jsp+servlet學習（三）監聽器實例

ServletContextListener監聽ServletContext一生中兩個關鍵的事件——創建和撤銷

下面的實例是通過監聽器獲取初始化參數，並創建一個Dog對象，存放在ServletContext中。

項目結構如下：

①web.xml配置如下：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
	<context-param>
		<param-name>dogName</param-name>
		<param-value>TomEgg</param-value>
	</context-param>
	<listener>
		<listener-class>com.zhongqian.listener.TestServletContextListener</listener-class>
	</listener>
	<servlet>
		<servlet-name>testServlet</servlet-name>
		<servlet-class>com.zhongqian.servlet.TestServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>testServlet</servlet-name>
		<url-pattern>/testServlet.do</url-pattern>
	</servlet-mapping>
	<welcome-file-list>
		<welcome-file>index.jsp</welcome-file>
	</welcome-file-list>
</web-app>

②創建TestServletContextListener和TestServlet。

public class TestServletContextListener implements ServletContextListener{

	@Override
	public void contextDestroyed(ServletContextEvent arg0) {
		System.out.println("get into testServletContextListener contextDestoryed method.");
	}

	@Override
	public void contextInitialized(ServletContextEvent arg0) {
		System.out.println("get into testServletContextListener's contextInitialized method.");
		ServletContext sc = arg0.getServletContext();
		String dogName = sc.getInitParameter("dogName");
		Dog d = new Dog(dogName);
		sc.setAttribute("dog", d);
	}
}

public class TestServlet extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		doPost(req, resp);
	}
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		Dog d = (Dog)this.getServletContext().getAttribute("dog");
		System.out.println(d.getDogName());
	}
}

③創建Dog類

服務器啓動時，即調用了監聽器。當客戶端訪問testServlet.do時，在控制檯打印TomEgg.

除了上下文監聽器外，還有很多其他的監聽器。常見的8個監聽器如下：

注意：監聽器的初始化是在服務器啓動時完成的。因此可以在監聽器中完成一些初始化工作。如：spring框架就是通過監聽器完成初始化工作，讀取配置文件，完成注入。