ServletContextListener 详解

1.首先来看一看源码 该类的源码

public interface ServletContextListener extends EventListener {

    /**
     * Receives notification that the web application initialization
     * process is starting.
     *
     * <p>All ServletContextListeners are notified of context
     * initialization before any filters or servlets in the web
     * application are initialized.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being initialized
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     * Receives notification that the ServletContext is about to be
     * shut down.
     *
     * <p>All servlets and filters will have been destroyed before any
     * ServletContextListeners are notified of context
     * destruction.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being destroyed
     */
    public void contextDestroyed(ServletContextEvent sce);
}

此接口中提供了两个方法，用于监听ServletContext  的创建和销毁，也就是监听ServletContext 的生命周期，可以说成是监听Web 应用的生命周期，当web应用启动后，就会触发ServletContextEvent 事件 当此事件执行时，就会被ServletContextListener 监听器监听到，会调用他的 contextInitialized(ServletContextEvent sce)  方法，通过sce 可以获取ServletContext 实例，初始化一些数据，例如缓存的应用，如，创建数据库连接，读取数据库数据，通过setAttribute(“”,obj) 方法设置数据，然后就是可通过servlet 获取servletContext 的实例，通过getAttribute("") 获取设置的数据

实现代码：

public class MyContextListener implements ServletContextListener {
    private ServletContext context = null;

    public void contextInitialized(ServletContextEvent event) {
        context = event.getServletContext();
        User user = DatabaseManager.getUserById(1);
        context.setAttribute("user1", user);
    }

    public void contextDestroyed(ServletContextEvent event) {
        User user = (User)context.getAttribute("user1");
        DatabaseManager.updateUserData(user);
        this.context = null;
    }
}

如果是web 项目 最后一步是使 ServletContext 生效，需要在web.xml 中配置监听器，并且web.xml　把它放在正确的WEB-INF/classes目录下，

<listener>
    <listener-class>MyServletContextListener</listener-class>
</listener>