Servlet,Listener和Filter如何获取ServletContext(既application)

最新推荐文章于 2024-06-01 18:45:08 发布

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最新推荐文章于 2024-06-01 18:45:08 发布

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本文介绍了Java Web应用中Listener、Filter和Servlet如何通过各自的配置对象（如ServletContextEvent、FilterConfig和ServletConfig）与容器进行交互，从而获取项目上下文（ServletContext）的过程。

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Listener的项目上下文（既ServletContext既application）是从event中获取的，event是Listener和容器之间交流的中间人

  


public interface ServletContextListener extends EventListener {
	/**
	 ** Notification that the web application initialization
	 ** process is starting.
	 ** All ServletContextListeners are notified of context
	 ** initialization before any filter or servlet in the web
	 ** application is initialized.
	 */

    public void contextInitialized ( ServletContextEvent sce );



--------------------------------------------

 ServletContext servletContext;


   public void contextInitialized(ServletContextEvent sce)
   {
      servletContext = sce.getServletContext();
   }

而Filter的项目上下文（既ServletContext既application）是从FilterConfig中获取的，FilterConfig是Filter和容器之间交流的中间人

  

public interface Filter {

	/** 
	* Called by the web container to indicate to a filter that it is being placed into
	* service. The servlet container calls the init method exactly once after instantiating the
	* filter. The init method must complete successfully before the filter is asked to do any
	* filtering work. <br><br>

     	* The web container cannot place the filter into service if the init method either<br>
        * 1.Throws a ServletException <br>
        * 2.Does not return within a time period defined by the web container 
	*/
	public void init(FilterConfig filterConfig) throws ServletException;
	


------------------------
filterConfig.getServletContext()

而Servlet的项目上下文（既ServletContext既application）是从ServletConfig中获取的，ServletConfig是Servlet和容器之间交流的中间人

public interface Servlet {

    /**
     * Called by the servlet container to indicate to a servlet that the 
     * servlet is being placed into service.
     *
     * <p>The servlet container calls the <code>init</code>
     * method exactly once after instantiating the servlet.
     * The <code>init</code> method must complete successfully
     * before the servlet can receive any requests.
     *
     * <p>The servlet container cannot place the servlet into service
     * if the <code>init</code> method
     * <ol>
     * <li>Throws a <code>ServletException</code>
     * <li>Does not return within a time period defined by the Web server
     * </ol>
     *
     *
     * @param config			a <code>ServletConfig</code> object 
     *					containing the servlet's
     * 					configuration and initialization parameters
     *
     * @exception ServletException 	if an exception has occurred that
     *					interferes with the servlet's normal
     *					operation
     *
     * @see 				UnavailableException
     * @see 				#getServletConfig
     *
     */

    public void init(ServletConfig config) throws ServletException;
    
---------------------------------------

 getServletConfig().getServletContext()

我们的应用程序组件只能被动的遵守一定的规则，和容器打交道，和其他组件通信，也必须借助于容器的力量。这里面其实已经有一点控制反转的味道，既然是组件生活在容器中，就必须被动的接受容器喂给他吃的东西，不能(要)自己创造（new）。

Spring之所以称为容器（号称轻量级），就是因为被他控制的组件，被动的吃他喂过来的东西，不能(要)自己创造（new）。