JSP-Servlet Interaction

Although coding JSP pages is convenient in many ways, some situations call for servlets. One example is when you are outputting binary data, as discussed in "Reasons to Avoid Binary Data in JSP Pages".

Therefore, it is sometimes necessary to go back and forth between servlets and JSP pages in an application. This section discusses how to accomplish this, covering the following topics:

• Invoking a Servlet from a JSP Page

• Passing Data to a Servlet Invoked from a JSP Page

• Invoking a JSP Page from a Servlet

• Passing Data Between a JSP Page and a Servlet

• JSP-Servlet Interaction Samples

  Important: This discussion assumes a servlet 2.2 environment. Appropriate reference is made to other sections of this document for related considerations for Apache/JServ and other servlet 2.0 environments.

Invoking a Servlet from a JSP Page

As when invoking one JSP page from another, you can invoke a servlet from a JSP page through the jsp:include and jsp:forward action tags. (See "JSP Actions and the <jsp: > Tag Set".) Following is an example:

<jsp:include page="/servlet/MyServlet" flush="true" />

When this statement is encountered during page execution, the page buffer is output to the browser and the servlet is executed. When the servlet has finished executing, control is transferred back to the JSP page and the page continues executing. This is the same functionality as for jsp:include actions from one JSP page to another.

And as with jsp:forward actions from one JSP page to another, the following statement would clear the page buffer, terminate the execution of the JSP page, and execute the servlet:

<jsp:forward page="/servlet/MyServlet" />

Important: You cannot include or forward to a servlet in Apache/JServ or other servlet 2.0 environments; you would have to write a JSP wrapper page instead. For information, see "Dynamic Includes and Forwards in Apache/JServ".

Passing Data to a Servlet Invoked from a JSP Page

When dynamically including or forwarding to a servlet from a JSP page, you can use a jsp:param tag to pass data to the servlet (the same as when including or forwarding to another JSP page).

A jsp:param tag is used within a jsp:include or jsp:forward tag. Consider the following example:

<jsp:include page="/servlet/MyServlet" flush="true" >
   <jsp:param name="username" value="Smith" />
   <jsp:param name="userempno" value="9876" />
</jsp:include>

For more information about the jsp:param tag, see "JSP Actions and the <jsp: > Tag Set".

Alternatively, you can pass data between a JSP page and a servlet through an appropriately scoped JavaBean or through attributes of the HTTP request object. Using attributes of the request object is discussed later, in "Passing Data Between a JSP Page and a Servlet".

Note: The jsp:param tag was introduced in the JSP 1.1 specification.

Invoking a JSP Page from a Servlet

You can invoke a JSP page from a servlet through functionality of the standard javax.servlet.RequestDispatcher interface. Complete the following steps in your code to use this mechanism:

1. Get a servlet context instance from the servlet instance:

   ServletContext sc = this.getServletContext();
   
   

2. Get a request dispatcher from the servlet context instance, specifying the page-relative or application-relative path of the target JSP page as input to the getRequestDispatcher() method:

   RequestDispatcher rd = sc.getRequestDispatcher("/jsp/mypage.jsp");
   
   

   Prior to or during this step, you can optionally make data available to the JSP page through attributes of the HTTP request object. See the next section, "Passing Data Between a JSP Page and a Servlet", for information.

3. Invoke the include() or forward() method of the request dispatcher, specifying the HTTP request and response objects as arguments. For example:

   rd.include(request, response);
   
   

   or:

   rd.forward(request, response);
   
   

   The functionality of these methods is similar to that of jsp:include and jsp:forward actions. The include() method only temporarily transfers control; execution returns to the invoking servlet afterward.

   Note that the forward() method clears the output buffer.

   Notes: The request and response objects would have been obtained earlier using standard servlet functionality, such as the doGet() method specified in the javax.servlet.http.HttpServlet class. This functionality was introduced in the servlet 2.1 specification.

Passing Data Between a JSP Page and a Servlet

The preceding section, "Invoking a JSP Page from a Servlet", notes that when you invoke a JSP page from a servlet through the request dispatcher, you can optionally pass data through the HTTP request object.

You can accomplish this using either of the following approaches:

• You can append a query string to the URL when you obtain the request dispatcher, using "?" syntax with name=value pairs.

  Here is an example:

  RequestDispatcher rd = 
         sc.getRequestDispatcher("/jsp/mypage.jsp?username=Smith");
  
  

  In the target JSP page (or servlet), you can use the getParameter() method of the implicit request object to obtain the value of a parameter set in this way.

• You can use the setAttribute() method of the HTTP request object.

  Here is an example:

  request.setAttribute("username", "Smith");
  RequestDispatcher rd = sc.getRequestDispatcher("/jsp/mypage.jsp");
  
  

  In the target JSP page (or servlet), you can use the getAttribute() method of the implicit request object to obtain the value of a parameter set in this way.

  Notes: This functionality was introduced in the servlet 2.1 specification. Be aware that the semantics are different between the servlet 2.1 specification and the servlet 2.2 specification--in a servlet 2.1 environment a given attribute can be set only once. Mechanisms discussed in this section can be used instead of the jsp:param tag to pass data from a JSP page to a servlet.

JSP-Servlet Interaction Samples

This section provides a JSP page and a servlet that use functionality described in the preceding sections. The JSP page Jsp2Servlet.jsp includes the servlet MyServlet, which includes another JSP page, welcome.jsp.

Code for Jsp2Servlet.jsp

<HTML>
<HEAD> <TITLE> JSP Calling Servlet Demo </TITLE> </HEAD>
<BODY>
<!-- Forward processing to a servlet -->
<% request.setAttribute("empid", "1234"); %>
<jsp:include page="/servlet/MyServlet?user=Smith" flush="true"/>
</BODY>
</HTML>

Code for MyServlet.java

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.PrintWriter;
import java.io.IOException;

public class MyServlet extends HttpServlet {

    public void doGet (HttpServletRequest request,
                       HttpServletResponse response) 
      throws IOException, ServletException {
      PrintWriter out= response.getWriter(); 
      out.println("<B><BR>User:" + request.getParameter("user"));
      out.println
          (", Employee number:" + request.getAttribute("empid") + "</B>");
      this.getServletContext().getRequestDispatcher("/jsp/welcome.jsp").
        include(request, response);
    }
}

Code for welcome.jsp

<%-----------------------------------------------------------
   Copyright © 1999, Oracle Corporation. All rights reserved.
------------------------------------------------------------%>

<HTML>
<HEAD> <TITLE> The Welcome JSP  </TITLE> </HEAD>
<BODY>

<H3> Welcome! </H3>
<P><B> Today is <%= new java.util.Date() %>.  Have a nice day! </B></P>
</BODY>
</HTML>

---