spring Context 创建

  使用spring框架的时候，在web.xml的配置文件中都会加入如下注释：

<!-- spring -->
  	<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>/WEB-INF/spring/spring.xml</param-value>
	</context-param>
	
	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>

   从上面的配置文件可知，spring容器的加载和创建是从org.springframework.web.context.ContextLoaderListener开始的。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener

  ContextLoaderListener实现了ServletContextListener接口，大家都知道ServletContextListener是Servlet的监听器，监听ServletEvent，

在加载web.xml的时候，会首先加载Listener，ContextLoaderListener实现了ServletContextListener的抽象方法contextInitialized（），contextDestroyed（）

spring容器的创建就是在方法contextInitialized（）中完成的，如下是contextInitialized（）的代码：

public void contextInitialized(ServletContextEvent event) {
		this.contextLoader = createContextLoader();
		if (this.contextLoader == null) {
			this.contextLoader = this;
		}
		this.contextLoader.initWebApplicationContext(event.getServletContext());
	}

  createContextLoader（）方法已经废弃了，会返回一个null，所以实际的创建在initWebApplicationContext方法中完成。

  initWebApplicationContext方法的实现：

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.
                        //创建WebApplicationContext
			if (this.context == null) {
				this.context = createWebApplicationContext(servletContext);
			}
                        
			if (this.context instanceof ConfigurableWebApplicationContext) {//对context进行包装
				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);//设置父context
						cwac.setParent(parent);
					}
                                        //配置和刷新context
					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;
		}
	}

    首先会进入方法：createWebApplicationContext（）

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
                //决定究竟创建哪种类型的context
		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);
	}

  当前方法会进入determineContextClass（）方法

protected Class<?> determineContextClass(ServletContext servletContext) {
                //先获取web.xml中配置的CONTEXT_CLASS_PARAM即“contextClass”
		String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
		if (contextClassName != null) {//分支1
			try {
				return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
			}
			catch (ClassNotFoundException ex) {
				throw new ApplicationContextException(
						"Failed to load custom context class [" + contextClassName + "]", ex);
			}
		}
		else {//分支2
			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);
			}
		}
	}

    在web.xml中我很少会配置contextClass这个配置项，所以代码会走分支2，defaultStrategies的内容是什么呢？

    在ContextLoader有如下代码

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 {
                        //获取DEFAULT_STRATEGIES_PATH路径下的配置文件，然后加载此配置文件内容
			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());
		}
	}

    DEFAULT_STRATEGIES_PATH为ContextLoader.properties，即是org.springframework.web.ContextLoader.properties

   ContextLoader.properties内容为：

# Default WebApplicationContext implementation class for ContextLoader.
# Used as fallback when no explicit context implementation has been specified as context-param.
# Not meant to be customized by application developers.
 
org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

  所以determineContextClass（）方法的返回值是XmlWebApplicationContext.class

  所以在方法createWebApplicationContext（）中，根据XmlWebApplicationContext.class创建XmlWebApplicationContext对象

  现在回到方法initWebApplicationContext中去,

  然后会给context设置父context，然后配置和刷新context，现在进入方法configureAndRefreshWebApplicationContext（）

  方法configureAndRefreshWebApplicationContext

      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id
			if (idParam != null) {
				wac.setId(idParam);
			}
			else {
				// Generate default id...
				if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {
					// Servlet <= 2.4: resort to name specified in web.xml, if any.
					wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
							ObjectUtils.getDisplayString(sc.getServletContextName()));
				}
				else {
					wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
							ObjectUtils.getDisplayString(sc.getContextPath()));
				}
			}
		}

		wac.setServletContext(sc);
                //CONFIG_LOCATION_PARAM即是：“contextConfigLocation”即web.xml配置的contextConfigLocation，即spring配置文件的位置
		String initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);
		if (initParameter != null) {
			wac.setConfigLocation(initParameter);
		}
		customizeContext(sc, wac);
		wac.refresh();
	}

   customizeContext(sc, wac);和wac.refresh();做了什么？ 

customizeContext（sc, wac)代码为：

protected void customizeContext(ServletContext servletContext, ConfigurableWebApplicationContext applicationContext) {
		List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
				determineContextInitializerClasses(servletContext);
		if (initializerClasses.size() == 0) {
			// no ApplicationContextInitializers have been declared -> nothing to do
			return;
		}

		Class<?> contextClass = applicationContext.getClass();
		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, contextClass, String.format(
						"Could not add context initializer [%s] as its generic parameter [%s] " +
						"is not assignable from the type of application context used by this " +
						"context loader [%s]: ", initializerClass.getName(), initializerContextClass.getName(),
						contextClass.getName()));
			}
			initializerInstances.add(BeanUtils.instantiateClass(initializerClass));
		}

		ConfigurableEnvironment env = applicationContext.getEnvironment();
		if (env instanceof ConfigurableWebEnvironment) {
			((ConfigurableWebEnvironment)env).initPropertySources(servletContext, null);
		}

		Collections.sort(initializerInstances, new AnnotationAwareOrderComparator());
		for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : initializerInstances) {
			initializer.initialize(applicationContext);
		}
	}

 首先代码会进入方法determineContextInitializerClasses(servletContext); determineContextInitializerClasses方法代码为：

protected List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>
			determineContextInitializerClasses(ServletContext servletContext) {
		String classNames = servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM);
		List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> classes =
			new ArrayList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>();
		if (classNames != null) {
			for (String className : StringUtils.tokenizeToStringArray(classNames, ",")) {
				try {
					Class<?> clazz = ClassUtils.forName(className, ClassUtils.getDefaultClassLoader());
					Assert.isAssignable(ApplicationContextInitializer.class, clazz,
							"class [" + className + "] must implement ApplicationContextInitializer");
					classes.add((Class<ApplicationContextInitializer<ConfigurableApplicationContext>>)clazz);
				}
				catch (ClassNotFoundException ex) {
					throw new ApplicationContextException(
							"Failed to load context initializer class [" + className + "]", ex);
				}
			}
		}
		return classes;
	}

 如上所示：首先会从web.xml的配置文件里查找名称为CONTEXT_INITIALIZER_CLASSES_PARAM（即contextInitializerClasses）的配置项，我们一般不会配置这个配置项的，所以classNames为null,所以如上方法返回为一个空的list，再回到方法customizeContext（）中，list为空，所以方法customizeContext（）返回值为空。

那么我们就进入最后一个方法：wac.refresh();代码为：

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();
			}

			catch (BeansException ex) {
				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

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

				// Propagate exception to caller.
				throw ex;
			}
		}
	}

 由上面的代码可以知道，context的加载配置项，初始化，创建beanFactory均在此方法中完成。