启动spring上下文容器的前奏

ContextLoaderListener ： 用来创建和销毁spring的根WebApplicationContext的, 继承了ContextLoader类，实现了ServletContextListener接口。

当服务器的ServletContext创建好后就会触发ServletContextEvent事件，ContextLoaderListener中的这个方法就会开始执行。

public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(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!");
		}

		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.
			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) {
						// The context instance was injected without an explicit parent ->
						// determine parent for root web application context, if any.
						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;
		}
	}

1.可以自定制WebApplicationContext

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
		Class<?> contextClass = determineContextClass(sc);
		if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
			throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
					"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
		}
		return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
	}

	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);
			}
		}
	}

ContextLoader中有一个定义了一个属性public static final String CONTEXT_CLASS_PARAM = "contextClass";
通过在web.xml文件中配置
<context-param>
    <param-name>contextClass</param-name>
    <param-value>context Class的路径</param-value>
  </context-param>
可以自定义spirng的根容器，没有定义的时候使用默认的spring上下文容器，比如XmlWebApplicationContext。

2.自定义初始化上下文容器

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()));
			}
		}

		wac.setServletContext(sc);
		String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
		if (configLocationParam != null) {
			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);
		wac.refresh();
	}

2.1

wac.setServletContext(sc)让spring上下文容器持有一个servletContext的引用，以便可以随时访问servletContext中的全局资源。

2.2

public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}

通过在web.xml中配置
<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>spring配置文件路径</param-value>
  </context-param>
获取用户的spring配置

2.3

protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
		List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
				determineContextInitializerClasses(sc);
		if (initializerClasses.isEmpty()) {
			// no ApplicationContextInitializers have been declared -> nothing to do
			return;
		}

		ArrayList<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerInstances =
				new ArrayList<ApplicationContextInitializer<ConfigurableApplicationContext>>();

		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()));
			}
			initializerInstances.add(BeanUtils.instantiateClass(initializerClass));
		}

		AnnotationAwareOrderComparator.sort(initializerInstances);
		for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : initializerInstances) {
			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;
	}

public static final String GLOBAL_INITIALIZER_CLASSES_PARAM = "globalInitializerClasses";
public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses";

只要在web.xml中配置
<context-param>
    <param-name>globalInitializerClasses</param-name>
    <param-value>类的路径</param-value>
</context-param>

<context-param>
    <param-name>contextInitializerClasses</param-name>
    <param-value>类的路径</param-value>
  </context-param>
这两个配置并没有发现有什么特别的区别

3.spring上下文容器的生命周期

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
这段代码让spring上下文容器的生命周期与servletContext的生命周期绑定，也就是说spring上下文容器的生命周期与服务器的生命周期一样。

4.运行时获取spring上下文容器

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

ContextLoader定义了一个属性和方法

private static final Map<ClassLoader, WebApplicationContext> currentContextPerThread =
			new ConcurrentHashMap<ClassLoader, WebApplicationContext>(1);

	public static WebApplicationContext getCurrentWebApplicationContext() {
		ClassLoader ccl = Thread.currentThread().getContextClassLoader();
		if (ccl != null) {
			WebApplicationContext ccpt = currentContextPerThread.get(ccl);
			if (ccpt != null) {
				return ccpt;
			}
		}
		return currentContext;
	}

属性currentContextPerThread 是一个以ClassLoader为key，WebApplicationContext为值的map，通过ContextLoader的静态方法getCurrentWebApplicationContext就可以在运行时得到spring上下文容器。