springMVC源码分析--容器初始化（一）ContextLoaderListener

在spring Web中，需要初始化IOC容器，用于存放我们注入的各种对象。当tomcat启动时首先会初始化一个web对应的IOC容器，用于初始化和注入各种我们在web运行过程中需要的对象。当tomcat启动的时候是如何初始化IOC容器的，我们先看一下在web.xml中经常看到的配置：

<context-param>
	<param-name>contextConfigLocation</param-name>
	<param-value>
		classpath:applicationContext.xml
	</param-value>
</context-param>
<listener>
	<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

ContextLoaderListener是一个监听器，其实现了ServletContextListener接口，其用来监听Servlet，当tomcat启动时会初始化一个Servlet容器，这样ContextLoaderListener会监听到Servlet的初始化，这样在Servlet初始化之后我们就可以在ContextLoaderListener中也进行一些初始化操作。看下面的ServletContextListener的源码也是比较简单的，ServletContextListener实现了ServletContextListener接口，所以会有两个方法contextInitialized和contextDestroyed。web容器初始化时会调用方法contextInitialized，web容器销毁时会调用方法contextDestroyed。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

	public ContextLoaderListener() {
	}

	public ContextLoaderListener(WebApplicationContext context) {
		super(context);
	}
	/**
	 * Initialize the root web application context.
	 */
	@Override
	public void contextInitialized(ServletContextEvent event) {
		//在父类ContextLoader中实现
		initWebApplicationContext(event.getServletContext());
	}
	/**
	 * Close the root web application context.
	 */
	@Override
	public void contextDestroyed(ServletContextEvent event) {
		closeWebApplicationContext(event.getServletContext());
		ContextCleanupListener.cleanupAttributes(event.getServletContext());
	}

}

ContextLoaderListener的方法contextInitialized（）的默认实现是在他的父类ContextLoader的initWebApplicationContext方法中实现的，意思就是初始化web应用上下文。他的主要流程就是创建一个IOC容器，并将创建的IOC容器存到servletContext中，ContextLoader的核心实现如下：

initWebApplicationContext函数：

//初始化WebApplicationContext，IOC容器
	public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
		//判断web容器中是否已经有WebApplicationContext
		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!");
		}

		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 {
			// Store context in local instance variable, to guarantee that
			// it is available on ServletContext shutdown.
			//创建WebApplicationContext
			if (this.context == null) {
				//最终得到的是XmlWebApplicationContext
				this.context = createWebApplicationContext(servletContext);
			}
			if (this.context instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
				//判断应用上下文是否激活
				if (!cwac.isActive()) {
					// The context has not yet been refreshed -> provide services such as
					// setting the parent context, setting the application context id, etc
					//判断是否已经有父容器
					if (cwac.getParent() == null) {
						// The context instance was injected without an explicit parent ->
						// determine parent for root web application context, if any.
						//获得父容器
						ApplicationContext parent = loadParentContext(servletContext);
						cwac.setParent(parent);
					}
					//设置并刷新WebApplicationContext容器
					configureAndRefreshWebApplicationContext(cwac, servletContext);
				}
			}
			//将初始化的WebApplicationContext设置到servletContext中
			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");
			}
			//初始化 WebApplicationContext完成并返回
			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;
		}
	}

springMVC父容器初始化流程图（盗图）如下：

完整的ContextLoader源码如下：

/**
 * 用于初始化ApplicationContext
 *
 */
public class ContextLoader {

	//WebApplicationContext id
	public static final String CONTEXT_ID_PARAM = "contextId";

	//配置文件
	public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";

	public static final String CONTEXT_CLASS_PARAM = "contextClass";

	public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses";

	public static final String GLOBAL_INITIALIZER_CLASSES_PARAM = "globalInitializerClasses";

	public static final String LOCATOR_FACTORY_SELECTOR_PARAM = "locatorFactorySelector";

	public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey";

	private static final String INIT_PARAM_DELIMITERS = ",; \t\n";

	private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";

	private static final Properties defaultStrategies;

	static {
		// Load default strategy implementations from properties file.
		// This is currently strictly internal and not meant to be customized
		// by application developers.
		try {
			ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
			defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
		}
		catch (IOException ex) {
			throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
		}
	}
	private static final Map<ClassLoader, WebApplicationContext> currentContextPerThread =
			new ConcurrentHashMap<ClassLoader, WebApplicationContext>(1);

	private static volatile WebApplicationContext currentContext;

	private WebApplicationContext context;

	private BeanFactoryReference parentContextRef;

	private final List<ApplicationContextInitializer<ConfigurableApplicationContext>> contextInitializers =
			new ArrayList<ApplicationContextInitializer<ConfigurableApplicationContext>>();

	public ContextLoader() {
	}

	public ContextLoader(WebApplicationContext context) {
		this.context = context;
	}

	@SuppressWarnings("unchecked")
	public void setContextInitializers(ApplicationContextInitializer<?>... initializers) {
		if (initializers != null) {
			for (ApplicationContextInitializer<?> initializer : initializers) {
				this.contextInitializers.add((ApplicationContextInitializer<ConfigurableApplicationContext>) initializer);
			}
		}
	}

	//初始化WebApplicationContext，IOC容器
	public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
		//判断web容器中是否已经有WebApplicationContext
		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!");
		}

