在web应用启动时,会去web.xml读取配置信息,并由Web容器实例化ContextLoaderListener。当ServletContext被初始化创建完毕后,会调用监听里的contextInitialized方法,
public void contextInitialized(ServletContextEvent event) { this.contextLoader = createContextLoader(); if (this.contextLoader == null) { this.contextLoader = this; } this.contextLoader.initWebApplicationContext(event.getServletContext()); }
这个方法中就对WebApplicationContext进行初始化操作。
// Store context in local instance variable, to guarantee that // it is available on ServletContext shutdown. if (this.context == null) {//context为空,则去创建一个context. 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);
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); }
这个方法会去获得我们创建的context应该是哪个具体类的实例。
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); } } }
可以看出,当我们的指定了ContextClass时,则使用我们所指定的。一般我们都没有去指定,所以为空,这样就会去获取默认的类名。defaultStrategies是在这个类的静态代码块中。
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()); } }
可以看出会去加载一个资源文件
private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
所以我们最后得到的结论是,webApplicationContext的实例是一个XmlWebApplicationContext的示例。