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>
    
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>

<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。J

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。