		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 {
			// Store context in local instance variable, to guarantee that
			// it is available on ServletContext shutdown.
			//创建WebApplicationContext
			if (this.context == null) {
				//最终得到的是XmlWebApplicationContext
				this.context = createWebApplicationContext(servletContext);
			}
			if (this.context instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
				//判断应用上下文是否激活
				if (!cwac.isActive()) {
					// The context has not yet been refreshed -> provide services such as
					// setting the parent context, setting the application context id, etc
					//判断是否已经有父容器
					if (cwac.getParent() == null) {
						// The context instance was injected without an explicit parent ->
						// determine parent for root web application context, if any.
						//获得父容器
						ApplicationContext parent = loadParentContext(servletContext);
						cwac.setParent(parent);
					}
					//设置并刷新WebApplicationContext容器
					configureAndRefreshWebApplicationContext(cwac, servletContext);
				}
			}
			//将初始化的WebApplicationContext设置到servletContext中
			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");
			}
			//初始化 WebApplicationContext完成并返回
			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;
		}
	}

	//在web容器加载的时候初始化根WebApplicationContext
	protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
		//通过反射获得上下文类
		Class<?> contextClass = determineContextClass(sc);
		if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
			throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
					"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
		}
		//初始化WebApplicationContext，是其实现类XmlWebApplicationContext
		return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
	}

	protected Class<?> determineContextClass(ServletContext servletContext) {
		//查看servlet是否已经存在应用上下文类名
		String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
		if (contextClassName != null) {
			try {
				//类名不为空则反射获得类
				return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
			}
			catch (ClassNotFoundException ex) {
				throw new ApplicationContextException(
						"Failed to load custom context class [" + contextClassName + "]", ex);
			}
		}
		else {
			//如果应用上下文类名为空，则获得默认的context上下文名
			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);
			}
		}
	}

	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);
			//设置应用上下文id,在web.xml中可以进行配置
			if (idParam != null) {
				wac.setId(idParam);
			}
			else {
				// Generate default id...
				//如果没有配置则设置默认名称
				wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
						ObjectUtils.getDisplayString(sc.getContextPath()));
			}
		}
		//设置ServletContext到容器中
		wac.setServletContext(sc);
		//获得web.xml中配置的contextConfigLocation的值，一般是classpath:applicationContext.xml
		String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
		if (configLocationParam != null) {
			//设置加载文件地址
			wac.setConfigLocation(configLocationParam);
		}

		// 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);
		//这一步是特别关键的，这样application.xml中的所有配置都会被初始化到XMLWebApplication中
		wac.refresh();
	}

	protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
		List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
				determineContextInitializerClasses(sc);

		for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
			Class<?> initializerContextClass =
					GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
			if (initializerContextClass != null) {
				Assert.isAssignable(initializerContextClass, wac.getClass(), String.format(
						"Could not add context initializer [%s] since its generic parameter [%s] " +
						"is not assignable from the type of application context used by this " +
						"context loader [%s]: ", initializerClass.getName(), initializerContextClass.getName(),
						wac.getClass().getName()));
			}
			this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));
		}

		AnnotationAwareOrderComparator.sort(this.contextInitializers);
		for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {
			initializer.initialize(wac);
		}
	}

	protected List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>
			determineContextInitializerClasses(ServletContext servletContext) {

		List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> classes =
				new ArrayList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>();

		String globalClassNames = servletContext.getInitParameter(GLOBAL_INITIALIZER_CLASSES_PARAM);
		if (globalClassNames != null) {
			for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) {
				classes.add(loadInitializerClass(className));
			}
		}

		String localClassNames = servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM);
		if (localClassNames != null) {
			for (String className : StringUtils.tokenizeToStringArray(localClassNames, INIT_PARAM_DELIMITERS)) {
				classes.add(loadInitializerClass(className));
			}
		}

		return classes;
	}

	@SuppressWarnings("unchecked")
	private Class<ApplicationContextInitializer<ConfigurableApplicationContext>> loadInitializerClass(String className) {
		try {
			Class<?> clazz = ClassUtils.forName(className, ClassUtils.getDefaultClassLoader());
			Assert.isAssignable(ApplicationContextInitializer.class, clazz);
			return (Class<ApplicationContextInitializer<ConfigurableApplicationContext>>) clazz;
		}
		catch (ClassNotFoundException ex) {
			throw new ApplicationContextException("Failed to load context initializer class [" + className + "]", ex);
		}
	}

	//从ServletContext中获取父容器
	protected ApplicationContext loadParentContext(ServletContext servletContext) {
		ApplicationContext parentContext = null;
		String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM);
		String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM);

		if (parentContextKey != null) {
			// locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml"
			BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
			Log logger = LogFactory.getLog(ContextLoader.class);
			if (logger.isDebugEnabled()) {
				logger.debug("Getting parent context definition: using parent context key of '" +
						parentContextKey + "' with BeanFactoryLocator");
			}
			this.parentContextRef = locator.useBeanFactory(parentContextKey);
			parentContext = (ApplicationContext) this.parentContextRef.getFactory();
		}

		return parentContext;
	}

	//通过给定的ServletContext来关闭IOC容器
	public void closeWebApplicationContext(ServletContext servletContext) {
		servletContext.log("Closing Spring root WebApplicationContext");
		try {
			if (this.context instanceof ConfigurableWebApplicationContext) {
				((ConfigurableWebApplicationContext) this.context).close();
			}
		}
		finally {
			ClassLoader ccl = Thread.currentThread().getContextClassLoader();
			if (ccl == ContextLoader.class.getClassLoader()) {
				currentContext = null;
			}
			else if (ccl != null) {
				currentContextPerThread.remove(ccl);
			}
			servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
			if (this.parentContextRef != null) {
				this.parentContextRef.release();
			}
		}
	}

	//在当前线程中获取WebApplicationContext
	public static WebApplicationContext getCurrentWebApplicationContext() {
		ClassLoader ccl = Thread.currentThread().getContextClassLoader();
		if (ccl != null) {
			WebApplicationContext ccpt = currentContextPerThread.get(ccl);
			if (ccpt != null) {
				return ccpt;
			}
		}
		return currentContext;
	}

}

http://blog.csdn.net/qq924862077/article/details/52769754