Recipe6.1.Creating a Base Action

Recipe 6.1. Creating a Base Action

Problem

You want to implement and enforce a common feature or behavior across all of your Actions.

Solution

Implement an abstract base class that incorporates the behavior in its execute( ) method. As shown by the class in Example 6-1, the execute( ) method calls abstract methods that are implemented by subclasses.

Example 6-1. Abstract base action

package com.oreilly.strutsckbk.ch06; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.struts.action.Action; import org.apache.struts.action.ActionForm; import org.apache.struts.action.ActionForward; import org.apache.struts.action.ActionMapping; public abstract class BaseAction extends Action {          public ActionForward execute( ActionMapping mapping,                                    ActionForm form,                                   HttpServletRequest request,                                    HttpServletResponse response)          throws Exception { executeBefore( );         // call the abstract method         ActionForward forward = executeAction( mapping, form,          request, response );         executeAfter( );         return forward;     }             protected abstract ActionForward executeAction( ActionMapping mapping,                                                         ActionForm form,                                                HttpServletRequest request,                                                HttpServletResponse response)                                      throws Exception;     private void executeBefore( ) {         //Real stuff goes here     }     private void executeAfter( ) {         //Real stuff goes here     }     protected CommonServices getCommonServices( ) {...} }

Discussion

For some applications, you may want to apply global business rules any time a user accesses the application, submits a form, or clicks on a link. Here are some common examples of these rules:

Users must be logged in.
Users can only access the page if they have been granted permission.
You want to track every time users hit a page.
The user's first name and last name should be displayed at the top of every page.

You can enforce these across-the-board features using an abstract base Action class that your custom Actions extend. The Solution above shows such a base Action. BaseAction extends org.apache.struts.action.Action and implements the standard execute( ) method. The execute() method calls the private method executeBefore( ) and then the abstract method executeAction(). executeAction( ) returns an ActionForward, which is stored in a local variable. Next, the private method executeAfter( ) is called. The execute( ) method ends by returning the ActionForward referenced by the local variable.

Your concrete actions extend this BaseAction. Each concrete action implements the executeAction( ) method as if it were the execute( ) method inherited from org.apache.struts.action.Action.

If you are familiar with design patterns, you will probably recognize the Solution as the Template Method from Design Patterns of Object-Oriented Software by Erich Gamma, et al. (Addison-Wesley). The Template Method pattern is characterized by a base abstract class that defines the steps of an algorithm. In the Solution, the steps are comprised of a pre-execution step, the execution step, and a post-execution step.

If you want subclasses to be able to override the common behavior in the pre- and post-execution steps, make the methods protected; otherwise, they should be private. The signature of the executeAction( ) method is identical to the execute( ) method; however, this is not required. In general, you'll want to pass through at least the set of parameters available to the execute( ) method. In addition, you may want to pass additional references to objects that are created in the base class, such as a user object representing the current user.

Base actions can provide access to global objects and services that concrete actions will need. For example, you might want to provide a getUser( ) method that returns a user object for the current session. Chapter 11 provides more detail on this specific case.