SprinBoot 内置了 tomcat啟動，原理你搞的清楚嗎？

前言

不得不說SpringBoot的開發者是在為大衆程式猿謀福利，把大家都慣成了懶漢，xml不配置了，連tomcat也懶的配置了，典型的一鍵啟動系統，那麼tomcat在springboot是怎麼啟動的呢？

内置tomcat

開發階段對我們來說使用内置的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("===應用啟動耗時："+(System.currentTimeMillis()-time)+"===");
    }
}      

這裡是main函數入口，兩句代碼最耀眼，分别是SpringBootApplication注解和SpringApplication.run()方法。

釋出生産

釋出的時候，目前大多數的做法還是排除内置的tomcat，打瓦包（war）然後部署在生産的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，并重寫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方法傳回的是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);
        
        //列印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);
        
        //建立應用上下文
        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);
        
        //重新整理上下文
        this.refreshContext(context);
        
        //再重新整理上下文
        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，重點關注建立應用上下文（createApplicationContext）和重新整理上下文（refreshContext）。

建立上下文

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

這裡會建立AnnotationConfigServletWebServerApplicationContext類。

而AnnotationConfigServletWebServerApplicationContext類繼承了ServletWebServerApplicationContext，而這個類是最終內建了AbstractApplicationContext。

重新整理上下文

//SpringApplication.java
//重新整理上下文
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
//在這個方法裡看到了熟悉的面孔，this.createWebServer，神秘的面紗就要揭開了。
protected void onRefresh() {
    super.onRefresh();
    try {
        this.createWebServer();
    } catch (Throwable var2) {
        
    }
}

//ServletWebServerApplicationContext.java
//這裡是建立webServer，但是還沒有啟動tomcat，這裡是通過ServletWebServerFactory建立，那麼接着看下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);
}

//實作
AbstractServletWebServerFactory
JettyServletWebServerFactory
TomcatServletWebServerFactory
UndertowServletWebServerFactory      

這裡ServletWebServerFactory接口有4個實作類

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

//TomcatServletWebServerFactory.java
//這裡我們使用的tomcat，是以我們檢視TomcatServletWebServerFactory。到這裡總算是看到了tomcat的蹤迹。
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
    Tomcat tomcat = new Tomcat();
    File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
    tomcat.setBaseDir(baseDir.getAbsolutePath());
    //建立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
//傳回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是最進階别容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器      

getWebServer這個方法建立了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方法傳回的是TomcatWebServer。

//TomcatWebServer.java
//這裡調用構造函數執行個體化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) {
                
            }
            
            //開啟阻塞非守護程序
            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;
            //在控制台列印這句日志，如果在yml設定了上下文，這裡會列印
            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，代表着整個伺服器，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用來處理連接配接相關的事情，并提供Socket到Request和Response相關轉化。Container用于封裝和管理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配接。

多個Connector和一個Container就形成了一個Service，有了Service就可以對外提供服務了，但是Service要提供服務又必須提供一個宿主環境，那就非Server莫屬了，是以整個tomcat的聲明周期都由Server控制。

總結

SpringBoot的啟動主要是通過執行個體化SpringApplication來啟動的，啟動過程主要做了以下幾件事情：配置屬性、擷取監聽器，釋出應用開始啟動事件初、始化輸入參數、配置環境，輸出banner、建立上下文、預處理上下文、重新整理上下文、再重新整理上下文、釋出應用已經啟動事件、釋出應用啟動完成事件。在SpringBoot中啟動tomcat的工作在重新整理上下這一步。而tomcat的啟動主要是執行個體化兩個元件：Connector、Container，一個tomcat執行個體就是一個Server，一個Server包含多個Service，也就是多個應用程式，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。

原作者：歪頭兒在帝都