這個J2EE小提示闡述了ServletContextListener的用法。這個事件類作為Web應(yīng)用服務(wù)的一部分，處理Web應(yīng)用的 servlet上下文
(context)的變化的通知。這可以解釋為，好像有個人在服務(wù)器旁不斷地通知我們服務(wù)器在發(fā)生什么事件。那當(dāng)然需要監(jiān)聽者了。
因 此，在通知上下文(context)初始化和銷毀的時候，ServletContextListner非常有用。
import javax.servlet.ServletContextListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.*;
public class MyListener implements ServletContextListener 
{
private ServletContext context = null;
/** *//** 這個方法在Web應(yīng)用服務(wù)被移除，沒有能力再接受請求的時候被調(diào)用。 */
public void contextDestroyed(ServletContextEvent event){
//Output a simple message to the server's console
System.out.println("The Simple Web App. Has Been Removed");
this.context = null; }
// 這個方法在Web應(yīng)用服務(wù)做好接受請求的時候被調(diào)用。
public void contextInitialized(ServletContextEvent event){
this.context = event.getServletContext();
//Output a simple message to the server's console
System.out.println("The Simple Web App. Is Ready");
}
}
<web-app>
<listener>
<listener-class>
com.listeners.MyContextListener
</listener-class>
</listener>
<servlet/>
<servlet-mapping/>
</web-app>
下面是spring中的ContextLoaderListener，我們可以看看里面的代碼
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());
}
/** *//**
* Create the ContextLoader to use. Can be overridden in subclasses.
* @return the new ContextLoader
*/
protected ContextLoader createContextLoader() {
return new ContextLoader();
}
/** *//**
* Return the ContextLoader used by this listener.
*/
public ContextLoader getContextLoader() {
return contextLoader;
}
/** *//**
* Close the root web application context.
*/
public void contextDestroyed(ServletContextEvent event) {
if (this.contextLoader != null) {
this.contextLoader.closeWebApplicationContext(event.getServletContext());
}
}
}
<listener>
<listener-class> org.springframework.web.context.ContextLoaderListener </listener-class>
</listener>
ServletContextListener接口有兩方需要實(shí)現(xiàn)的方法:contextInitialized()和contextDestroyed();
Listener,譯為監(jiān)聽者.顧名思義,它會監(jiān)聽Servlet容器,當(dāng)應(yīng)用開始的時候它會調(diào)用contextInitialized()方法;
當(dāng)應(yīng)用關(guān)閉的時候,它同樣會調(diào)用contextDestroyed()方法.我們可以利用這個特性初始化一些信息,當(dāng)然我們
也可以利用Servlet類init()方法,并在配置文件中讓它啟動應(yīng)用的時候就執(zhí)行,并且在關(guān)閉的時候執(zhí)行destroy()方
法.但是繼承此接口應(yīng)該更符合容器的應(yīng)用.
舉個簡單的例子:在一些論壇,社區(qū)及聊天室當(dāng)中,刪除在線的超時用戶就可以利用這個接口來實(shí)現(xiàn).
可以利用JAVA的TimerTask及Timer類來實(shí)現(xiàn)每隔一定的時間進(jìn)行自動檢測.
實(shí)例代碼如下:
UserOnlineTimerTask.java
-----------------
package com.bcxy.servlet;
import java.util.TimerTask;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
public class UserOnlineTimerTask extends TimerTask {
Log log = LogFactory.getLog(UserOnlineTimerTask.class);
public void run() {
// 刪除超時在線用戶
log.info("刪除在線的超時用戶.");
}
}
SysListener.java
-----------------------------------
package com.bcxy.servlet;
import java.io.IOException;
import java.util.Timer;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
public class SysListener
extends HttpServlet
implements ServletContextListener {
Log log = LogFactory.getLog(SysListener.class);
Timer timer = new Timer();
public void service(ServletRequest request, ServletResponse response)
throws ServletException, IOException {
//
}
public void contextInitialized(ServletContextEvent sce) {
log.info("initial context.");
timer.schedule(new UserOnlineTimerTask(), 0, 10000);
}
public void contextDestroyed(ServletContextEvent sce) {
log.info("destory context.");
timer.cancel();
}
}
HttpSessionListener
繼承HttpSessionListener接口的類，來監(jiān)聽Session創(chuàng)建和銷毀的事件
import javax.servlet.*;
import javax.servlet.http.*;
import jp.co.sysmex.sps.app.web.WebAccountBean;
//①HttpSessionListener 接口的實(shí)現(xiàn)。
public class CheckSessionServlet implements HttpSessionListener {
private static int sesCount = 0;
//②session生成時觸發(fā)sessionCreated方法
public void sessionCreated(HttpSessionEvent hse) {
sesCount++;
//ServletContext sc = hse.getSession().getServletContext();
String sessid = hse.getSession().getId();
System.out.println(" session Created " + sesCount);
System.out.println(" session ++ " + sessid);
}
//③session無效時觸發(fā)sessionDestroyed方法
//此時session中的內(nèi)容還可以正常取道
public void sessionDestroyed(HttpSessionEvent hse) {
String sessid = hse.getSession().getId();
System.out.println(" session Destroyed " + sesCount);
System.out.println(" session -- " + sessid);
WebAccountBean account = (WebAccountBean)(hse.getSession().getAttribute("ACCOUNT_KEY"));
System.out.println(account.getEnterpriseCode());
System.out.println(account.getEnterpriseFullKanjiName());
sesCount--;
}
}
web.xml文件中增加配置信息.
<listener>
<listener-class>jp.co.sysmex.sps.util.CheckSessionServlet</listener-class>
</listener>