用JSP程序实现统计当前在线人数

/**这是管理user信息的类 

文件名为onLineUser.java 

*/ 
import javax.servlet.http.*; 
import javax.servlet.*; 
import java.util.*; 
public class onLineUser implements HttpSessionBindingListener { 
public onLineUser(){ 
} 
private Vector users=new Vector(); 
public int getCount(){ 
users.trimToSize(); 
return users.capacity(); 
} 
public boolean existUser(String userName){ 
users.trimToSize(); 
boolean existUser=false; 
for (int i=0;i<users.capacity();i++ ) 
{ 
if (userName.equals((String)users.get(i))) 
{ 
existUser=true; 
break; 
} 
} 
return existUser; 
} 
public boolean deleteUser(String userName) { 

users.trimToSize(); 
if(existUser(userName)){ 
int currUserIndex=-1; 
for(int i=0;i<users.capacity();i++){ 
if(userName.equals((String)users.get(i))){ 
currUserIndex=i; 
break; 
} 
} 
if (currUserIndex!=-1){ 
users.remove(currUserIndex); 
users.trimToSize(); 
return true; 
} 
} 
return false; 
} 
public Vector getOnLineUser() 
{ 
return users; 
} 
public void valueBound(HttpSessionBindingEvent e) { 
users.trimToSize(); 
if(!existUser(e.getName())){ 
users.add(e.getName()); 
System.out.print(e.getName()+"\t 登入到系统\t"+(new Date())); 
System.out.println(" 在线用户数为："+getCount()); 
}else 
System.out.println(e.getName()+"已经存在"); 
}