Spring源码分析 从web.xml 中ContextLoaderListener看Spring 容器与web容器联系

Spring IOC容器如何与web容器创建联系，使得在web环境下能运用Spring 容器去管理对象，这要从web.xml配置文件中的ContextLoaderListener提及。它是Spring容器与web容器创建联系的入口，这里就先抛砖引玉啦。

先来看看web.xml配置文件中的

<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>
			classpath:spring/spring-context.xml,
			classpath:spring/spring-datasource.xml,
			classpath:spring/spring-context-shiro.xml
		</param-value>
	</context-param>
	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>

这个你们一字很熟悉，这即是Spring容器与web容器创建联系的入口。来看看ContextLoaderListener的源码

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
	public ContextLoaderListener() {
	}
	public ContextLoaderListener(WebApplicationContext context) {
		super(context);
	}
	@Override
	public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}
	@Override
	public void contextDestroyed(ServletContextEvent event) {
		closeWebApplicationContext(event.getServletContext());
		ContextCleanupListener.cleanupAttributes(event.getServletContext());
	}
}

能够看到其实现了ServletContextListener 接口，这样即可以从web.xml文件中加载ServletContext一些配置信息，兼听ServletContext上下文的变化状况。ServletContextListener 有两个方法contextInitialized 与contextDestroyed，contextInitialized 在容器启动时调用，contextDestroyed在容器关闭前调用。 能够看到在ContextLoaderListener中 contextInitialized 内部调用了父类ContextLoader 的initWebApplicationContext方法，这即是Spring容器初始化的入口。

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
	//判断是否已初始化过Spring容器，若是有抛出异常
	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) {
                               //建立Spring容器，默认状况下为XmlWebApplicationContext
				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);
					}
                                        //配置Spring容器，加载servletContext的一些配置文件
					configureAndRefreshWebApplicationContext(cwac, servletContext);
				}
			}
//将WebApplicationContext对象保存在ServletContext上下文中以便使用
			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;
		}

这里忽视一些细节，重点看看initWebApplicationContext方法中

1. this.context = createWebApplicationContext(servletContext);
2. configureAndRefreshWebApplicationContext(cwac, servletContext); 3.servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

createWebApplicationContext //建立Spring容器，默认状况下为XmlWebApplicationContext this.context = createWebApplicationContext(servletContext) context是ContextLoader的成员变量，其类型为WebApplicationContext，WebApplicationContext续承ApplicationContext为Spring的容器。 再来看看源码

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
	//采用什么类型的FactoryBean建立Spring 容器
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);
	}

再看determineContextClass源码

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);
			}
		}
	}

默认状况下在web.xml中不会设置SevletContext的contextClass参数，便根据defaultStrategies.getProperty(WebApplicationContext.class.getName())查找Spring容器对应的实现类。 defaultStrategies为ContextLoader的类型为Properties的静态成员变量，ContextLoader内有一个静态初始化块对其进行初始化。

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());
		}
	}

ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class)

这一行代码能够看出，初始化块将找DEFAULT_STRATEGIES_PATH这个路径的文件，其实是ClassPathResource将到ContextLoader包对应的路径下面找到ContextLoader.properties文件。

再打ContextLoader.properties看看其内容：

能够看到前面说的Spring容器的默认实现类XmlWebApplicationContext，最后这个经过反射createWebApplicationContext将返回XmlWebApplicationContext的对象给ContextLoader的成员变量context。 到这里**1. this.context = createWebApplicationContext(servletContext);**分析结束。

再来看看**2. configureAndRefreshWebApplicationContext(cwac, servletContext);**的分析 configureAndRefreshWebApplicationContext源码：

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
		if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
			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);
                //获得web.xml中ServletContext参数contextConfigLocation对应的配置信息
		String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
		if (configLocationParam != null) {
                        //将配置信息进行解析并设置
			wac.setConfigLocation(configLocationParam);
		}
		ConfigurableEnvironment env = wac.getEnvironment();
		if (env instanceof ConfigurableWebEnvironment) {
			((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
		}
		customizeContext(sc, wac);
                //Spring容器真正初始化的地方
		wac.refresh();
	}

String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);

这行代码将获得在web.xml中的Spring配置文件的信息，就是一开始 的web.xml的这部分

<context-param>
 		<param-name>contextConfigLocation</param-name>
		<param-value>
			classpath:spring/spring-context.xml,
			classpath:spring/spring-datasource.xml,
			classpath:spring/spring-context-shiro.xml
		</param-value>
	</context-param>

再来分析一下**wac.setConfigLocation(configLocationParam)**这行代码 从上面的分析知道默认状况下wac为XmlWebApplicationContext类型的对象。 在XmlWebApplicationContext的父类AbstractRefreshableWebApplicationContext实现了ConfigurableWebApplicationContext接口，而另外一方面AbstractRefreshableWebApplicationContext又续承了AbstractRefreshableConfigApplicationContext类。AbstractRefreshableConfigApplicationContext类里面实现了setConfigLocation方法。

public void setConfigLocation(String location) {
		setConfigLocations(StringUtils.tokenizeToStringArray(location, CONFIG_LOCATION_DELIMITERS));
	}

String CONFIG_LOCATION_DELIMITERS = ",; \t\n"; 从这里能够看出，在web.xml里面设置contextConfigLocation的param-value时，能够用",; \t\n"来分隔多个配置文件。

public void setConfigLocations(String... locations) {
		if (locations != null) {
			Assert.noNullElements(locations, "Config locations must not be null");
			this.configLocations = new String[locations.length];
			for (int i = 0; i < locations.length; i++) {
				this.configLocations[i] = resolvePath(locations[i]).trim();
			}
		}
		else {
			this.configLocations = null;
		}
	}

再回到configureAndRefreshWebApplicationContext方法中wac.refresh()这一行，这才是Spring容器真正初始化的地方，因为后面涉及的东西比较多将在后面专门分析一下。 到这里2. configureAndRefreshWebApplicationContext(cwac, servletContext);的分析结束。 再来看看3.servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context); 这个比较简单就是将上面用反射方法建立的XmlWebApplicationContext对象保存在ServletContext上下文中。因为本人水平有限不正确的地方，欢迎你们指正。