現(xiàn)在來說說Servlet的監(jiān)聽器Listener眉菱，它是實(shí)現(xiàn)了javax.servlet.ServletContextListener 接口的服務(wù)器端程序缴渊，它也是隨web應(yīng)用的啟動(dòng)
而啟動(dòng)绣檬，只初始化一次模狭，隨web應(yīng)用的停止而銷毀渠鸽。主要作用是：做一些初始化的內(nèi)容添加工作叫乌、設(shè)置一些基本的內(nèi)容、比如一些參數(shù)或者是一些
固定的對(duì)象等等徽缚。首先來看一下ServletContextListener接口的源代碼：
1. public abstract interface ServletContextListener extends EventListener{
2. public abstract void contextInitialized(ServletContextEvent paramServletContextEvent);
3. public abstract void contextDestroyed(ServletContextEvent paramServletContextEvent);
4. }
下面利用監(jiān)聽器對(duì)數(shù)據(jù)庫連接池DataSource的初始化演示它的使用：ListenerTest.java
1. import javax.servlet.ServletContext;
2. import javax.servlet.ServletContextEvent;
3. import javax.servlet.ServletContextListener;
4. import org.apache.commons.dbcp.BasicDataSource;
5. /**
6. * 現(xiàn)在來說說Servlet的監(jiān)聽器Listener憨奸，它是實(shí)現(xiàn)了javax.servlet.ServletContextListener 接口的
7. * 服務(wù)器端程序，它也是隨web應(yīng)用的啟動(dòng)而啟動(dòng)凿试，只初始化一次排宰，隨web應(yīng)用的停止而銷毀似芝。主要作用是：做一些初始化
8. * 的內(nèi)容添加工作、設(shè)置一些基本的內(nèi)容板甘、比如一些參數(shù)或者是一些固定的對(duì)象等等党瓮。
9. *
10. * 示例代碼：使用監(jiān)聽器對(duì)數(shù)據(jù)庫連接池DataSource進(jìn)行初始化
11. */
12. public class ListenerTest implements ServletContextListener{
13. // 應(yīng)用監(jiān)聽器的銷毀方法
14. public void contextDestroyed(ServletContextEvent servletContextEvent) {
15. ServletContext servletContext = servletContextEvent.getServletContext();
16. // 在整個(gè)web應(yīng)用銷毀之前調(diào)用，將所有應(yīng)用空間所設(shè)置的內(nèi)容清空
17. servletContext.removeAttribute("dataSource");
18. System.out.println("銷毀工作完成...");
19. }
20. // 應(yīng)用監(jiān)聽器的初始化方法
21. public void contextInitialized(ServletContextEvent servletContextEvent) {
22. // 通過這個(gè)事件可以獲取整個(gè)應(yīng)用的空間
23. // 在整個(gè)web應(yīng)用下面啟動(dòng)的時(shí)候做一些初始化的內(nèi)容添加工作
24. ServletContext servletContext = servletContextEvent.getServletContext();
25. // 設(shè)置一些基本的內(nèi)容盐类；比如一些參數(shù)或者是一些固定的對(duì)象
26. // 創(chuàng)建DataSource對(duì)象寞奸，連接池技術(shù) dbcp
27. BasicDataSource basicDataSource = new BasicDataSource();
28. basicDataSource.setDriverClassName("com.jdbc.Driver");
29. basicDataSource.setUrl("jdbc:mysqlocalhost:3306/");
30. basicDataSource.setUsername("root");
31. basicDataSource.setPassword("root");
32. basicDataSource.setMaxActive(10);//最大連接數(shù)
33. basicDataSource.setMaxIdle(5);//最大管理數(shù)
34. //bds.setMaxWait(maxWait); 最大等待時(shí)間
35. // 把 DataSource 放入ServletContext空間中，
36. // 供整個(gè)web應(yīng)用的使用(獲取數(shù)據(jù)庫連接)
37. servletContext.setAttribute("dataSource", basicDataSource);
38. System.out.println("應(yīng)用監(jiān)聽器初始化工作完成...");
39. System.out.println("已經(jīng)創(chuàng)建DataSource...");
40. }
41. }
web.xml中配置如下在跳，很簡單：
1. <!-- 配置應(yīng)用監(jiān)聽器 -->
2. <listener>
3. <listener-class>com.ycq.ListenerTest</listener-class>
4. </listener>
這樣配置好了之后枪萄，以后在web應(yīng)用中就可以通過ServletContext取得BasicDataSource對(duì)象，從而獲取與數(shù)據(jù)庫的連接猫妙，提高性能呻引，方便使用。
示例代碼二：
1. import java.io.File;
2. import javax.servlet.ServletContextEvent;
3. import javax.servlet.ServletContextListener;
4. import com.i2f.fsp.deploy.TransactionDeployer;
5. /**
6. * 監(jiān)聽器隨著項(xiàng)目的啟動(dòng)而啟動(dòng)
7. *
8. */
9. public class ListenerTest2 implements ServletContextListener{
10. // 銷毀監(jiān)聽器
