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Recipe 16.11 Accessing or Removing Request Attributes in Servlets


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


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


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(

    java.io.PrintWriter out = response.getWriter( );
        "<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. 

} //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.

See Also

Recipe 16.1-Recipe 16.4 on handling ServletContext attributes in servlets and JSPs; Recipe 16.5-Recipe 16.8 on handling session attributes in servlets and JSPs; Recipe 16.12 on accessing or removing request attributes in JSPs; Chapter 6 on including content in servlets and JSPs; the Javadoc for javax.servlet. ServletRequestAttributeListener: http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletRequestAttributeListener.html.

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