session是使用bean的一個生存期限,一般為page,session意思是在這個用戶沒有離開網站之前一直有效,如果無法判斷用戶何時離開,一般依據系統設定,tomcat中設定為30分鐘
<%@page import = "java.util.*" session="true"%>
<HTML>
<HEAD>
<TITLE>Using Sessions to Track Users</TITLE>
</HEAD><BODY>
<%
Integer counter = (Integer)session.getAttribute("counter");
if (counter == null) {
counter = new Integer(1);
} else {
counter = new Integer(counter.intValue() + 1);
}session.setAttribute("counter", counter);
%>
<H1>Using Sessions to Track Users</H1>
Session ID: <%=session.getId()%>
<BR>
Session creation time: <%=new Date(session.getCreationTime())%>
<BR>
Last accessed time: <%=new Date(session.getLastAccessedTime())%>
<BR>
Number of times you've been here: <%=counter%>
</BODY>
</HTML>
獲取或設置session值
<HTML>
<HEAD>
<TITLE>Using the Application Object</TITLE>
</HEAD><BODY>
<H1>Using the Application Object</H1>
<%
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.
<BR>
This page has been visited by all users <%=i%> times.
</BODY>
</HTML>
我們的test.jsp教程 和test1.jsp都需要得到一個參數userdir,這個userdir是從數據庫教程中得知,使用session將大大優化性能,程序如下:
設計一個javabean 存儲userdir.
public class UserEnv {
private String userdir = "";
private String userurl = "";public UserEnv(){
//構建方法初始化userdir,可以從數據庫中讀取,這裡簡單給值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" %>
<jsp:useBean id="myenv" scope="session" class="mysite.UserEnv"/>
<html>
<head>
<title>Untitled</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
</head><body>
this is test1.jsp:<%=myenv.getUserdir()%>
</body>
</html>
test2.jsp程序:
<%@ page contentType="text/html;charset=ISO8859_1" %><jsp:useBean id="myenv" scope="session" class="mysite.UserEnv"/>
<html>
<head>
<title>Untitled</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
</head><body>
this is test2.jsp:<%=myenv.getUserdir()%>
</body>
</html>
無論用戶先調用test1.jsp還是test2.jsp, java bean UserEnv總是先初始化一次, 由於這個bean存在周期是seesion,因此該用戶第二次以後只要在seesion有效期內再調用,myenv.getUserdir()將直接從bean內存中讀取變量,不必再初始化.這樣提高速度,又減少數據庫訪問量.
