监听器listener
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转载 :http://blog.csdn.net/reggergdsg/article/details/52891311
原博文补充:
servlet中有4个域对象Application、Session、Request、pageContext
listener监听器监听3个ServletContext即(Application)、HttpSession即(Session)、ServletRequest即(Request)
现在来说说Servlet的监听器Listener,它是实现了javax.servlet.ServletContextListener 接口的服务器端程序,它也是随web应用的启动
而启动,只初始化一次,随web应用的停止而销毁。主要作用是:做一些初始化的内容添加工作、设置一些基本的内容、比如一些参数或者是一些固定的对象等等。首先来看一下ServletContextListener接口的源代码:
- public abstract interface ServletContextListener extends EventListener{
- public abstract void contextInitialized(ServletContextEvent paramServletContextEvent);
- public abstract void contextDestroyed(ServletContextEvent paramServletContextEvent);
- }
下面利用监听器对数据库连接池DataSource的初始化演示它的使用:ListenerTest.java - import javax.servlet.ServletContext;
- import javax.servlet.ServletContextEvent;
- import javax.servlet.ServletContextListener;
- import org.apache.commons.dbcp.BasicDataSource;
-
-
-
-
-
-
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- public class ListenerTest implements ServletContextListener{
-
- public void contextDestroyed(ServletContextEvent servletContextEvent) {
- ServletContext servletContext = servletContextEvent.getServletContext();
-
- servletContext.removeAttribute("dataSource");
- System.out.println("销毁工作完成...");
- }
-
- public void contextInitialized(ServletContextEvent servletContextEvent) {
-
-
- ServletContext servletContext = servletContextEvent.getServletContext();
-
-
- BasicDataSource basicDataSource = new BasicDataSource();
- basicDataSource.setDriverClassName("com.jdbc.Driver");
- basicDataSource.setUrl("jdbc:mysqlocalhost:3306/");
- basicDataSource.setUsername("root");
- basicDataSource.setPassword("root");
- basicDataSource.setMaxActive(10);
- basicDataSource.setMaxIdle(5);
-
-
-
- servletContext.setAttribute("dataSource", basicDataSource);
- System.out.println("应用监听器初始化工作完成...");
- System.out.println("已经创建DataSource...");
- }
- }
web.xml中配置如下,很简单:-
- <listener>
- <listener-class>com.ycq.ListenerTest</listener-class>
- </listener>
这样配置好了之后,以后在web应用中就可以通过ServletContext取得BasicDataSource对象,从而获取与数据库的连接,提高性能,方便使用。示例代码二:
- import java.io.File;
- import javax.servlet.ServletContextEvent;
- import javax.servlet.ServletContextListener;
- import com.i2f.fsp.deploy.TransactionDeployer;
-
-
-
-
- public class ListenerTest2 implements ServletContextListener{
-
- public void contextDestroyed(ServletContextEvent servletContextEvent) {
- System.out.println("date20161020095500 :" + servletContextEvent.getServletContext());
- }
- public void contextInitialized(ServletContextEvent servletContextEvent) {
- try{
-
- String basePath = servletContextEvent.getServletContext().getRealPath("/");
-
- System.out.println("basePath20161020094700 :" + basePath);
- if (!(basePath.endsWith(File.separator))){
- basePath = basePath + File.separator;
- }
- basePath = basePath + "WEB-INF" + File.separator + "classes" + File.separator;
- new TransactionDeployer(basePath).deploy();
-
- System.out.println("basePath20161020094701 :" + basePath);
- }
- catch (Exception e){
- e.printStackTrace();
- System.exit(-1);
- }
- }
- }
示例代码三:- import javax.servlet.http.HttpSession;
- import javax.servlet.http.HttpSessionEvent;
- import javax.servlet.http.HttpSessionListener;
- import org.apache.commons.logging.Log;
- import org.apache.commons.logging.LogFactory;
- import org.springframework.context.ApplicationContext;
- import org.springframework.web.context.support.WebApplicationContextUtils;
- public class UserLogoutListener implements HttpSessionListener{
- protected final Log log = LogFactory.getLog(super.getClass());
- public void sessionCreated(HttpSessionEvent event){
- this.log.error("session created. id = " + event.getSession().getId());
- }
