SpringBoot內置tomcat啟動原理詳解

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前言

不得不說SpringBoot的開發者是在為大眾程序猿謀福利，把大傢都慣成瞭懶漢，xml不配置瞭，連tomcat也懶的配置瞭，典型的一鍵啟動系統，那麼tomcat在springboot是怎麼啟動的呢？

內置tomcat

開發階段對我們來說使用內置的tomcat是非常夠用瞭，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("===應用啟動耗時："+(System.currentTimeMillis()-time)+"===");
    }
}

這裡是main函數入口，兩句代碼最耀眼，分別是SpringBootApplication註解和SpringApplication.run()方法。

發佈生產

發佈的時候，目前大多數的做法還是排除內置的tomcat，打瓦包（war）然後部署在生產的tomcat中，好吧，那打包的時候應該怎麼處理？

<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--->
<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，並重寫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("===應用啟動耗時："+(System.currentTimeMillis()-time)+"===");
    }

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

從main函數說起

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

–這裡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，這裡你可以自己塗鴉一下，換成自己項目的logo
  Banner printedBanner = this.printBanner(environment);
  
  //創建應用上下文
  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中是怎麼啟動的，那麼run方法中，重點關註創建應用上下文（createApplicationContext）和刷新上下文（refreshContext）。

創建上下文

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

這裡會創建AnnotationConfigServletWebServerApplicationContext類。
而AnnotationConfigServletWebServerApplicationContext類繼承瞭ServletWebServerApplicationContext，而這個類是最終集成瞭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) {
  }
 }
}

//這裡直接調用最終父類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();
   //調用各個子類的onRefresh()方法，也就說這裡要回到子類：ServletWebServerApplicationContext，調用該類的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
//在這個方法裡看到瞭熟悉的面孔，this.createWebServer，神秘的面紗就要揭開瞭。
protected void onRefresh() {
 super.onRefresh();
 try {
  this.createWebServer();
 } catch (Throwable var2) {
  
 }
}

//ServletWebServerApplicationContext.java
//這裡是創建webServer，但是還沒有啟動tomcat，這裡是通過ServletWebServerFactory創建，那麼接著看下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);
}

//實現
AbstractServletWebServerFactory
JettyServletWebServerFactory
TomcatServletWebServerFactory
UndertowServletWebServerFactory

這裡ServletWebServerFactory接口有4個實現類

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

//TomcatServletWebServerFactory.java
//這裡我們使用的tomcat，所以我們查看TomcatServletWebServerFactory。到這裡總算是看到瞭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，看到這裡，如果熟悉tomcat源碼的話，對engine不會感到陌生。
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這個方法創建瞭Tomcat對象，並且做瞭兩件重要的事情：把Connector對象添加到tomcat中，configureEngine(tomcat.getEngine());
           getWebServer方法返回的是TomcatWebServer。

//TomcatWebServer.java
//這裡調用構造函數實例化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tomcat服務===
   this.tomcat.start();

   rethrowDeferredStartupExceptions();

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

//TomcatWebServer.java
//啟動tomcat服務
@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設置瞭上下文，這裡會打印
   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());
  }
 }
}

//關閉tomcat服務
@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，代表著整個服務器，一個Server包含多個Service。從上圖可以看除Service主要包括多個Connector和一個Container。Connector用來處理連接相關的事情，並提供Socket到Request和Response相關轉化。Container用於封裝和管理Servlet，以及處理具體的Request請求。那麼上文提到的Engine>Host>Context>Wrapper容器又是怎麼回事呢？ 我們來看下圖：

      綜上所述，一個tomcat隻包含一個Server，一個Server可以包含多個Service，一個Service隻有一個Container，但有多個Connector，這樣一個服務可以處理多個連接。
      多個Connector和一個Container就形成瞭一個Service，有瞭Service就可以對外提供服務瞭，但是Service要提供服務又必須提供一個宿主環境，那就非Server莫屬瞭，所以整個tomcat的聲明周期都由Server控制。

總結

SpringBoot的啟動主要是通過實例化SpringApplication來啟動的，啟動過程主要做瞭以下幾件事情：配置屬性、獲取監聽器，發佈應用開始啟動事件初、始化輸入參數、配置環境，輸出banner、創建上下文、預處理上下文、刷新上下文、再刷新上下文、發佈應用已經啟動事件、發佈應用啟動完成事件。在SpringBoot中啟動tomcat的工作在刷新上下這一步。而tomcat的啟動主要是實例化兩個組件：Connector、Container，一個tomcat實例就是一個Server，一個Server包含多個Service，也就是多個應用程序，每個Service包含多個Connector和一個Container，而一個Container下又包含多個子容器。

到此這篇關於SpringBoot內置tomcat啟動原理詳解的文章就介紹到這瞭,更多相關SpringBoot內置tomcat啟動內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet！

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