理论

类名	域名	功能
ServletContext	context域	 在整个服务器上保存，所有用户都可使用。重启服务器后无效JSP内置对象
HttpSession	session域	再一次会话中有效。服务器跳转、客户端跳转都有效。网页关闭重新打开无效
HttpServletRequet	request域	只在一次请求中有效，服务器跳转之后有效。客户端跳无效
PageContext	page域	只在一个页面中保存属性。跳转之后无效。

域对象的作用：保存数据,获取数据，共享数据.

可以往这4个属性范围对象内部放置东西，然后在别的地方取出来使用。使用的时候谁的范围小谁优先。

示例1：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
	<body>
	    <%
	        application.setAttribute("application","application");
	        session.setAttribute("session","session");
	        request.setAttribute("request","request");
	        pageContext.setAttribute("pageContext","pageContext");
	
	        System.out.println(application.getAttribute("application"));
	        System.out.println(session.getAttribute("session"));
	        System.out.println(request.getAttribute("request"));
	        System.out.println(pageContext.getAttribute("pageContext"));
	    %>
	</body>
</html>

示例2：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
	<head>
	    <title>login</title>
	</head>
	<body>
	    <%
	        System.out.println(application.getAttribute("application"));
	        System.out.println(session.getAttribute("session"));
	        System.out.println(request.getAttribute("request"));
	        System.out.println(pageContext.getAttribute("pageContext"));
	    %>
	</body>
</html>

示例3：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
	<body>
	    <%
	        application.setAttribute("k","application");
	        session.setAttribute("k","session");
	        request.setAttribute("k","request");
	        pageContext.setAttribute("k","pageContext");
	
	        System.out.println(application.getAttribute("k"));
	        System.out.println(session.getAttribute("k"));
	        System.out.println(request.getAttribute("k"));
	        System.out.println(pageContext.getAttribute("k"));
	    %>
	</body>
</html>

示例4：

Servlet代码：

@WebServlet("/demo")
public class DemoServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //this指当前类的对象
        //this.getServletContext这个方法的返回值对象就是application对象
        ServletContext application = this.getServletContext();
        application.setAttribute("k","application");

        //获取Session对象
        HttpSession session = request.getSession();
        session.setAttribute("k","session");

        request.setAttribute("k","request");

        request.getRequestDispatcher("demo.jsp").forward(request,response);
    }
}

JSP页面代码

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
	<head>
	    <title>Title</title>
	</head>
	<body>
	fdsfdsfsd<br>
	    <%
	        out.println("application "+application.getAttribute("k"));
	        out.println("session "+session.getAttribute("k"));
	        out.println("request "+request.getAttribute("k"));
	        out.println("pageContext "+pageContext.getAttribute("k"));
	    %>
	</body>
</html>

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JSP中四大域对象

理论

示例1：

示例2：

示例3：

示例4：

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