Web User Interface for a Java application

Question

I'm trying to create a web user interface for a Java application. The user interface is going to be very simple, consisting of a single page with a form for users to pose their queries, and a results page -- sort of like Google's search engine or Ask.com.

I'm quite familiar with the base API of Java, but I don't have much experience in using Java for Web environments (though I've used ASP.NET), so I'm looking for some advice:

• What web application server should I use? Note that my interface is very light, and I just want something that is fast, easy to start/reset/stop and (re)deploy my application. Also, I need it to work on multiple environments, namely, GNU/Linux, Mac OS X, and Windows XP/Vista. Additionally, I'm using ant and Eclipse, so it would be great if I could easily add some ant targets for server management, and/or manage the server using the IDE. I've looked into Tomcat and Jetty, and the latter seems to be very light and easy to install and deploy. This is ideal, because the GUI is just for demonstration purposes, and I'll probably need to deploy it in different computers. However, Tomcat has been around for a very long time, and it seems more mature.

• As for the web pages, Java Server Pages look like a good fit, as they seem sufficiently simple for what I'm trying to accomplish (processing a form and outputting the result), but I'm all ears for suggestions.

• I also have another requirement, which requires me to explain the "basic" workflow of the application: Basically, I have a class Engine which has a method run(String) which will process the user's input and return the results for display. This class is the core of the application. Now, I'd like to instantiate this class only once, as it requires a lot of memory, and takes a very long time to startup, so I'd like to create it when the application/server starts, and store that reference for the entire span of the application (i.e., until I stop the server). Then, for each user request, I'd simply invoke the run method of the Engine instance, and display its results. How can this be accomplished in Java?

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Answer 1

1. App Server. You see Tomcat as heavy in terms of runtime footprint, or amount of learning or ...? I would tend to chose something that has well established integration with an IDE. So Eclipse + Tomcat or Apache Geronimo, perhaps in it's WebSphere Community Edition guise would do the job. From what I've seen these are sufficient for what you want, and the learning curves are really pretty manageable.
2. Yes JSPs. You may yet find that your presentation needs become a tad more complex. The extra effort of going to JSF might yet pay off - nice widgets such as Date pickers.
3. In your processing you will have a servlet (or an action class if you're using JSF) that class can have a member variable of type Engine initialised on startup and then used for every request. The thing to bear in mind is that many users will hit that servlet, and hence that engine at the same time. Is your Engine safe to be used from more than one thread at the same time?

To expand in this point. When implementing JSPs, there are two models refered to as (with some inventiveness) as Model 1 and Model 2. See this explanation.

In the case of model 1 you tend to put code directly into the JSP, it's acting in a controller role. Persoanlly, even when dealing with small, quickly developed apps, I do not so this. I always use Model 2. However if you choose you can just put some Java into your JSP.

<%  MyWorker theWorker = MyWorkerFactory.getWorker();
    // theWorker.work();
%>

I woudl favour having a factory like this so that you can control the creation of the worker. The factory would have something like (to give a really simple example)

private static MyWorker s_worker = new MyWorker();
public static synchronized getWorker() {
       return s_worker;
}

Alternatively you could create the worker when that method is first called.

In the case of model 2 you naturally have a servlet into which you are going to put some code, so you can just have

private MyWorker m_worker = MyWorkerFactory.getWorker();

This will be initialised when the servlet is loaded. No need to worry about setting it to load on startup, you just know that it will be initialsed before the first request is run. Better still, use the init() method of the servlet. This is guranteed to be called before any requests are processed and is the servlet API architected place for such work.

public class EngineServlet extends HttpServlet {

private Engine engine;

// init is the "official" place for initialisation
public void init(ServletConfig config) throws ServletException {
     super.init(config);
     engine = new Engine();
} 

Answer 2

The technology you need to learn is the Sun Java Servlet specification since that is what ALL non-trivial java webservers implement. This enables you to write servlets which can do all the things you need server side. You can then develop against any container working well with your iDe, and deploy on any other container working well in production.

You also need to learn basic HTML as you otherwise would need to learn JavaServer Faces or similar which is a rather big mouthfull to create the submit button you need with the other entries in a HTML form.

For your Engine to work you can create a servlet with a singleton in web.xml, which you can then call. Be absolutely certain it is thread safe otherwise you will have a lot of pain. For starters you can declare your invoking servlet synchronized to ensure that at most one call of run() is active at any time.