11. public void contextDestroyed(ServletContextEvent servletContextEvent) {
12. System.out.println("date20161020095500 :" + servletContextEvent.getServletContext());
13. }
14. public void contextInitialized(ServletContextEvent servletContextEvent) {
15. try{
16. // 獲取項(xiàng)目跟路徑
17. String basePath = servletContextEvent.getServletContext().getRealPath("/");
18. // D:\apache-tomcat-6.0.41\webapps\i2money\ 絕對(duì)路徑
19. System.out.println("basePath20161020094700 :" + basePath);
20. if (!(basePath.endsWith(File.separator))){
21. basePath = basePath + File.separator;
22. }
23. basePath = basePath + "WEB-INF" + File.separator + "classes" + File.separator;
24. new TransactionDeployer(basePath).deploy();
25. // D:\apache-tomcat-6.0.41\webapps\i2money\WEB-INF\classes\
26. System.out.println("basePath20161020094701 :" + basePath);
27. }
28. catch (Exception e){
29. e.printStackTrace();
30. System.exit(-1);
31. }
32. }
33. }
示例代碼三：
1. import javax.servlet.http.HttpSession;
2. import javax.servlet.http.HttpSessionEvent;
3. import javax.servlet.http.HttpSessionListener;
4. import org.apache.commons.logging.Log;
5. import org.apache.commons.logging.LogFactory;
6. import org.springframework.context.ApplicationContext;
7. import org.springframework.web.context.support.WebApplicationContextUtils;
8. public class UserLogoutListener implements HttpSessionListener{
9. protected final Log log = LogFactory.getLog(super.getClass());
10. public void sessionCreated(HttpSessionEvent event){
11. this.log.error("session created. id = " + event.getSession().getId());
12. }
13. public void sessionDestroyed(HttpSessionEvent event){
14. this.log.error("session destroyed.id = " + event.getSession().getId());
15. HttpSession session = event.getSession();
16. ApplicationContext context = WebApplicationContextUtils.getWebApplicationContext(session.getServletContext());
17. OnlineUserMonitorClient client = (OnlineUserMonitorClient)context.getBean("onlineUserMonitorClient");
18. client.afterSessionDestroyed(session);
19. }
20. }
監(jiān)聽器在實(shí)際項(xiàng)目中的應(yīng)用吐咳，監(jiān)聽器在java web中應(yīng)用的較多色罚，比如：統(tǒng)計(jì)當(dāng)前在線人數(shù)、自定義session掃描器沿盅。
--------------------- 應(yīng)用一：統(tǒng)計(jì)當(dāng)前在線人數(shù) ---------------------
1. import javax.servlet.ServletContext;
2. import javax.servlet.http.HttpSessionEvent;
3. import javax.servlet.http.HttpSessionListener;
4. /**
5. * @description HttpSessionListener監(jiān)聽器實(shí)現(xiàn)統(tǒng)計(jì)網(wǎng)站在線人數(shù)的功能
6. */
7. public class SessionListener implements HttpSessionListener{
9. public static int TOTAL_ONLINE_USERS = 0;
10. public void sessionCreated(HttpSessionEvent httpSessionEvent) {
11. ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
12. TOTAL_ONLINE_USERS = (Integer) servletContext.getAttribute("TOTAL_ONLINE_USERS");
13. // 如果用戶退出塞帐，TOTAL_ONLINE_USERS自減1
14. if(TOTAL_ONLINE_USERS == 0){
15. servletContext.setAttribute("TOTAL_ONLINE_USERS", 1);
16. }
17. else{
18. TOTAL_ONLINE_USERS--;
19. servletContext.setAttribute("TOTAL_ONLINE_USERS", TOTAL_ONLINE_USERS);
20. }
21. }
23. public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
24. ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
