Implicit Objects

A JSP element may refer to several pre-declared implicit objects. These objects are classified according to their scope.

Application Scope

Objects with application scope persist as long as the server is running and are available to all servlets and JSP pages.

ServletContext application; // the container

Session Scope

A session refers to a sequence of requests from the same client and that are part of the same transaction. A session ends when

1. It has been inactive for a period of time
2. The session has been invalidated by the programmer
3. The client closes his/her browser

Objects with session scope persist for the duration of a client's session. They are available only to servlets and JSP pages that participate in the session.

HttpSession session; // client's session data

Request Scope

Several servlets or JSP pages may contribute to the response to a single request (through forward and include directives). Objects with request scope are visible to only these pages.

HttpServletRequest request; // client's request

Page Scope

Although several pages and servlets may contribute to a single response, objects with page scope are only visible to a single page.

HttpServletResponse response; // response to client

JspWriter out; // outputs text to response

PageContext pageContext; // provides access to all implicit objects

Throwable exception; // passed to the error page

Attribute Maps

The most important thing about the application, session, and request objects is that each provides an attribute map. The following methods can be used to manipulate the attribute map:

Object getAttribute(String name);
void setAttribute(String name, Object val);
void removeAttribute(String name);
Enumeration getAttributeNames();

Programmers use these maps to store data. Of course data stored in the application attribute map will have application scope, etc.

Example 1

<%!
   class CounterBean implements java.io.Serializable {
      private int count = 0;
      public CounterBean(int n) { count = n; }
      public CounterBean() { count = 0; }
      public void inc() { count++; }
      public int getCount() { return count; }
      public String toString() { return "" + count; }
   }
%>

<html>
<head> <title> Counters </title> </head>
<body>

<% CounterBean hitCount;
   hitCount = (CounterBean)application.getAttribute("hit count");
   if (hitCount == null) {
      hitCount = new CounterBean();
      application.setAttribute("hit count", hitCount);
   }
   hitCount.inc();

%>
Hit count = <%= hitCount %> <br/>

<% CounterBean sessionCount;
   sessionCount = (CounterBean)session.getAttribute("session count");
   if (sessionCount == null) {
      sessionCount = new CounterBean();
      session.setAttribute("session count", sessionCount);
   }
   sessionCount.inc();
   %>
Session count = <%= sessionCount %> <br/>

</body>
</html>

Try experimenting with this page using several browser sessions simultaneously.

Example 2

class CounterBean
   implements java.io.Serializable, HttpSessionBindingListener {
   private int count = 0;
   public CounterBean(int n) { count = n; }
   public CounterBean() { count = 0; }
   public void inc() { count++; }
   public int getCount() { return count; }
   public String toString() { return "" + count; }

   // automatically called when this counter is bound to a session
   public void valueBound(HttpSessionBindingEvent e) {
      System.out.println(e.getName() + " bound");
   }
   // automatically called when this counter
   // is unbound from a session
   public void valueUnbound(HttpSessionBindingEvent e) {
      System.out.println(e.getName() + " unbound");
   }
}

Add the following to the end of Counter.jsp body:

Session created at
   <%= new java.util.Date(session.getCreationTime()) %> <br />
Session last accessed at
   <%= new java.util.Date(session.getLastAccessedTime()) %> <br />

<% long timeOut = 25; // < 0 indicates no expiration
   session.setMaxInactiveInterval(timeOut); %>
Session max inactive interval is
   <%= session.getMaxInactiveInterval() + " secs" %> <br />

<% if (sessionCount.getCount() > 5) session.invalidate(); %>

Example 3

A Form

The Form Implementation

User info form: <hr />
<form action = "/myapp/request.jsp" >

   
   <p>
   <input type = "hidden" name = "arg" value = "42">
   Name: <input type = "text" name = "client" value = "???" size = "30"/> <br />
   Password: <input type = "password" name = "password" size = "8"/>
   </p>

   
   <p>
   Party: <br />
    <input type = "radio" name = "party" value="Democrat" checked/> Democrat <br />
    <input type = "radio" name = "party" value="Republican"/> Republican
   </p>

   
   <p>
   State: <br />
   <select name = "state">
     <option value = "CA">California</option>
     <option value = "OR">Oregon</option>
     <option value = "NM">New Mexico</option>
   </select>
   </p>

   
   <p>
   Comments: <br />
     <textarea name = "comments" rows = "5" cols = "40" >
     Your comments would be appreciated
     </textarea>
   </p>

   
   <p>
   Send me more information:
      <input type = "checkbox" name = "more info" />
   </p>

   
   <p>
   <input type = "reset" value = "Clear Form" />
   <input type = "submit" value = "Send Info" />
   </p>

</form>

The Response

request.jsp

<html>
<head> <title> Display HTTP Request </title> </head>
<body>

<p> Here's the request you sent to the server: </p>
<%
   java.util.Enumeration attNames
         = request.getAttributeNames();
   java.util.Enumeration paramNames
         = request.getParameterNames();
   java.util.Enumeration hdrNames
         = request.getHeaderNames();
%>
Request =
<%= request.getMethod() %>
<%= request.getServerName() %>:
<%= request.getServerPort() %>
<%= request.getProtocol() %>

<hr />
Request headers: <br/>
<% while (hdrNames.hasMoreElements()) {
      String name = (String) hdrNames.nextElement();
      String value = request.getHeader(name);
      out.println("..." + name + ": " + value + "<br />");
      } %>

<hr />
Request parameters: <br/>
<% while (paramNames.hasMoreElements()) {
      String name = (String) paramNames.nextElement();
      String value = request.getParameter(name);
      out.println("..." + name + ": " + value + "<br />");
      } %>

<hr />
Request attributes: <br/>
<% while (attNames.hasMoreElements()) {
      String name = (String) attNames.nextElement();
      Object value = request.getAttribute(name);
      out.println("..." + name + ": " + value + "<br />");
      } %>
<hr />
</body>
</html>