下面是学习时，发现书上有段不错的代码，是说只允许一台机器的一个帐号登陆的，另外一个上线的话，会被注销掉。下面是代码；
<%
String action = request.getParameter("action");
String account = request.getParameter("account");
if("login".equals(action) && account.trim().length() > 0){
// 登录，将personInfo放入session
PersonInfo personInfo = new PersonInfo();
personInfo.setAccount(account.trim().toLowerCase());
personInfo.setIp(request.getRemoteAddr());
personInfo.setLoginDate(new java.util.Date());
session.setAttribute("personInfo", personInfo);
response.sendRedirect(response.encodeRedirectURL(request.getRequestURI()));
return;
}
else if("logout".equals(action)){
// 注销，将personInfo从session中移除
session.removeAttribute("personInfo");
response.sendRedirect(response.encodeRedirectURL(request.getRequestURI()));
return;
}
%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
<style type="text/css">
body {
font-size:12px;
}
</style>
</head>
<body>
<c:choose>
<c:when test="${ personInfo != null }">
<!-- 已经登录，将显示帐号信息 -->
欢迎您，${ personInfo.account }。<br/>
您的登录IP为${ personInfo.ip }，<br/>
登录时间为<fmt:formatDate value="${ personInfo.loginDate }" pattern="yyyy-MM-dd HH:mm"/>。
<a href="${ pageContext.request.requestURI }?action=logout">退出</a>
<!-- 每5秒钟刷新一次页面 -->
<script>setTimeout("location=location; ", 5000); </script>
</c:when>
<c:otherwise>
<!-- 没有登录，将显示登录页面 -->
${ msg }
<c:remove var="msg" scope="session" />
<form action="${ pageContext.request.requestURI }?action=login" method="post">
帐号：
<input name="account" />
<input type="submit" value="登录">
</form>
</c:otherwise>
</c:choose>
PersonInfo.java:
public class PersonInfo implements Serializable {
private static final long serialVersionUID = 4063725584941336123L;
// 帐号
private String account;
// 登录IP地址
private String ip;
// 登录时间
private Date loginDate;
public String getAccount() {
return account;
}
public void setAccount(String account) {
this.account = account;
}
public String getIp() {
return ip;
}
public void setIp(String ip) {
this.ip = ip;
}
public Date getLoginDate() {
return loginDate;
}
public void setLoginDate(Date loginDate) {
this.loginDate = loginDate;
}
@Override
public boolean equals(Object obj) {
if (obj == null || !(obj instanceof PersonInfo)) {
return false;
}
return account.equalsIgnoreCase(((PersonInfo) obj).getAccount());
}
listener.java:用MAP把登陆的session保存
public class LoginSessionListener implements HttpSessionAttributeListener {
Log log = LogFactory.getLog(this.getClass());
Map<String, HttpSession> map = new HashMap<String, HttpSession>();
public void attributeAdded(HttpSessionBindingEvent event) {
String name = event.getName();
// 登录
if (name.equals("personInfo")) {
PersonInfo personInfo = (PersonInfo) event.getValue();
if (map.get(personInfo.getAccount()) != null) {
// map 中有记录，表明该帐号在其他机器上登录过，将以前的登录失效
HttpSession session = map.get(personInfo.getAccount());
PersonInfo oldPersonInfo = (PersonInfo) session
.getAttribute("personInfo");
log.info("帐号" + oldPersonInfo.getAccount() + "在"
+ oldPersonInfo.getIp() + "已经登录，该登录将被迫下线。");
session.removeAttribute("personInfo");
session.setAttribute("msg", "您的帐号已经在其他机器上登录，您被迫下线。");
}
// 将session以用户名为索引，放入map中
map.put(personInfo.getAccount(), event.getSession());
log.info("帐号" + personInfo.getAccount() + "在" + personInfo.getIp()
+ "登录。");
}
}
public void attributeRemoved(HttpSessionBindingEvent event) {
String name = event.getName();
// 注销
if (name.equals("personInfo")) {
// 将该session从map中移除
PersonInfo personInfo = (PersonInfo) event.getValue();
map.remove(personInfo.getAccount());
log.info("帐号" + personInfo.getAccount() + "注销。");
}
}
public void attributeReplaced(HttpSessionBindingEvent event) {
String name = event.getName();
// 没有注销的情况下，用另一个帐号登录
if (name.equals("personInfo")) {
// 移除旧的的登录信息
PersonInfo oldPersonInfo = (PersonInfo) event.getValue();
map.remove(oldPersonInfo.getAccount());
// 新的登录信息
PersonInfo personInfo = (PersonInfo) event.getSession()
.getAttribute("personInfo");
// 也要检查新登录的帐号是否在别的机器上登录过
if (map.get(personInfo.getAccount()) != null) {
// map 中有记录，表明该帐号在其他机器上登录过，将以前的登录失效
HttpSession session = map.get(personInfo.getAccount());
session.removeAttribute("personInfo");
session.setAttribute("msg", "您的帐号已经在其他机器上登录，您被迫下线。");
}
map.put("personInfo", event.getSession());
}
}
还有一个小技巧就是：
MyContextListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent event) {
// 启动时，记录服务器启动时间
ApplicationConstants.START_DATE = new Date();
}
public void contextDestroyed(ServletContextEvent event) {
// 关闭时，将结果清除。也可以将结果保存到硬盘上。
}
ServletContextListener ：监听context的创建与销毁的，当服务器启动或执行war包时，执行contextInitialized，关闭时执行
contextDestroyed
