Spring ContextLoaderListener源碼分析 .

http://blog.csdn.net/hz_blog/article/details/7688921

我們要自動裝配ApplicationContext配置信息時候，首先在web.xml配置ContextLoaderListener，下面是部分源代碼：

public class ContextLoaderListener implements ServletContextListener {

    private ContextLoader contextLoader;


    /**
     * Initialize the root web application context.
     */
    public void contextInitialized(ServletContextEvent event) {
        this.contextLoader = createContextLoader();
        this.contextLoader.initWebApplicationContext(event.getServletContext());//注意此處
    }

    protected ContextLoader createContextLoader() {
        return new ContextLoader();
    }

    public ContextLoader getContextLoader() {
        return this.contextLoader;
    }

    public void contextDestroyed(ServletContextEvent event) {
        if (this.contextLoader != null) {
            this.contextLoader.closeWebApplicationContext(event.getServletContext());
        }
    }

}

我們主要了解下WebApplicationContext是怎麼初始化出來的，首先contextInitialized(ServletContextEvent event)方法的this.contextLoader.initWebApplicationContext(event.getServletContext());進去之後我們看到

public WebApplicationContext initWebApplicationContext(ServletContext servletContext)
        throws IllegalStateException, BeansException {

    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!");
    }

    servletContext.log("Initializing Spring root WebApplicationContext");
    if (logger.isInfoEnabled()) {
        logger.info("Root WebApplicationContext: initialization started");
    }
    long startTime = System.currentTimeMillis();

    try {
        // Determine parent for root web application context, if any.
        ApplicationContext parent = loadParentContext(servletContext);

        // 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);//注意此處
        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;
    }
}

注意注意此處部分，說明當我們要得到一個WebApplicationContext只需要在ServletContext中獲取屬性名爲WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE的常量值就可以得到一個ApplicationContext，最終可以獲取spring配置文件中任意一個Bean對象

eg:

public String execute() throws Exception {
    // TODO Auto-generated method stub
    ApplicationContext conn=(ApplicationContext)ServletActionContext.getServletContext().getAttribute(WebApplicationContext.class.getName() + ".ROOT");
    System.out.println(conn.getBean("biz"));
    return SUCCESS;
}

或者直接使用org.springframework.web.context.support.WebApplicationContextUtils這個抽象的工具類來獲得WebApplicationContext,該類的方法

public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
    return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
}

還有更多方法就不多介紹了，自行查看這個類的源碼。