SpringBoot內置tomcat啓動原理

前言

         不得不說SpringBoot的開發者是在爲大衆程序猿謀福利，把你們都慣成了懶漢，xml不配置了，連tomcat也懶的配置了，典型的一鍵啓動系統，那麼tomcat在springboot是怎麼啓動的呢？ 

內置tomcat

         開發階段對咱們來講使用內置的tomcat是很是夠用了，固然也可使用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("===應用啓動耗時："+(System.currentTimeMillis()-time)+"===");
    }
}

        這裏是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，目前大多數的作法仍是排除內置的tomcat，打瓦包（war）而後部署在生產的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("===應用啓動耗時："+(System.currentTimeMillis()-time)+"===");
    }

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

從main函數提及

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

--這裏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，這裏你能夠本身塗鴉一下，換成本身項目的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);
        
        //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) {
        
    }
}

        既然咱們想知道tomcat在SpringBoot中是怎麼啓動的，那麼run方法中，重點關注建立應用上下文（createApplicationContext）和刷新上下文（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);
}

這裏會建立AnnotationConfigServletWebServerApplicationContext類。
而AnnotationConfigServletWebServerApplicationContext類繼承了ServletWebServerApplicationContext，而這個類是最終集成了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) {
        }
    }
}

//這裏直接調用最終父類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，也就說這裏要回到子類：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
//在這個方法裏看到了熟悉的面孔，this.createWebServer，神祕的面紗就要揭開了。
protected void onRefresh() {
    super.onRefresh();
    try {
        this.createWebServer();
    } catch (Throwable var2) {
        
    }
}

//ServletWebServerApplicationContext.java
//這裏是建立webServer，可是尚未啓動tomcat，這裏是經過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);
}

//實現
AbstractServletWebServerFactory
JettyServletWebServerFactory
TomcatServletWebServerFactory
UndertowServletWebServerFactory

 這裏ServletWebServerFactory接口有4個實現類

而其中咱們經常使用的有兩個：TomcatServletWebServerFactory和JettyServletWebServerFactory。

//TomcatServletWebServerFactory.java
//這裏咱們使用的tomcat，因此咱們查看TomcatServletWebServerFactory。到這裏總算是看到了tomcat的蹤影。
@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
//返回Engine容器，看到這裏，若是熟悉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這個方法建立了Tomcat對象，而且作了兩件重要的事情：把Connector對象添加到tomcat中，configureEngine(tomcat.getEngine());
           getWebServer方法返回的是TomcatWebServer。

//TomcatWebServer.java
//這裏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 {
    //在控制檯會看到這句日誌
    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();
                }
            });

            //===啓動tomcat服務===
            this.tomcat.start();

            rethrowDeferredStartupExceptions();

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

//TomcatWebServer.java
//啓動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;
            //在控制檯打印這句日誌，若是在yml設置了上下文，這裏會打印
            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，表明着整個服務器，一個Server包含多個Service。從上圖能夠看除Service主要包括多個Connector和一個Container。Connector用來處理鏈接相關的事情，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？ 咱們來看下圖：

      綜上所述，一個tomcat只包含一個Server，一個Server能夠包含多個Service，一個Service只有一個Container，但有多個Connector，這樣一個服務能夠處理多個鏈接。
      多個Connector和一個Container就造成了一個Service，有了Service就能夠對外提供服務了，可是Service要提供服務又必須提供一個宿主環境，那就非Server莫屬了，因此整個tomcat的聲明週期都由Server控制。

總結

   SpringBoot的啓動主要是經過實例化SpringApplication來啓動的，啓動過程主要作了如下幾件事情：配置屬性、獲取監聽器，發佈應用開始啓動事件初、始化輸入參數、配置環境，輸出banner、建立上下文、預處理上下文、刷新上下文、再刷新上下文、發佈應用已經啓動事件、發佈應用啓動完成事件。在SpringBoot中啓動tomcat的工做在刷新上下這一步。而tomcat的啓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，每一個Service包含多個Connector和一個Container，而一個Container下又包含多個子容器。