Jsp内置对象

重点：

各个对象的生命周期

各种页面间传值的方式

Jsp有九个内置对象，包括：

对象名类型作用域

request javax.servlet.ServletRequest Request

response javax.servlet.ServletResponse Page

pageContext javax.servlet.jsp.PageContext Page

session javax.servlet.http.HttpSession Session

application javax.servlet.SevletContext Application

out javax.servlet.jsp.JspWriter Page

config javax.servlet.ServletConfig Page

page java.lang.Object Page

exception java.lang.Throwable Page

***********************************************************************

主要的三个对象　

request

主要的方法有：

String getParameter(String name) 　多用于值唯一的情况　

Enumeration getParameterNames()

String[] getParameterValues(String name)　　多用于值有多个时，比如复选框里的勾选内容

Object getAttribute(String name) 用于用户向服务器端发出请求时，传递参数，必须是同一个请求时才有效。可以一次请求多页面传递。

用户的每一个请求都会产生一个request对象。请求结束时，request对象被销毁，也就没有attribute了。当发出下一个请求时，生成了另外一个request对象。

void setAttribute(String name,Object o)

request1.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'request1.jsp' starting page</title>

</head>

<body>

username:

</form>

</body>

</html>

Request2.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'request2.jsp' starting page</title>

</head>

<body>

String username = request.getParameter("username");

request.setAttribute("username",username);

username: <%=username %>

<jsp:forward page="request3.jsp"></jsp:forward>

</body>

</html>

Request3.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'request3.jsp' starting page</title>

</head>

<body>

String username = (String)request.getAttribute("username");

<%=username %>

</body>

</html>

session

http是无状态的协议

web server对每一个客户端的请求都没有历史记忆

session用来保存当前客户端状态信息

主要的方法有：

Object getAttribute(String name)

void setAttribute(String name,Object value)

只要当前连接没有关闭，或session没有到期，session里的数据就不丢失（序列化到硬盘的情况除外）

session1.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'session1.jsp' starting page</title>

</head>

<body>

username:

</form>

</body>

</html>

Session2.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'session2.jsp' starting page</title>

</head>

<body>

String username = request.getParameter("username");

session.setAttribute("name",username);

your name "<%=username %>" is saved in session<br>

<a href="session3.jsp?name=<%=username %>">check</a>

</body>

</html>

Session3.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'session3.jsp' starting page</title>

</head>

<body>

String username = (String)session.getAttribute("name");

if(username == null)

{

out.println("你还没有登录");

}

else

{

out.println(username + "已经登录");

}

</body>

</html>

执行结果

application

Object getAttribute(String name)

void setAttribute(String name,Object object)

application.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'application.jsp' starting page</title>

</head>

<body>

int count = 0;

if(application.getAttribute("count")==null)

{

application.setAttribute("count",1);

count = 1;

}

else

{

String icount = application.getAttribute("count").toString();

count = Integer.parseInt(icount);

count++;

application.setAttribute("count",count);

}

out.println("你是第"+count +"个访问者");

</body>

</html>

存活范围最大的对象，只要服务器没有关闭，application的数据会一直存在，会被所有的用户共享。

其他常用的方法

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'application2.jsp' starting page</title>

</head>

<body>

out.println("java servlet API version: <b>"

+ application.getMajorVersion() + "."

+ application.getMinorVersion() + "</b><br>");

out.println("URL of 'application2.jsp' is: <b>"

+ application.getResource("/application2.jsp")

+ "</b><br>");

out.println("getServerInfo() = <b>" + application.getServerInfo() + "</b><br>");

out.println("<b>" + application.getRealPath(" ") + "</b>");

</body>

</html>

输出结果为：

java servlet API version: 2.5
URL of 'application2.jsp' is: jndi:/localhost/test/application2.jsp
getServerInfo() = Apache Tomcat/6.0.16
D:\MyEclipse\projects\webtest\WebRoot\

Forward机制

<jsp:forward>

会将用户的请求转发到其他页面，传递的是始终是同一个请求。

在请求期间，可以用request对象传递数据，但是请求结束，数据就会销毁。
myForward.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'myForward.jsp' starting page</title>

</head>

<body>

username:

</form>

</body>

</html>

MyFormServlet.jsp

public class MyFormServlet extends HttpServlet

{

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException

{

String username= request.getParameter("username");

request.setAttribute("username",username);

List<String> list = new ArrayList<String>();

for(int i=0;i<100;i++)

{

list.add(i+"");

}

request.setAttribute("list",list);

RequestDispatcher rd = request.getRequestDispatcher("myresult.jsp");

rd.forward(request,response);

}

Myresult.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'myresult.jsp' starting page</title>

</head>

<body>

username:

<%=request.getAttribute("username")%>

<br>

List<String> list = (List<String>) request.getAttribute("list");

for (int i = 0; i < list.size(); i++)

{

out.println(list.get(i) + "<br>");

}

</body>

</html>

Servlet主要进行流程处理，包括派发请求并增加额外处理等。

jsp主要处理页面显示的问题。

************************************************************************

Asp.net常用的页面传值的方式：(也是实际开发中常用的方式)

Hidden1.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'hidden1.jsp' starting page</title>

</head>

<body>

please input your name:

</form>

</body>

</html>

Hidden2.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'hidden2.jsp' starting page</title>

</head>

<body>

String username = request.getParameter("username");

username:

<%=username%>

please input your like:

</form>

</body>

</html>

Hidden3.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>My JSP 'hidden3.jsp' starting page</title>

</head>

<body>

String username = request.getParameter("username");

String like = request.getParameter("like");

your name is :

<%=username%>

<br/>

your like is :

<%=like%>

</body>

</html>