spring 源码阅读之 WebApplicationContext 初始化

web.xml

<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>

public class ContextLoaderListener extends ContextLoader

private ContextLoader contextLoader;

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

	@Deprecated
	protected ContextLoader createContextLoader() {
		return null;
	}

public class ContextLoader

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!");
		}

		long startTime = System.currentTimeMillis();

		try {
			// Determine parent for root web application context, if any.
			ApplicationContext parent = loadParentContext(servletContext);// 这个地方一般会为null


			// Store context in local instance variable, to guarantee that
			// it is available on ServletContext shutdown.
			this.context = createWebApplicationContext(servletContext, parent);
			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
			currentContextPerThread.put(Thread.currentThread().getContextClassLoader(), 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;
		}
	}


	protected ApplicationContext loadParentContext(ServletContext servletContext) {
		ApplicationContext parentContext = null;
		String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM);
		String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM);
//   public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey"; // 一般都没在web.xml设这个参数 所以最后会返回null


		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;
	}


	protected WebApplicationContext createWebApplicationContext(ServletContext sc, ApplicationContext parent) {
		Class<?> contextClass = determineContextClass(sc);// 这里要好好看下

		if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
			throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
					"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
		}// 看下contextClass 是不是 ConfigurableWebApplicationContext的子类

		ConfigurableWebApplicationContext wac =
				(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

		// Assign the best possible id value.
		if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {
			// Servlet <= 2.4: resort to name specified in web.xml, if any.
			String servletContextName = sc.getServletContextName();
			if (servletContextName != null) {
				wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + servletContextName);
			}
			else {
				wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX);
			}
		}
		else {
			// Servlet 2.5's getContextPath available!
			wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + sc.getContextPath());
		}

		wac.setParent(parent);
		wac.setServletContext(sc);
		wac.setConfigLocation(sc.getInitParameter(CONFIG_LOCATION_PARAM));
		customizeContext(sc, wac);
		wac.refresh();
		return wac;
	}


	protected Class<?> determineContextClass(ServletContext servletContext) {
		String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
//    public static final String CONTEXT_CLASS_PARAM = "contextClass"; //你想要加载自己的扩张applicationContext类的话可以把类名写在这个参数里 

		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);
			}
		}
	}


	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);
//   private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";  
//我们没有在类路径放这个文件啊 哦 原来他在spring-web.jar 的  org/springframework/web/context/ContextLoader.properties

			defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
		}
		catch (IOException ex) {
			throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
		}
	}

ContextLoader.properties

# 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

所以最后加载出来的beanFactory 是XmlWebApplicationContext