Purpose of ContextLoaderListener in Spring

The ApplicationContext is where your Spring beans live. The purpose of the ContextLoaderListener is two-fold:

1. to tie the lifecycle of the ApplicationContext to the lifecycle of the ServletContext and

2. to automate the creation of the ApplicationContext, so you don't have to write explicit code to do create it - it's a convenience function.

Another convenient thing about the ContextLoaderListener is that it creates a WebApplicationContext and WebApplicationContext provides

access to the ServletContextvia ServletContextAware beans and the getServletContext method.

ServletContext定义了一些方法方便Servlet和Servlet容器进行通讯，在一个web应用中所有的Servlet都共用一个ServletContext，Spring在和web应用结合使用的时候，

是将Spring的容器存到ServletContext中的；而ServletContextListener用于监听ServletContext一些事件。

Bootstrap listener to start up and shut down Spring's root WebApplicationContext. Simply delegates to ContextLoader as well as to ContextCleanupListener.

 

1.分析

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    /**
     * Initialize the root web application context.
     */
    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }

    /**
     * Close the root web application context.
     */
    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }

在initWebApplicationContext里面有详细信息

            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);

由最后一句可知，WeApplicationContext的实例，存放在servletContext中。

文件内容：

# 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