- public void sessionDestroyed(HttpSessionEvent event){
- this.log.error("session destroyed.id = " + event.getSession().getId());
- HttpSession session = event.getSession();
- ApplicationContext context = WebApplicationContextUtils.getWebApplicationContext(session.getServletContext());
- OnlineUserMonitorClient client = (OnlineUserMonitorClient)context.getBean("onlineUserMonitorClient");
- client.afterSessionDestroyed(session);
- }
- }
监听器在实际项目中的应用,监听器在java web中应用的较多,比如:统计当前在线人数、自定义session扫描器。--------------------- 应用一:统计当前在线人数 ---------------------- import javax.servlet.ServletContext;
- import javax.servlet.http.HttpSessionEvent;
- import javax.servlet.http.HttpSessionListener;
-
-
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- public class SessionListener implements HttpSessionListener{
-
- public static int TOTAL_ONLINE_USERS = 0;
- public void sessionCreated(HttpSessionEvent httpSessionEvent) {
- ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
- TOTAL_ONLINE_USERS = (Integer) servletContext.getAttribute("TOTAL_ONLINE_USERS");
-
- if(TOTAL_ONLINE_USERS == 0){
- servletContext.setAttribute("TOTAL_ONLINE_USERS", 1);
- }
- else{
- TOTAL_ONLINE_USERS--;
- servletContext.setAttribute("TOTAL_ONLINE_USERS", TOTAL_ONLINE_USERS);
- }
- }
-
- public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
- ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
- TOTAL_ONLINE_USERS = (Integer) servletContext.getAttribute("TOTAL_ONLINE_USERS");
-
- if(TOTAL_ONLINE_USERS == 0){
- servletContext.setAttribute("TOTAL_ONLINE_USERS", 1);
- }
- else{
- TOTAL_ONLINE_USERS++;
- servletContext.setAttribute("TOTAL_ONLINE_USERS", TOTAL_ONLINE_USERS);
- }
- }
- }
--------------------- 应用二:自定义session扫描器 ---------------------- import java.util.LinkedList;
- import java.util.List;
- import java.util.Timer;
- import javax.servlet.ServletContextEvent;
- import javax.servlet.ServletContextListener;
- import javax.servlet.http.HttpSession;
- import javax.servlet.http.HttpSessionEvent;
- import javax.servlet.http.HttpSessionListener;
- import jeus.util.concurrent50.Collections;
-
-
-
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- public class SessionScanerListener implements HttpSessionListener,ServletContextListener{
-
- @SuppressWarnings("unchecked")
- List<HttpSession> sessionList = Collections.synchronizedList(new LinkedList<HttpSession>());
- private Object lock = new Object();
-
- public void sessionCreated(HttpSessionEvent httpSessionEvent) {
- System.out.println("session 创建成功...");
- HttpSession httpSession = httpSessionEvent.getSession();
- synchronized (lock){
- sessionList.add(httpSession);
- }
- }
-
- public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
- System.out.println("session 销毁成功...");
- }
-
- public void contextDestroyed(ServletContextEvent servletContextEvent) {
- System.out.println("web应用关闭...");
- }
-
-
- public void contextInitialized(ServletContextEvent servletContextEvent) {
- System.out.println("web应用初始化...");
-
- Timer timer = new Timer();
-
- timer.schedule(new MyTask(sessionList,lock), 0, 1000*30);
- }
- }
- import java.util.List;
- import java.util.ListIterator;
- import java.util.TimerTask;
- import javax.servlet.http.HttpSession;
-
-
-
- public class MyTask extends TimerTask{
- private List<HttpSession> list;
-
- private Object lock;
-
- MyTask(List<HttpSession> list, Object lock){
- this.list = list;
- this.lock = lock;
- }
- @Override
- public void run() {
-
- synchronized (lock){
- System.out.println("定时器开始执行...");
- ListIterator<HttpSession> listIterator = list.listIterator();
- while(listIterator.hasNext()){
- HttpSession httpSession = listIterator.next();
-
- if(System.currentTimeMillis() - httpSession.getLastAccessedTime() > 1000*30){
-
- httpSession.invalidate();
-
- listIterator.remove();
- }
- }
- }
- }
- }