//先建立數(shù)據(jù)庫(kù)連接,執(zhí)行查詢語(yǔ)句
Connection conn = DriverManager.getConnection(URL, USER, PassWord);
Statement st=conn.createStatement();
ResultSet rs =st.executeQuery("select count(*) as result from tablename");
//創(chuàng)建變量存取個(gè)數(shù)
int count=0;
while(rs.next())
{
count=getInt(1);
}

public class ApplicationListener implements ServletContextListener {	
	//Servlet上下文對(duì)象創(chuàng)建的時(shí)候被調(diào)用
	@Override
	public void contextInitialized(ServletContextEvent contextEvent) {
		System.out.println("Servlet上下文對(duì)象被創(chuàng)建啦...");
		
    //項(xiàng)目一旦啟動(dòng)，此處代碼運(yùn)行！
		Timer timer=new Timer();
		//5秒鐘之后開(kāi)始執(zhí)行，以后每間隔2秒發(fā)送一封郵件！
		timer.schedule(new TimerTask() {
			@Override
			public void run() {
				//System.out.println("發(fā)郵件...."+new Date());
			}
		}, 5000, 2000);
	}
	//Servlet上下文對(duì)象銷(xiāo)毀的時(shí)候被調(diào)用
	@Override
	public void contextDestroyed(ServletContextEvent contextEvent) {
		System.out.println("Servlet上下文對(duì)象被銷(xiāo)毀啦...");
		//服務(wù)器在停止的時(shí)候，要執(zhí)行某些動(dòng)作，那么就可以把代碼寫(xiě)在這個(gè)位置?。?！	
	}
}

<!-- web.xml中配置 -->
<listener>
	<listener-class>com.dream.listener.ApplicationListener</listener-class>
</listener>

@WebListener
public class SessionListener implements HttpSessionListener{
 @Override
 public void sessionCreated(HttpSessionEvent event) {
 HttpSession session = event.getSession();
 System.out.println("session對(duì)象創(chuàng)建啦...."+session.getId());
 }
 @Override
 public void sessionDestroyed(HttpSessionEvent event) {
 HttpSession session = event.getSession();
 System.out.println("session對(duì)象銷(xiāo)毀啦...."+session.getId());
 }
}

@WebListener
public class RequestListener implements ServletRequestListener{

 @Override
 public void requestInitialized(ServletRequestEvent event) {
 ServletRequest request = event.getServletRequest();
 System.out.println("Request對(duì)象的創(chuàng)建...."+request);
 }
 @Override
 public void requestDestroyed(ServletRequestEvent event) {
 ServletRequest request = event.getServletRequest();
 System.out.println("Request對(duì)象的銷(xiāo)毀...."+request);
 }

}

@WebListener
public class ApplicationListener implements ServletContextListener{
 @Override
 public void contextInitialized(ServletContextEvent event) {
 //項(xiàng)目啟動(dòng),向application對(duì)象中存一個(gè)變量，初始值0
 ServletContext application = event.getServletContext(); 
 application.setAttribute("count", 0);
 }
 @Override
 public void contextDestroyed(ServletContextEvent event) {
 }
}

@WebListener
public class SessionListener implements HttpSessionListener {

 @Override
 public void sessionCreated(HttpSessionEvent event) {
 // 有人訪問(wèn)了 count++
 HttpSession session = event.getSession();
 ServletContext application = session.getServletContext();

 int count =(Integer) application.getAttribute("count");
 count++;
 application.setAttribute("count", count);
 }
 @Override
 public void sessionDestroyed(HttpSessionEvent event) {
 // 有人離開(kāi)了 count--
 HttpSession session = event.getSession();
 ServletContext application = session.getServletContext();
 
 Integer count =(Integer) application.getAttribute("count");
 count--;
 application.setAttribute("count", count);
 }
}

@WebListener
public class ApplicationAttributeListener implements ServletContextAttributeListener{

  //Servlet上下文對(duì)象新增值的時(shí)候被調(diào)用
 @Override
 public void attributeAdded(ServletContextAttributeEvent event) {
 String str = "Servlet上下文對(duì)象中添加了屬性:"+event.getName()
      +"，屬性值是:"+event.getValue();
 System.out.println(str);
 }
 //Servlet上下文對(duì)象刪除值的時(shí)候被調(diào)用
 @Override
 public void attributeRemoved(ServletContextAttributeEvent event) {
 String str = "Servlet上下文對(duì)象中刪除了屬性:"+event.getName()
      +"，屬性值是:"+event.getValue();
 System.out.println(str);
 }
 //Servlet上下文對(duì)象替換值的時(shí)候被調(diào)用
 @Override
 public void attributeReplaced(ServletContextAttributeEvent event) {
 String str = "Servlet上下文對(duì)象中替換了屬性:"+event.getName()
      +"，屬性值是:"+event.getValue();
 System.out.println(str);
 }
}

public class User implements HttpSessionBindingListener {
 private int id;
 private String name;

 public User() {
 }
 public User(int id, String name) {
 this.id = id;
 this.name = name;
 }
 public int getId() {
 return id;
 }
 public void setId(int id) {
 this.id = id;
 }
 public String getName() {
 return name;
 }
 public void setName(String name) {
 this.name = name;
 }
 public void valueBound(HttpSessionBindingEvent event) {
 System.out.println("對(duì)象綁定到了Session中");
 }
 public void valueUnbound(HttpSessionBindingEvent event) {
 System.out.println("對(duì)象從Session中移除");
 }
}

<%@ page import="com.dream.vo.User"%>
<%@ page language="java" pageEncoding="UTF-8"%>
<!DOCTYPE HTML>
<html>
<head>
<title>ServletContextAttributeListener監(jiān)聽(tīng)器測(cè)試</title>
</head>
<body>
 <%
 User user = new User(1, "aaa");
 session.setAttribute("user", user);
 session.removeAttribute("user");
 %>
</body>
</html>

public class User implements Serializable, HttpSessionActivationListener{
 private static final long serialVersionUID = -1566395353697458460L;
 private int id;
 private String name;
 public User() {
 }
 public User(int id, String name) {
 this.id = id;
 this.name = name;
 }
 public int getId() {
 return id;
 }
 public void setId(int id) {
 this.id = id;
 }
 public String getName() {
 return name;
 }
 public void setName(String name) {
 this.name = name;
 }
 //鈍化
 @Override
 public void sessionWillPassivate(HttpSessionEvent event) {
 System.out.println("對(duì)象被鈍化......." + event.getSource());
 }
 //活化
 @Override
 public void sessionDidActivate(HttpSessionEvent event) {
 System.out.println("對(duì)象被活化......");
 }
}

<?xml version="1.0" encoding="UTF-8"?>
<Context>
 <!-- 
 maxIdleSwap:"1": session如果1分鐘沒(méi)有使用就序列化
 directory: 序列化后文件所保存的路徑 
 -->
 <Manager className="org.apache.catalina.session.PersistentManager"
 maxIdleSwap="1">
 <Store className="org.apache.catalina.session.FileStore"  
  directory="C:\\text" />
 </Manager>
</Context>