Spring Boot内嵌容器

       使用Spring Boot工程时默认web容器是Tomcat，但是可以根据需要进行修改，例如使用Jetty或者Undertow。只需要排除spring-boot-starter-web包中的spring-boot-starter-tomcat，然后添加其它的容器即可，例如spring-boot-starter-jetty。
      不用修改任何代码即可完成容器的切换，那么Spring Boot到底是如何实现的呢？关键在于EmbeddedServletContainerAutoConfiguration这个自动注解类：

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication
@Import(BeanPostProcessorsRegistrar.class)
public class EmbeddedServletContainerAutoConfiguration {

	/**
	 * Nested configuration if Tomcat is being used.
	 */
	@Configuration
	@ConditionalOnClass({ Servlet.class, Tomcat.class })
	@ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedTomcat {

		@Bean
		public TomcatEmbeddedServletContainerFactory tomcatEmbeddedServletContainerFactory() {
			return new TomcatEmbeddedServletContainerFactory();
		}
	}

	/**
	 * Nested configuration if Jetty is being used.
	 */
	@Configuration
	@ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
			WebAppContext.class })
	@ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedJetty {

		@Bean
		public JettyEmbeddedServletContainerFactory jettyEmbeddedServletContainerFactory() {
			return new JettyEmbeddedServletContainerFactory();
		}
	}

	/**
	 * Nested configuration if Undertow is being used.
	 */
	@Configuration
	@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
	@ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedUndertow {

		@Bean
		public UndertowEmbeddedServletContainerFactory undertowEmbeddedServletContainerFactory() {
			return new UndertowEmbeddedServletContainerFactory();
		}
	}

      @ConditionalOnClass 表示对应的类在classpath目录下存在时，才会实例化一个Bean；@ConditionalOnMisssingClass表示对应的类在classpath目录下找不到，才会实例化一个Bean。也就是说Spring Boot是根据在classpath下扫描到的容器类的类型决定使用哪个web容器。
      当引入了Undertow容器的jar包，classpath目录下一定会有Undertow相关的class，满足限制条件，那么Spring Boot将使用UndertowEmbeddedServletContainerFactory来生成Undertow容器。

    那是怎么调用到这的呢？Spring中很核心的方法是AbstractApplicationContext#refresh这个方法：

	@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();
			}
                        //...
		}
	}

      其中的onRefresh()实际调用的是EmbeddedWebApplicationContext的onRefresh(),EmbeddedWebApplicationContext是AnnotationConfigEmbeddedWebApplicationContext的父类。

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

      在createEmbeddedServletContainer()方法中：

	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();
	}

      getEmbeddedServletContainerFactory()即从beanFactory获取EmbeddedServletContainerFactory.class类型的bean，调用getEmbeddedServletContainer()方法启动容器。