Friday, 14 November 2014

jsp pageContext implicit object

pageContext is an implicit object of type PageContext class.
It can be used to get,set and remove attributes from these scopes-
  •  page
  •  request
  •  session
  •  application

 Why pageContext??
  •  a single API to manage the various scoped namespaces
  •  a mechanism to obtain the JspWriter for output
  •  a mechanism to manage session usage by the page
  •  mechanisms to forward or include the current request to other active components in the application
  •  a mechanism to handle errorpage exception processing

Popular Methods of implicit object pageContext-

  1.Object findAttribute (String AttributeName)
 It searches for the attribute at all the level in the order specified below
  •  page
  •  request
  •  session
  •  application
 If it does not anything till the last,it returns NULL.

  2.Object getAttribute (String attributename, int scope)
 This one is also kind of specific version of above one.
 It only simply checks in the specified scope.

  String username = (String)pageContext.getAttribute("user", PageContext.SESSION_CONTEXT);
                           
 in place of PageContext.SESSION_CONTEXT,we can specify other scopes as well.

  3. void removeAttribute(String attributename, int scope)
 This is used to remove an attribute from the specified scope.
 pageContext.removeAttribute(“user”, PageContext. APPLICATION_CONTEXT);

  4. void setAttribute(String attributename, Object attributevalue, int scope)
 Again as name suggests,it sets the attribute for the specified scope.
 pageContext.setAttribute(“user”, “Prince”, PageContext. APPLICATION_CONTEXT);

Some Key Points
  • It stores referece to the implicit objects. 
        By using this object,we can populate other objects as shown below
        ...
        application = pageContext.getServletContext ();
        config = pageContext.getServletConfig ();
        session = pageContext.getSession ();
        out = pageContext.getOut ();
         ...
  • Provides convenience methods to get and set attributes in different scopes.
PageContextExample.jsp 
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
 <%-- Saving the data into various scopes --%>
<%
  pageContext.setAttribute("attribute1", "value1");  // PAGE_SCOPE is the default
  pageContext.setAttribute("attribute2", "value2", PageContext.PAGE_SCOPE);
  pageContext.setAttribute("attribute3", "value3", PageContext.REQUEST_SCOPE);
  pageContext.setAttribute("attribute4", "value4", PageContext.SESSION_SCOPE);
  pageContext.setAttribute("attribute5", "value5", PageContext.APPLICATION_SCOPE);
%>
  <%-- Retrieving the data from  scopes --%>
Arrribute1 is <%= pageContext.getAttribute("attribute1") %>
Arrribute2 is <%= pageContext.getAttribute("attribute2", PageContext.PAGE_SCOPE) %>
Arrribute3 is <%= pageContext.getAttribute("attribute3", PageContext.REQUEST_SCOPE) %>
Arrribute4 is <%= pageContext.getAttribute("attribute4", PageContext.SESSION_SCOPE) %>
Arrribute5 is <%= pageContext.getAttribute("attribute5", PageContext.APPLICATION_SCOPE) %>
</body>
</html>
output screen
  •  Transferring requests to other resources in your application:
 void include (String url)-Includes the output of another resource defined by the relativeURL in the output of the current page.
 void forward (String url)-Forwards the request to another resource defined by the relativeURL.

 example-
 pageContext.forward ("anyOtherResource.jsp");
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