ServletContextListener 詳解

1.首先來看一看源碼 該類的源碼

 

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);
}

 

此接口中提供了兩個方法，用於監聽ServletContext  的建立和銷燬，也就是監聽ServletContext 的生命週期，能夠說成是監聽Web 應用的生命週期，當web應用啓動後，就會觸發ServletContextEvent 事件 當此事件執行時，就會被ServletContextListener 監聽器監聽到，會調用他的 contextInitialized(ServletContextEvent sce)  方法，經過sce 能夠獲取ServletContext 實例，初始化一些數據，例如緩存的應用，如，建立數據庫鏈接，讀取數據庫數據，經過setAttribute(「」,obj) 方法設置數據，而後就是可經過servlet 獲取servletContext 的實例，經過getAttribute("") 獲取設置的數據

實現代碼：

 

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;
    }
}

若是是web 項目 最後一步是使 ServletContext 生效，須要在web.xml 中配置監聽器，而且web.xml　把它放在正確的WEB-INF/classes目錄下，

 

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