Spring Boot嵌入式Servlet容器启动原理

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 2019/04/21   Share

嵌入式Servlet容器启动原理

前面我们总结了Spring Boot中嵌入式Servlet容器自动配置原理，也提到了Spring Boot2.0版本中使用ServletWebServerFactoryConfiguration替换了EmbeddedServletContainerAutoConfiguration，那么什么时候创建嵌入式的Servlet容器工厂以及什么时候获取嵌入式的Servlet容器并启动Tomcat呢？我们进入到ServletWebServerFactoryConfiguration中查看。

获取嵌入式的Servlet容器从Spring Boot应用启动运行run()方法开始

以Tomcat为例，在new TomcatServletWebServerFactory()处打断点，以Debug方式运行。

Tomcat

我们发现第一步是从执行SpringBootRestfulcrudApplication的run()方法开始的。

SpringBootApplication

Spring Boot刷新IOC容器

Spring Boot刷新IOC容器，创建IOC容器对象，并且初始化容器，创建容器中每一个组件。

并进行判断，如果是web应用就创建AnnotationConfigServletWebServerApplicationContext，否则就创建默认的AnnotationConfigApplicationContext。此处Spring Boot2.0中也进行了更新，将原来的AnnotationConfigEmbeddedWebApplicationContext替换为AnnotationConfigServletWebServerApplicationContext。

刷新刚创建好的IOC容器

调用refresh()方法刷新刚刚创建好的IOC容器。

refresh

web的IOC容器重写onRefresh()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.
       //重写此处的onRefresh()方法
			onRefresh();

			// Check for listener beans and register them.
			registerListeners();

			// Instantiate all remaining (non-lazy-init) singletons.
			finishBeanFactoryInitialization(beanFactory);

			// Last step: publish corresponding event.
			finishRefresh();
		}

		catch (BeansException ex) {
			if (logger.isWarnEnabled()) {
				logger.warn("Exception encountered during context initialization - " +
						"cancelling refresh attempt: " + ex);
			}

			// Destroy already created singletons to avoid dangling resources.
			destroyBeans();

			// Reset 'active' flag.
			cancelRefresh(ex);

			// Propagate exception to caller.
			throw ex;
		}

		finally {
			// Reset common introspection caches in Spring's core, since we
			// might not ever need metadata for singleton beans anymore...
			resetCommonCaches();
		}
	}
}

web的IOC容器会创建嵌入式的Servlet容器

上一步会调用此处的createWebServer()方法。

private void createWebServer() {
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		if (webServer == null && servletContext == null) {
			ServletWebServerFactory factory = getWebServerFactory();
			this.webServer = factory.getWebServer(getSelfInitializer());
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context",
						ex);
			}
		}
		initPropertySources();
	}