Recipe 16.11 Accessing or Removing Request Attributes in Servlets

Problem

You want a servlet to access or remove a request attribute.

Solution

Use the javax.servlet.ServletRequest.getAttribute( ) and javax.servlet.ServletRequest.removeAttribute( ) methods, including the name of the attribute as the method parameter.

Discussion

Example 16-13 is derived from the doGet( ) method of Example 16-11 in Recipe 16.9 (refer to that class if you need to review the complete code of a servlet handling request attributes). Example 16-13 gets an object attribute from the HttpServletRequest object, which is the doGet( ) method's first parameter.

The servlet container creates an HttpServletRequest object and passes it as the first parameter to all of the HttpServlet's service methods, including doGet( ) and doPost( ).

Example 16-13 calls one of the attribute's methods, then removes the request attribute.

Example 16-13. A servlet accesses and removes a request attribute

public void doGet(HttpServletRequest request, HttpServletResponse response) 
    throws ServletException, java.io.IOException { 

    ContextObject obj = (ContextObject) request.getAttribute(
        "com.jspservletcookbook.ContextObject");

    response.setContentType("text/html");
    java.io.PrintWriter out = response.getWriter( );
    out.println(
        "<html><head><title>Request Attribute</title></head><body>");

    //display the attribute's Map keys
    out.println("<h2>Request attribute values</h2>");

    if (obj != null)
        out.println( obj.getValues( ) );

    //This method call may not be necessary as request attributes
    //persist only as long as the request is being handled, 
    //according to the ServletRequest API documentation. 
    request.removeAttribute("com.jspservletcookbook.ContextObject");

    out.println("</body></html>");
      
} //doGet

If the attribute does not exist in the request (because it was not bound to the request in the first place), ServletRequest.getAttribute( ) returns null. Make sure the servlet code checks for a null value before it calls the object's methods. In addition, the ServletRequest.getAttribute( ) method returns an Object type, so ensure that the servlet code casts the return value to the proper type before calling the expected type's methods.

Recipe 16.11 Accessing or Removing Request Attributes in Servlets

Problem

Solution

Discussion

Example 16-13. A servlet accesses and removes a request attribute

See Also