWebServer，神秘的面纱就要揭开了。

protected void onRefresh() {

	super.onRefresh();

	try {

		this.createWebServer();

	} catch (Throwable var2) {

	}

}

//ServletWebServerApplicationContext.java

//这里是创建webServer，但是还没有启动tomcat，这里是通过ServletWebServerFactory创建，那么接着看下ServletWebServerFactory

private void createWebServer() {

	WebServer webServer = this.webServer;

	ServletContext servletContext = this.getServletContext();

	if (webServer == null && servletContext == null) {

		ServletWebServerFactory factory = this.getWebServerFactory();

		this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});

	} else if (servletContext != null) {

		try {

			this.getSelfInitializer().onStartup(servletContext);

		} catch (ServletException var4) {

		}

	}

	this.initPropertySources();

}

//接口

public interface ServletWebServerFactory {

    WebServer getWebServer(ServletContextInitializer... initializers);

}

//实现

AbstractServletWebServerFactory

JettyServletWebServerFactory

TomcatServletWebServerFactory

UndertowServletWebServerFactory

这里ServletWebServerFactory接口有4个实现类

而其中我们常用的有两个：TomcatServletWebServerFactory和JettyServletWebServerFactory。

//TomcatServletWebServerFactory.java

//这里我们使用的tomcat，所以我们查看TomcatServletWebServerFactory。到这里总算是看到了tomcat的踪迹。

@Override

public WebServer getWebServer(ServletContextInitializer... initializers) {

	Tomcat tomcat = new Tomcat();

	File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");

	tomcat.setBaseDir(baseDir.getAbsolutePath());

    //创建Connector对象

	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);

}

//Tomcat.java

//返回Engine容器，看到这里，如果熟悉tomcat源码的话，对engine不会感到陌生。

public Engine getEngine() {

    Service service = getServer().findServices()[0];

    if (service.getContainer() != null) {

        return service.getContainer();

    }

    Engine engine = new StandardEngine();

    engine.setName( "Tomcat" );

    engine.setDefaultHost(hostname);

    engine.setRealm(createDefaultRealm());

    service.setContainer(engine);

    return engine;

}

//Engine是最高级别容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器

getWebServer这个方法创建了Tomcat对象，并且做了两件重要的事情：把Connector对象添加到tomcat中，configureEngine(tomcat.getEngine());

getWebServer方法返回的是TomcatWebServer。

//TomcatWebServer.java

//这里调用构造函数实例化TomcatWebServer

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 {

    //在控制台会看到这句日志

	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())) {

					removeServiceConnectors();

				}

			});

			//===启动tomcat服务===

			this.tomcat.start();

			rethrowDeferredStartupExceptions();

			try {

				ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());

			}

			catch (NamingException ex) {

			}

            //开启阻塞非守护进程

			startDaemonAwaitThread();

		}

		catch (Exception ex) {

			stopSilently();

			destroySilently();

			throw new WebServerException("Unable to start embedded Tomcat", ex);

		}

	}

}

//Tomcat.java

public void start() throws LifecycleException {

	getServer();

	server.start();

}

//这里server.start又会回到TomcatWebServer的

public void stop() throws LifecycleException {

	getServer();

	server.stop();

}

//TomcatWebServer.java

//启动tomcat服务

@Override

public void start() throws WebServerException {

	synchronized (this.monitor) {

		if (this.started) {

			return;

		}

		try {

			addPreviouslyRemovedConnectors();

			Connector connector = this.tomcat.getConnector();

			if (connector != null && this.autoStart) {

				performDeferredLoadOnStartup();

			}

			checkThatConnectorsHaveStarted();

			this.started = true;

			//在控制台打印这句日志，如果在yml设置了上下文，这里会打印

			logger.info("Tomcat started on port(s): " + getPortsDescription(true) + " with context path '"

					+ getContextPath() + "'");

		}

		catch (ConnectorStartFailedException ex) {

			stopSilently();

			throw ex;

		}

		catch (Exception ex) {

			throw new WebServerException("Unable to start embedded Tomcat server", ex);

		}

		finally {

			Context context = findContext();

			ContextBindings.unbindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());

		}

	}

}

//关闭tomcat服务

@Override

public void stop() throws WebServerException {

	synchronized (this.monitor) {

		boolean wasStarted = this.started;

		try {

			this.started = false;

			try {

				stopTomcat();

				this.tomcat.destroy();

			}

			catch (LifecycleException ex) {

			}

		}

		catch (Exception ex) {

			throw new WebServerException("Unable to stop embedded Tomcat", ex);

		}

		finally {

			if (wasStarted) {

				containerCounter.decrementAndGet();

			}

		}

	}

}

附：tomcat顶层结构图

tomcat最顶层容器是Server，代表着整个服务器，一个Server包含多个Service。从上图可以看除Service主要包括多个Connector和一个Container。Connector用来处理连接相关的事情，并提供Socket到Request和Response相关转化。Container用于封装和管理Servlet，以及处理具体的Request请求。那么上文提到的Engine>Host>Context>Wrapper容器又是怎么回事呢？我们来看下图：

综上所述，一个tomcat只包含一个Server，一个Server可以包含多个Service，一个Service只有一个Container，但有多个Connector，这样一个服务可以处理多个连接。

多个Connector和一个Container就形成了一个Service，有了Service就可以对外提供服务了，但是Service要提供服务又必须提供一个宿主环境，那就非Server莫属了，所以整个tomcat的声明周期都由Server控制。

总结

SpringBoot的启动主要是通过实例化SpringApplication来启动的，启动过程主要做了以下几件事情：配置属性、获取监听器，发布应用开始启动事件初、始化输入参数、配置环境，输出banner、创建上下文、预处理上下文、刷新上下文、再刷新上下文、发布应用已经启动事件、发布应用启动完成事件。在SpringBoot中启动tomcat的工作在刷新上下这一步。而tomcat的启动主要是实例化两个组件：Connector、Container，一个tomcat实例就是一个Server，一个Server包含多个Service，也就是多个应用程序，每个Service包含多个Connector和一个Container，而一个Container下又包含多个子容器。

巴特西