SpringBoot內置tomcat啟動(dòng)原理詳解

前言

不得不說(shuō)SpringBoot的開(kāi)發(fā)者是在為大眾程序猿謀福利，把大家都慣成了懶漢，xml不配置了，連tomcat也懶的配置了，典型的一鍵啟動(dòng)系統，那么tomcat在springboot是怎么啟動(dòng)的呢？

內置tomcat

開(kāi)發(fā)階段對我們來(lái)說(shuō)使用內置的tomcat是非常夠用了，當然也可以使用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("===應用啟動(dòng)耗時(shí)："+(System.currentTimeMillis()-time)+"===");
    }
}

這里是main函數入口，兩句代碼最耀眼，分別是SpringBootApplication注解和SpringApplication.run()方法。

發(fā)布生產(chǎn)

發(fā)布的時(shí)候，目前大多數的做法還是排除內置的tomcat，打瓦包（war）然后部署在生產(chǎn)的tomcat中，好吧，那打包的時(shí)候應該怎么處理？

<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依賴(lài)--->
<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>3.1.0</version>
    <scope>provided</scope>
</dependency>

更新main函數，主要是繼承SpringBootServletInitializer，并重寫(xiě)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("===應用啟動(dòng)耗時(shí)："+(System.currentTimeMillis()-time)+"===");
    }

    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(this.getClass());
    }
}

從main函數說(shuō)起

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

--這里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，這里你可以自己涂鴉一下，換成自己項目的logo
  Banner printedBanner = this.printBanner(environment);
  
  //創(chuàng  )建應用上下文
  context = this.createApplicationContext();
  exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);

  //預處理上下文
  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) {
  
 }
}

既然我們想知道tomcat在SpringBoot中是怎么啟動(dòng)的，那么run方法中，重點(diǎn)關(guān)注創(chuàng )建應用上下文（createApplicationContext）和刷新上下文（refreshContext）。

創(chuàng )建上下文

//創(chuàng  )建上下文
protected ConfigurableApplicationContext createApplicationContext() {
 Class<?> contextClass = this.applicationContextClass;
 if (contextClass == null) {
  try {
   switch(this.webApplicationType) {
    case SERVLET:
                    //創(chuàng  )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);
}

這里會(huì )創(chuàng )建AnnotationConfigServletWebServerApplicationContext類(lèi)。
而AnnotationConfigServletWebServerApplicationContext類(lèi)繼承了ServletWebServerApplicationContext，而這個(gè)類(lèi)是最終集成了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) {
  }
 }
}

//這里直接調用最終父類(lèi)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();
   //調用各個(gè)子類(lèi)的onRefresh()方法，也就說(shuō)這里要回到子類(lèi)：ServletWebServerApplicationContext，調用該類(lèi)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
//在這個(gè)方法里看到了熟悉的面孔，this.createWebServer，神秘的面紗就要揭開(kāi)了。
protected void onRefresh() {
 super.onRefresh();
 try {
  this.createWebServer();
 } catch (Throwable var2) {
  
 }
}

//ServletWebServerApplicationContext.java
//這里是創(chuàng  )建webServer，但是還沒(méi)有啟動(dòng)tomcat，這里是通過(guò)ServletWebServerFactory創(chuàng  )建，那么接著(zhù)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);
}

//實(shí)現
AbstractServletWebServerFactory
JettyServletWebServerFactory
TomcatServletWebServerFactory
UndertowServletWebServerFactory

這里ServletWebServerFactory接口有4個(gè)實(shí)現類(lèi)

而其中我們常用的有兩個(gè)：TomcatServletWebServerFactory和JettyServletWebServerFactory。

//TomcatServletWebServerFactory.java
//這里我們使用的tomcat，所以我們查看TomcatServletWebServerFactory。到這里總算是看到了tomcat的蹤跡。
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
 Tomcat tomcat = new Tomcat();
 File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
 tomcat.setBaseDir(baseDir.getAbsolutePath());
    //創(chuàng  )建Connector對象
 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，看到這里，如果熟悉tomcat源碼的話(huà)，對engine不會(huì )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是最高級別容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器

getWebServer這個(gè)方法創(chuàng )建了Tomcat對象，并且做了兩件重要的事情：把Connector對象添加到tomcat中，configureEngine(tomcat.getEngine());
           getWebServer方法返回的是TomcatWebServer。

//TomcatWebServer.java
//這里調用構造函數實(shí)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 {
    //在控制臺會(huì )看到這句日志
 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();
    }
   });

   //===啟動(dòng)tomcat服務(wù)===
   this.tomcat.start();

   rethrowDeferredStartupExceptions();

   try {
    ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
   }
   catch (NamingException ex) {
                
   }
            
            //開(kāi)啟阻塞非守護進(jìn)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();
}
//這里server.start又會(huì )回到TomcatWebServer的
public void stop() throws LifecycleException {
 getServer();
 server.stop();
}

//TomcatWebServer.java
//啟動(dòng)tomcat服務(wù)
@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;
   //在控制臺打印這句日志，如果在yml設置了上下文，這里會(huì )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());
  }
 }
}

//關(guān)閉tomcat服務(wù)
@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頂層結構圖

      tomcat最頂層容器是Server，代表著(zhù)整個(gè)服務(wù)器，一個(gè)Server包含多個(gè)Service。從上圖可以看除Service主要包括多個(gè)Connector和一個(gè)Container。Connector用來(lái)處理連接相關(guān)的事情，并提供Socket到Request和Response相關(guān)轉化。Container用于封裝和管理Servlet，以及處理具體的Request請求。那么上文提到的Engine>Host>Context>Wrapper容器又是怎么回事呢？ 我們來(lái)看下圖：

      綜上所述，一個(gè)tomcat只包含一個(gè)Server，一個(gè)Server可以包含多個(gè)Service，一個(gè)Service只有一個(gè)Container，但有多個(gè)Connector，這樣一個(gè)服務(wù)可以處理多個(gè)連接。
      多個(gè)Connector和一個(gè)Container就形成了一個(gè)Service，有了Service就可以對外提供服務(wù)了，但是Service要提供服務(wù)又必須提供一個(gè)宿主環(huán)境，那就非Server莫屬了，所以整個(gè)tomcat的聲明周期都由Server控制。

總結

SpringBoot的啟動(dòng)主要是通過(guò)實(shí)例化SpringApplication來(lái)啟動(dòng)的，啟動(dòng)過(guò)程主要做了以下幾件事情：配置屬性、獲取監聽(tīng)器，發(fā)布應用開(kāi)始啟動(dòng)事件初、始化輸入參數、配置環(huán)境，輸出banner、創(chuàng )建上下文、預處理上下文、刷新上下文、再刷新上下文、發(fā)布應用已經(jīng)啟動(dòng)事件、發(fā)布應用啟動(dòng)完成事件。在SpringBoot中啟動(dòng)tomcat的工作在刷新上下這一步。而tomcat的啟動(dòng)主要是實(shí)例化兩個(gè)組件：Connector、Container，一個(gè)tomcat實(shí)例就是一個(gè)Server，一個(gè)Server包含多個(gè)Service，也就是多個(gè)應用程序，每個(gè)Service包含多個(gè)Connector和一個(gè)Container，而一個(gè)Container下又包含多個(gè)子容器。

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