public class ContextListener implements ServletContextListener {public static final String NAME = "name";public static final String Talks = "talks";//static final 静态最终啥的玩意 其实就是放在application里的数据//Talks是服务器内部调用的 ContextListener.Talks//talks是web前端调用的 applicationScope.talksprivate StringBuilder talks = new StringBuilder();@Overridepublic void contextDestroyed(ServletContextEvent sce) {// TODO Auto-generated method stub
        sce.getServletContext().removeAttribute(NAME);sce.getServletContext().removeAttribute(Talks);}@Overridepublic void contextInitialized(ServletContextEvent sce) {// TODO Auto-generated method stubCounter c = new Counter();sce.getServletContext().setAttribute(NAME, c);//在服务器创建的时候，建立了一个countersce.getServletContext().setAttribute(Talks, talks);}}public class SessionListener implements HttpSessionListener{public static final String NAME = "name";private int count = 0;@Overridepublic void sessionCreated(HttpSessionEvent hse) {// TODO Auto-generated method stubHttpSession httpSession = hse.getSession();ServletContext sc = httpSession.getServletContext();Counter c = (Counter) sc.getAttribute(ContextListener.NAME);String name = "游客" + count;while (!c.addNowNames(name)){//加入成功则在counter里改变数据//加入不成功，就改变count知道加入成功count++;name = "游客" + count;}httpSession.setAttribute(NAME,name);StringBuilder sb = (StringBuilder)sc.getAttribute(ContextListener.Talks);sb.append(String.format("%s加入了！%n",name));}@Overridepublic void sessionDestroyed(HttpSessionEvent hse) {// TODO Auto-generated method stubHttpSession httpSession = hse.getSession();ServletContext sc = httpSession.getServletContext();Counter c = (Counter) sc.getAttribute(ContextListener.NAME);String name = httpSession.getAttribute(NAME).toString();c.removeNowNames(name);StringBuilder chatInfo = (StringBuilder) sc.getAttribute(ContextListener.Talks);chatInfo.append(String.format("%s离开了！%n", name));}}

用Serializable序列化来排序
public class Counter implements Serializable {
private static final long serialVersionUID = 45354343L;private int allNames = 0;private Set<String> nowNames = new LinkedHashSet<String>();public Counter() {}public boolean addNowNames(String name) {boolean r = nowNames.add(name);//如果原来有这个名字 就已上线人数增加if (r) {allNames++;}return r;}public boolean renameNowNames(String oldName, String newName) {if (nowNames.contains(oldName)) {nowNames.remove(oldName);} else {return false;}if (nowNames.add(newName)) {return true;} else {nowNames.add(oldName);return false;}}public void removeNowNames(String name) {nowNames.remove(name);}
//必须要有getxxxxxx的方法才能在jsp里被提取到public int getNowNamesSize() {//这个就是在线人数return nowNames.size();}public int getAllNames() {return allNames;}public Set<String> getNowNames() {return Collections.unmodifiableSet(nowNames);}
}

    <table align="center"><tr><td colspan="2"><table align="center" cellspacing="10"><tr><td>历史访问人数：${applicationScope.name.allNames }</td><td>在线人数：${applicationScope.name.nowNamesSize }<select><c:forEach items="${applicationScope.name.nowNames}" var="line"><!-- line是nowNames的其中一个值 --><option>${line}</option></c:forEach></select></td><td>欢迎：${sessionScope.name}</td></tr><tr><td colspan="3"><textarea rows="20" cols="80">${applicationScope.talks}</textarea></td></tr></table>