Spring Boot与嵌入式servlet容器

传统的Spring MVC工程部署时需要将WAR文件放置在servlet容器的文档目录内，而Spring Boot工程使用嵌入式servlet容器省去了这一步骤，本文分析Spring Boot中嵌入式servlet容器的创建和启动过程。

刷新应用上下文

Spring Boot的启动过程一文指出在Spring Boot工程中，Web环境下默认创建AnnotationConfigEmbeddedWebApplicationContext类型的应用上下文，它的刷新方法定义在它的父类EmbeddedWebApplicationContext中，相关代码如下：

@Override
public final void refresh() throws BeansException, IllegalStateException {
    try {
        super.refresh();
    }
    catch (RuntimeException ex) {
        stopAndReleaseEmbeddedServletContainer();
        throw ex;
    }
}

EmbeddedWebApplicationContext类重写的refresh方法在内部调用了基类AbstractApplicationContext的refresh方法，其代码如下所示：

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();
        // Tell the subclass to refresh the internal bean factory.
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);
        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);
            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);
            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);
            // Initialize message source for this context.
            initMessageSource();
            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();
            // Initialize other special beans in specific context subclasses.
            onRefresh();
            // Check for listener beans and register them.
            registerListeners();
            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);
            // Last step: publish corresponding event.
            finishRefresh();
        }
        // 省略一些代码
    }
}

• refresh方法可以看成是模板方法，子类可以重写prepareRefresh、onRefresh和finishRefresh等方法。

EmbeddedWebApplicationContext类重写的onRefresh和finishRefresh方法如下：

@Override
protected void onRefresh() {
    super.onRefresh();
    try {
        createEmbeddedServletContainer();
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start embedded container",
                ex);
    }
}

@Override
protected void finishRefresh() {
    super.finishRefresh();
    EmbeddedServletContainer localContainer = startEmbeddedServletContainer();
    if (localContainer != null) {
        publishEvent(
                new EmbeddedServletContainerInitializedEvent(this, localContainer));
    }
}

• onRefresh方法首先调用基类的onRefresh方法，然后创建嵌入式servlet容器；
• finishRefresh方法首先调用基类的finishRefresh方法，然后启动嵌入式servlet容器。

创建嵌入式servlet容器

EmbeddedWebApplicationContext类的createEmbeddedServletContainer方法创建嵌入式servlet容器，代码如下：

private void createEmbeddedServletContainer() {
    EmbeddedServletContainer localContainer = this.embeddedServletContainer;
    ServletContext localServletContext = getServletContext();
    if (localContainer == null && localServletContext == null) {
        EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();
        this.embeddedServletContainer = containerFactory
                .getEmbeddedServletContainer(getSelfInitializer());
    }
    else if (localServletContext != null) {
        try {
            getSelfInitializer().onStartup(localServletContext);
        }
        catch (ServletException ex) {
            throw new ApplicationContextException("Cannot initialize servlet context",
                    ex);
        }
    }
    initPropertySources();
}

• 嵌入式servlet容器由EmbeddedServletContainer接口抽象，该接口的实现类有TomcatEmbeddedServletContainer、JettyEmbeddedServletContainer和UndertowEmbeddedServletContainer，分别包装了嵌入式Tomcat、Jetty和Undertow；
• EmbeddedServletContainerFactory接口用于实际创建嵌入式servlet容器，该接口的实现类有TomcatEmbeddedServletContainerFactory、JettyEmbeddedServletContainerFactory和UndertowEmbeddedServletContainerFactory，分别用于创建上述三种嵌入式servlet容器；
• getEmbeddedServletContainerFactory方法从当前应用上下文中取得唯一的EmbeddedServletContainerFactory类型的bean，若有多个则报错。使用默认的自动配置时，该bean一定是TomcatEmbeddedServletContainerFactory、JettyEmbeddedServletContainerFactory或者UndertowEmbeddedServletContainerFactory中的一个。

ServletContextInitializer接口

在上述创建嵌入式servlet容器的过程中，EmbeddedServletContainerFactory接口方法的实参是getSelfInitializer方法的返回值，类型是ServletContextInitializer。ServletContextInitializer接口用于动态配置ServletContext，只有一个回调方法onStartup在容器启动时被调用。

public interface ServletContextInitializer {

    void onStartup(ServletContext servletContext) throws ServletException;
}

ServletContextInitializer的类层次结构如下图所示，可见ServletRegistrationBean和FilterRegistrationBean都实现了该接口，它们分别向容器添加新的servlet和过滤器。

