session是使用bean的一个生存期限,一般为page,session意思是在这个用户没有离开网站之前一直有效,如果无法判断用户何时离开,一般依据系统设定,tomcat中设定为30分钟

<%@page import = "java.util.*" session="true"%>

   
       
   

   
        <%
        Integer counter = (Integer)session.getAttribute("counter");
        if (counter == null) {
            counter = new Integer(1);
        } else {
            counter = new Integer(counter.intValue() + 1);
        }

        session.setAttribute("counter", counter);
        %>
       

Using Sessions to Track Users

        Session ID: <%=session.getId()%>
       

        Session creation time: <%=new Date(session.getCreationTime())%>
       

        Last accessed time: <%=new Date(session.getLastAccessedTime())%>
       

        Number of times you've been here: <%=counter%>
   

获取或设置session值

   
       
   

   
       

Using the Application Object

        <%
        Integer counter = (Integer)session.getAttribute("counter");
        String heading = null;
        if (counter == null) {
            counter = new Integer(1);
        } else {
            counter = new Integer(counter.intValue() + 1);
        }

        session.setAttribute("counter", counter);

        Integer i = (Integer)application.getAttribute("i");
        if (i == null) {
            i = new Integer(1);
        } else {
            i = new Integer(i.intValue() + 1);
        }

        application.setAttribute("i", i);
        %>

        You have visited this page <%=counter%> times.
       

        This page has been visited by all users <%=i%> times.
   

我们的test.jsp教程 和test1.jsp都需要得到一个参数userdir,这个userdir是从数据库教程中得知,使用session将大大优化性能,程序如下:

设计一个javabean 存储userdir.

public class UserEnv {

private String userdir = "";
private String userurl = "";

public UserEnv(){
//构建方法初始化userdir,可以从数据库中读取,这里简单给值ppp
userdir="pppp";
System.out.println("init userdir, one time");
}

public String getUserdir() throws Exception{
return userdir;

}

}

test1.jsp程序:

<%@ page contentType="text/html;charset=ISO8859_1" %>

this is test1.jsp:<%=myenv.getUserdir()%>

test2.jsp程序:
<%@ page contentType="text/html;charset=ISO8859_1" %>

this is test2.jsp:<%=myenv.getUserdir()%>

 

无论用户先调用test1.jsp还是test2.jsp, java bean UserEnv总是先初始化一次, 由于这个bean存在周期是seesion,因此该用户第二次以后只要在seesion有效期内再调用,myenv.getUserdir()将直接从bean内存中读取变量,不必再初始化.这样提高速度,又减少数据库访问量.