IOC容器启动的基本过程

本节一起学习IOC容器的启动过程

IOC容器的启动过程就是建立上下文的过程。该上下文是与ServletContext相伴而生的，同时也是IOC容器在web环境中的具体表现之一。由ContextLoaderListener启动的上下文为跟上下文。在根上下文的基础上，还有一个与Web MVC 相关的上下文用来保存控制器DispatcherServlet需要的MVC对象。在web容器中启动Spring 应用程序时，首先建立根上下文，然后建立上下文体系，这个上下文体系的建立由ContextLoader完成的。

在web.xml中，已经配置了ContextLoaderListener，这个ContextLoaderListener是为在web容器中建立IOC容器服务的，它实现了ServletContextListener接口。这个接口是在Servlet API中定义的，提供了与Servlet生命周期结合的回调，比如contextInitialized方法和contextDestroyed方法。而在web容器中，建立WebApplicationContext的过程，是在contextInitialized的接口实现中完成的。具体的载入IOC容器的过程由ContextLoaderListener交给ContextLoader完成的，ContextLoader本身就是ContextLoaderListener的基类。

在ContextLoader中，完成了两个IOC容器建立的基本过程，一个是在web容器中建立双亲IOC容器，一个是生成相应的WebApplicationContext并将其初始化。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

	
	public ContextLoaderListener() {
	}

	public ContextLoaderListener(WebApplicationContext context) {
		super(context);
	}


	/**
	 * 容器初始化，具体的操作交给基类处理
	 */
	@Override
	public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}


	/**
	 * Close the root web application context.
	 */
	@Override
	public void contextDestroyed(ServletContextEvent event) {
		closeWebApplicationContext(event.getServletContext());
		ContextCleanupListener.cleanupAttributes(event.getServletContext());
	}

}

	public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
		if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
			throw new IllegalStateException(
					"Cannot initialize context because there is already a root application context present - " +
					"check whether you have multiple ContextLoader* definitions in your web.xml!");
		}//判断在ServletContext中是否已经有上下文存在，如果存在，则异常

		Log logger = LogFactory.getLog(ContextLoader.class);
		servletContext.log("Initializing Spring root WebApplicationContext");
		if (logger.isInfoEnabled()) {
			logger.info("Root WebApplicationContext: initialization started");
		}
		long startTime = System.currentTimeMillis();

		try {
			// 创建根上下文，并存储到servletContext
			if (this.context == null) {
				this.context = createWebApplicationContext(servletContext);
			}
			if (this.context instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
				if (!cwac.isActive()) {
					// The context has not yet been refreshed -> provide services such as
					// setting the parent context, setting the application context id, etc
					if (cwac.getParent() == null) {
						// 载入根上下文的双亲上下文
						ApplicationContext parent = loadParentContext(servletContext);
						cwac.setParent(parent);
					}
					configureAndRefreshWebApplicationContext(cwac, servletContext);
				}
			}
			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

			ClassLoader ccl = Thread.currentThread().getContextClassLoader();
			if (ccl == ContextLoader.class.getClassLoader()) {
				currentContext = this.context;
			}
			else if (ccl != null) {
				currentContextPerThread.put(ccl, this.context);
			}

			if (logger.isDebugEnabled()) {
				logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
						WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
			}
			if (logger.isInfoEnabled()) {
				long elapsedTime = System.currentTimeMillis() - startTime;
				logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
			}

			return this.context;
		}
		catch (RuntimeException ex) {
			logger.error("Context initialization failed", ex);
			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
			throw ex;
		}
		catch (Error err) {
			logger.error("Context initialization failed", err);
			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
			throw err;
		}
	}

创建根上下文createWebApplicationContext方法的源码：

	protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
		Class contextClass = determineContextClass(sc);//这里判断使用什么样的类在web容器中作为IOC容器
		if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
			throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
					"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
		}//实例化需要产生的IOC容器
		return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
	}

determineContextClass方法确定使用什么样的类作为上下文，如果没有指定特定的IOC容器，则使用XmlWebApplicationContext对象作为在Web环境中使用的IOC容器

	protected Class determineContextClass(ServletContext servletContext) {
		String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
		if (contextClassName != null) {
			try {
				return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
			}
			catch (ClassNotFoundException ex) {
				throw new ApplicationContextException(
						"Failed to load custom context class [" + contextClassName + "]", ex);
			}
		}
		else {
			contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
			try {
				return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
			}
			catch (ClassNotFoundException ex) {
				throw new ApplicationContextException(
						"Failed to load default context class [" + contextClassName + "]", ex);
			}
		}
	}

configureAndRefreshWebApplicationContext用来设置IOC容器的属性以及refresh启动容器的初始化

	protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
		if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
			// The application context id is still set to its original default value
			// -> assign a more useful id based on available information
			String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
			if (idParam != null) {
				wac.setId(idParam);
			}
			else {
				// Generate default id...
				wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
						ObjectUtils.getDisplayString(sc.getContextPath()));
			}
		}
                //设置servletcontext
		wac.setServletContext(sc);
		String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
		if (configLocationParam != null) {
			wac.setConfigLocation(configLocationParam);//设置配置文件的位置参数
		}

		// The wac environment's #initPropertySources will be called in any case when the context
		// is refreshed; do it eagerly here to ensure servlet property sources are in place for
		// use in any post-processing or initialization that occurs below prior to #refresh
		ConfigurableEnvironment env = wac.getEnvironment();
		if (env instanceof ConfigurableWebEnvironment) {
			((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
		}

		customizeContext(sc, wac);
		wac.refresh();//启动容器的初始化
	}