ServletContextInitializer接口.png

配置ServletContext

getSelfInitializer方法的代码如下，只是调用了selfInitialize方法。

private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
    return new ServletContextInitializer() {
        @Override
        public void onStartup(ServletContext servletContext) throws ServletException {
            selfInitialize(servletContext);
        }
    };
}

容器启动时具体的配置动作由selfInitialize方法完成，其代码如下：

private void selfInitialize(ServletContext servletContext) throws ServletException {
    prepareEmbeddedWebApplicationContext(servletContext);
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    ExistingWebApplicationScopes existingScopes = new ExistingWebApplicationScopes(
            beanFactory);
    WebApplicationContextUtils.registerWebApplicationScopes(beanFactory,
            getServletContext());
    existingScopes.restore();
    WebApplicationContextUtils.registerEnvironmentBeans(beanFactory,
            getServletContext());
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
        beans.onStartup(servletContext);
    }
}

该方法主要做了以下工作：

• prepareEmbeddedWebApplicationContext方法将应用上下文设置到ServletContext的属性中，过程与Spring MVC的启动过程一文中分析的ContextLoader初始化根应用上下文非常相似；

  protected void prepareEmbeddedWebApplicationContext(ServletContext servletContext) {
      Object rootContext = servletContext.getAttribute(
              WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
      if (rootContext != null) {
          if (rootContext == this) {
              throw new IllegalStateException(
                      "Cannot initialize context because there is already a root application context present - "
                              + "check whether you have multiple ServletContextInitializers!");
          }
          return;
      }
      Log logger = LogFactory.getLog(ContextLoader.class);
      servletContext.log("Initializing Spring embedded WebApplicationContext");
      try {
          servletContext.setAttribute(
                  WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this);
          if (logger.isDebugEnabled()) {
              logger.debug(
                      "Published root WebApplicationContext as ServletContext attribute with name ["
                              + WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
                              + "]");
          }
          setServletContext(servletContext);
          if (logger.isInfoEnabled()) {
              long elapsedTime = System.currentTimeMillis() - getStartupDate();
              logger.info("Root WebApplicationContext: initialization completed in "
                      + elapsedTime + " ms");
          }
      }
      catch (RuntimeException ex) {
          logger.error("Context initialization failed", ex);
          servletContext.setAttribute(
                  WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
          throw ex;
      }
      catch (Error ex) {
          logger.error("Context initialization failed", ex);
          servletContext.setAttribute(
                  WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
          throw ex;
      }
  }
  

• 调用其他ServletContextInitializer的回调方法，如ServletRegistrationBean和FilterRegistrationBean分别向容器添加新的servlet和过滤器。

启动嵌入式servlet容器

EmbeddedWebApplicationContext类的startEmbeddedServletContainer方法启动先前创建的嵌入式servlet容器，在内部调用EmbeddedServletContainer的start接口方法：

private EmbeddedServletContainer startEmbeddedServletContainer() {
    EmbeddedServletContainer localContainer = this.embeddedServletContainer;
    if (localContainer != null) {
        localContainer.start();
    }
    return localContainer;
}

嵌入式Tomcat

在Spring Boot中，嵌入式Tomcat由TomcatEmbeddedServletContainer类包装，该类的实例创建于TomcatEmbeddedServletContainerFactory。
TomcatEmbeddedServletContainerFactory类实现了EmbeddedServletContainerFactory接口，实现的接口方法如下：

@Override
public EmbeddedServletContainer getEmbeddedServletContainer(
        ServletContextInitializer... initializers) {
    Tomcat tomcat = new Tomcat();
    File baseDir = (this.baseDirectory != null ? this.baseDirectory
            : createTempDir("tomcat"));
    tomcat.setBaseDir(baseDir.getAbsolutePath());
    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 getTomcatEmbeddedServletContainer(tomcat);
}

protected TomcatEmbeddedServletContainer getTomcatEmbeddedServletContainer(
        Tomcat tomcat) {
    return new TomcatEmbeddedServletContainer(tomcat, getPort() >= 0);
}

• 首先新建Tomcat实例，然后设置工作目录，最后绑定并自定义Connector、Engine和Context等Tomcat组件，关于这些组件的功能可以参考笔者以前的Tomcat分析系列；
• getTomcatEmbeddedServletContainer方法返回包装有嵌入式Tomcat的TomcatEmbeddedServletContainer实例。