How EJB home implements the Factory Method pattern

Written by Chintan Rajyaguru

Saturday, 28 March 2009 06:07

We had an interesting discussion at work about design patterns and EJB Home interface was used as an example of the Factory Method pattern as defined in the GoF book. I have heard this before. Upon googling, I found more places where EJB Home interface is used as the example of the Factory Method pattern. See here, here and here.

The factory method UML diagram is shown below.

factory_method

The explanation typically given is, "home interface provides methods to create bean instances" or "EJB home interface, which creates new EJBObjects" or "XxxHome creates instances of Xxx and YyyHome creates instances of Yyy." This is incomplete and even inaccurate explanation of the Factory Method pattern.

To make my point, I turn to the design pattern bible - the GoF book. The book clearly says, "Use the Factory Method pattern when, a class wants its subclasses to specify the objects it creates." It is easy to think that in case of EJB Home, javax.ejb.EJBHome wants its subclass (say) AccountHome to specify the objects (say) AccountRemote it creates.

So far so good. However, as shown in the figure above, the factory method is defined by the abstract super class or interface and implemented by the subclass. In case of EJBHome, it does NOT define any create methods for its subclasses to implement. See for yourself here. Create method is a 'special' method defined by the J2EE specification as a means to return an implementation of the business interface.

There is also another issue.

factory_method2

The figure above shows multiple subclasses that provide factory methods. They all create objects of type Figure (LineFigure or TextFigure). When you apply this concept to EJBHome interface, multiple subclasses such as AccountHome and OrderHome do NOT create objects of the same super type. AccountHome creates AccountRemote and OrderHome creates OrderRemote. These business interfaces don't implement the same contract.

The correct explanation

With EJB home interfaces, factory method is used when the 'container' implements the interface.

accounthome

When you define the AccountHome interface, you don't know how to create the objects that implement Account business interface. So, you leave it up to the subclass or the implementation of your interface to decide how to create those objects. You define a factory method and let the implementation provide the method. When you deploy your bean in a 'different container,' you may have Container2AccountHome that implements the factory method create () and creates Container2Account classes when invoked by the client. In yet another container, you may have Container3AccountHome and so on.

In case of the home interface example of the Factory Method Pattern, you will NOT have a situation shown in the second figure (Figure and Manipulator classes) above.

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