Programming Jakarta Struts By Chuck Cavaness - Part2

Yesterday i started reading Programming Jakarta Struts By Chuck Cavaness.

Nice book written by Chuck Cavaness.

I wanted to share few quotations found from the next 2 chapters(3-4) from this book.

1) The ActionServlet extends the javax.servlet.http.HttpServlet class and is responsible for packaging and routing HTTP traffic to the appropriate handler in the framework. The ActionServlet class is not abstract and therefore can be used as a concrete controller by your applications. Prior to Version 1.1 of the Struts framework, the ActionServlet was solely responsible for receiving the request and processing it by calling the appropriate handler. In Version 1.1, a new class, called org.apache.struts.action.RequestProcessor , was introduced to process the request for the controller. The main reason for decoupling the request processing from the ActionServlet is to provide you with the flexibility to subclass the RequestProcessor with your own version and modify how the request is processed.

2) Once the controller receives a client request, it delegates the handling of the request to a helper class. This helper knows how to execute the business operation associated with the requested action. In the Struts framework, this helper class is a descendant of the org.apache.struts.action.Action class.

3) The various articles, tutorials, and other resources available on the Struts framework disagree about whether the Action class is part of the controller or the model. The argument for it being part of the controller is that it isn't part of the "real" business logic. If Struts were replaced with an alternative framework, chances are the Action class would be replaced with something else. Therefore, it really isn't part of the model domain, but rather is tightly coupled to the Struts controller. It doesn't make sense to put business logic into the Action, because other types of clients can't easily reuse it.

Another reason to consider the Struts Action class part of the controller is that it has access to the ActionServlet, and therefore all of the controller resources, which the model domain shouldn't know about. Hypothetically, the Action class's behavior could have been left in the servlet, and the servlet would call the appropriate method on itself. If this were the case, there would be no doubt about whether this was controller or model functionality.

With all of that said, the Action class may invoke operations on the business model, and many developers end up trying to insert too much of their business logic into the Action classes. Eventually, the line becomes blurry. Perhaps this is why some developers consider it part of the model. However, this book will take the approach that the Action class is part of the controller.

4) An org.apache.struts.action.Action class in the Struts framework is an extension of the controller component. It acts as a bridge between a client-side user action and a business operation. The Action class decouples the client request from the business model. This decoupling allows for more than a one-to-one mapping between the user request and an Action. The Action class also can perform other functions, such as authorization, logging, and session validation, before invoking the business operation.

The execute( ) method is called by the controller when a request is received from a client. The controller creates an instance of the Action class if one doesn't already exist. The Struts framework will create only a single instance of each Action class in your application. Because there is only one instance for all users, you must ensure that all of your Action classes operate properly in a multithreaded environment, just as you would do when developing a servlet.

5) The format for both the web application deployment descriptor and the Struts configuration file is based on a Document Type Definition (DTD), which defines the legal building blocks that may be used in the XML files. From the DTD point of view, all XML documents, including the web application deployment descriptor and the Struts configuration file, are made up of the following elements:

· Elements

· Tags

· Attributes

· Entities

· PCDATA

· CDATA

6) Because the Struts ActionServlet class is not abstract, you are free to use that class and avoid having to create a subclass of the ActionServlet for your application. With earlier versions of the Struts framework, it was more important to extend the ActionServlet class with one of your own because most of the processing occurred there, and subclassing allowed you to override that functionality with your own. With Version 1.1, however, most of the processing functionality has been moved to another class, which you can configure declaratively (as you'll see later in this chapter). There is little reason to create your own ActionServlet class, although you are still free to do so.

7) If the container is distributable and uses more than one JVM, the web application may have a separate ServletContext instance for each JVM.

8) The ServletContext provides an external view of the web container environment for a servlet. A servlet can use the ServletContext object to gain access to external resources, log events, and store attributes and objects that other servlet instances in the same context can access. It's essentially an application-scope shared resource.


About the Author

Chuck Cavaness is a graduate from Georgia Tech with degrees in computer engineering and computer science, has built Java-based enterprise systems in the healthcare, banking, and B2B sectors. Working at an Internet company to design and develop software architecture, Chuck has spent many frustrating hours figuring out the dos and the don'ts of web applications. With each enterprise system he's developed, Chuck has learned several valuable lessons about building "real-world" web applications, information that he's made available to developers who haven't had the opportunity to work on large systems.

Chuck is the co-author of Special Edition Using Java 1.3 and Special Edition Using EJB 2.0, both available from QUE.