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
publicclassMySpringbootTomcatStarterextendsSpringBootServletInitializer {
   publicstaticvoidmain(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

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

--这里run方法返回的是ConfigurableApplicationContext
publicstatic 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) {
  thrownew 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
privatevoidrefreshContext(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
protectedvoidrefresh(ApplicationContext applicationContext) {
((AbstractApplicationContext)applicationContext).refresh();
}
//AbstractApplicationContext.java
publicvoidrefresh()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。
protectedvoidonRefresh() {
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
privatevoidcreateWebServer() {
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()});
} elseif (servletContext != null) {
 try {
  this.getSelfInitializer().onStartup(servletContext);
 } catch (ServletException var4) {

 }
}

this.initPropertySources();
}

//接口
publicinterfaceServletWebServerFactory {
   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) {
returnnew 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
publicTomcatWebServer(Tomcat tomcat, boolean autoStart) {
Assert.notNull(tomcat, "Tomcat Server must not be null");
this.tomcat = tomcat;
this.autoStart = autoStart;
initialize();
}

privatevoidinitialize()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();
  thrownew WebServerException("Unable to start embedded Tomcat", ex);
 }
}
}
//Tomcat.java
publicvoidstart()throws LifecycleException {
getServer();
server.start();
}
//这里server.start又会回到TomcatWebServer的
publicvoidstop()throws LifecycleException {
getServer();
server.stop();
}
//TomcatWebServer.java
//启动tomcat服务
@Override
publicvoidstart()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) {
  thrownew 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
publicvoidstop()throws WebServerException {
synchronized (this.monitor) {
 boolean wasStarted = this.started;
 try {
  this.started = false;
  try {
   stopTomcat();
   this.tomcat.destroy();
  }
  catch (LifecycleException ex) {

  }
 }
 catch (Exception ex) {
  thrownew 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。