25. TOTAL_ONLINE_USERS = (Integer) servletContext.getAttribute("TOTAL_ONLINE_USERS");
26. // 如果用戶登錄，TOTAL_ONLINE_USERS自增1
27. if(TOTAL_ONLINE_USERS == 0){
28. servletContext.setAttribute("TOTAL_ONLINE_USERS", 1);
29. }
30. else{
31. TOTAL_ONLINE_USERS++;
32. servletContext.setAttribute("TOTAL_ONLINE_USERS", TOTAL_ONLINE_USERS);
33. }
34. }
35. }
--------------------- 應(yīng)用二：自定義session掃描器 ---------------------
1. import java.util.LinkedList;
2. import java.util.List;
3. import java.util.Timer;
4. import javax.servlet.ServletContextEvent;
5. import javax.servlet.ServletContextListener;
6. import javax.servlet.http.HttpSession;
7. import javax.servlet.http.HttpSessionEvent;
8. import javax.servlet.http.HttpSessionListener;
9. import jeus.util.concurrent50.Collections;
10. /**
11. * @description 當(dāng)網(wǎng)站用戶量增加時(shí)沪羔，session占用的內(nèi)存會(huì)越來越大饥伊，這時(shí)session的管理，將會(huì)是一項(xiàng)很大的
12. * 系統(tǒng)開銷蔫饰，為了高效的管理session琅豆，我們可以寫一個(gè)監(jiān)聽器，定期清理掉過期的session
13. */
14. public class SessionScanerListener implements HttpSessionListener,ServletContextListener{
15. // 創(chuàng)建一個(gè)線程安全的集合篓吁，用來存儲(chǔ)session
16. @SuppressWarnings("unchecked")
17. List<HttpSession> sessionList = Collections.synchronizedList(new LinkedList<HttpSession>());
18. private Object lock = new Object();
20. public void sessionCreated(HttpSessionEvent httpSessionEvent) {
21. System.out.println("session 創(chuàng)建成功...");
22. HttpSession httpSession = httpSessionEvent.getSession();
23. synchronized (lock){
24. sessionList.add(httpSession);
25. }
26. }
28. public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
29. System.out.println("session 銷毀成功...");
30. }
31. // web應(yīng)用關(guān)閉時(shí)觸發(fā)contextDestroyed事件
32. public void contextDestroyed(ServletContextEvent servletContextEvent) {
33. System.out.println("web應(yīng)用關(guān)閉...");
34. }
36. // web應(yīng)用啟動(dòng)時(shí)觸發(fā)contextInitialized事件
37. public void contextInitialized(ServletContextEvent servletContextEvent) {
38. System.out.println("web應(yīng)用初始化...");
39. // 創(chuàng)建定時(shí)器
40. Timer timer = new Timer();
41. // 每隔30秒就定時(shí)執(zhí)行任務(wù)
42. timer.schedule(new MyTask(sessionList,lock), 0, 1000*30);
43. }
44. }
1. import java.util.List;
2. import java.util.ListIterator;
3. import java.util.TimerTask;
4. import javax.servlet.http.HttpSession;
5. /**
6. * 定時(shí)器茫因，定義定時(shí)任務(wù)的具體內(nèi)容
7. */
8. public class MyTask extends TimerTask{
9. private List<HttpSession> list;
10. // 存儲(chǔ)傳遞過來的鎖
11. private Object lock;
12. // 構(gòu)造方法
13. MyTask(List<HttpSession> list, Object lock){
14. this.list = list;
15. this.lock = lock;
16. }
17. @Override
18. public void run() {
19. // 考慮到多線程的情況，這里必須要同步
20. synchronized (lock){
21. System.out.println("定時(shí)器開始執(zhí)行...");
22. ListIterator<HttpSession> listIterator = list.listIterator();
23. while(listIterator.hasNext()){
24. HttpSession httpSession = listIterator.next();
25. // httpSession.getLastAccessedTime() = session的最后訪問時(shí)間
26. if(System.currentTimeMillis() - httpSession.getLastAccessedTime() > 1000*30){
27. // 手動(dòng)銷毀session
28. httpSession.invalidate();
29. // 從集合中移除已經(jīng)被銷毀的session
30. listIterator.remove();
31. }
32. }
33. }
34. }
35. }
