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Pierre Delisle's BlogResource Injection and the Java Persistence API in web applicationsPosted by pierre on January 09, 2006 at 05:16 PM | Comments (5)The Java EE tutorial constitutes a great resource to help developers better understand what the Java EE platform is all about. It covers the technologies you will be interested in if you write enterprise applications using EJBs as well as simpler web applications. See the tutorial roadmap diagram to get an idea of the key topics that are addressed by the tutorial. Lately, I've been helping Jennifer (one of the tutorial writers) update the "Duke's Bookstore" sample application of the tutorial so it leverages some of the new Java EE 5 features. Here is the tutorial's description of the Duke's Bookstore sample app: In Chapters 11 through 22 a common example--Duke's Bookstore--is used to illustrate the elements of Java Servlet technology, JavaServer Pages technology, the JSP Standard Tag Library, and JavaServer Faces technology. The example emulates a simple online shopping application. It provides a book catalog from which users can select books and add them to a shopping cart. Users can view and modify the shopping cart. When users are finished shopping, they can purchase the books in the cart.More specifically, we've been looking at migrating the 'database access' portion of the app from JNDI and JDBC to the new Java EE 5 Resource Injection and Java Persistence API (from the EJB 3.0 specification). I must admit we've struggled a little bit with some of the concepts that one must master to properly use this new combo. Even if Resource Injection and the Java Persistence API can be used independently of EJBs, it appeared to us that familiarity with EJB usage patterns makes it easier to understand how to properly use these new features in the context of webtier applications. Here are therefore a few thoughts on the topic. Basic concepts, the EJB wayIn the EJB world, an entity bean represents a business object in a persistent storage mechanism. In Duke's bookstore, 'Book' is the typical business object. In the Application Server, the persistent storage mechanism is a relational database. Each entity bean has an underlying table in a relational database, and each instance of the bean corresponds to a row in that table.A session bean, be it stateless or stateful, is an entry point into the business logic of the enterprise application. So when a user wants to add a book to the shopping cart, the ShoppingCart session bean is invoked and it takes care of properly handling the operation. Finally, all of the resources exposed by an enterprise application need to to be accessible. Getting a handle to these resources is typically handled through JNDI. Most will agree that the above concepts are simple and straightforward. Unfortunately, the complexity associated with the implementation of EJBs have scared more than one developer in the past. Luckily, much of this is changing with the Ease of Development focus of Java EE 5. Not only is it easier to work with EJBs, but benefits associated with traditional EJBs can be now be leveraged without even using them, as is the case with web applications. The new order: Java Persistence API and Resource InjectionAs stated in the EJB 3.0 specification, the Java Persistence API is "the Java API for the management of persistence and object/relational mapping with Java EE and Java SE". Entity beans are therefore no longer an exclusive feature of the Enterprise platform. The Java Persistence API is totally independent of EJBs and provides the Object-Relational Mapping layer that is so convenient to persist business objects.As can be seen in the tech tip Converting a POJO to a Persistent Entity, taking advantage of Java Persistence is quite easy. The only issue though is that using Java Persistence is much easier if you can rely on a container that supports "Resource Injection", a new feature also introduced in Java EE 5. Note though that Resource Injection is limited to components that are managed by a Java EE compliant container. Session beans therefore fit the model perfectly. But web applications live in a world that is EJB-free. While there are no session beans in a webapp, the need to implement business logic that is accessible over the web still exists. To fill that need, web app developers can choose from a variety of technologies. This choice is clearly showcased with the Dukes Bookstore web application. The webapp is implemented using three different technologies: pure Servlets, JSP, and finally JSF (and one could easily think of a Struts-based implementation to complement the offering). The challenge for a webapp developer that uses the Java Persistence API is therefore to design the application such that Resource Injection can be used (go with the flow young man, don't fight it!). As mentioned above, Resource Injection can only be used by classes that are managed by a Java EE compliant container. This is for two reasons. First, for performance considerations, a container can restrict its search of annotations only to the components it manages, which are defined in a deployment descriptor or accessible in specific locations of the classpath. Second, the container must have control over the creation of the component to be able to transparently perform the injection into the component. While it is true that Resource Injection would be most useful in the webtier if it were applicable to any POJO (Plain Old Java Objects), this will have to wait for a future release. In Java EE 5, the support for resource injection in the webtier is limited to the following webtier-managed components:
If the entry point into the business logic of a web application is a webtier-managed component, then all resources required to support the Java Persistence API can be injected transparently. This therefore works very well in a Servlet environment because servlets act as controllers into the business logic. The same holds in a JSF environment where managed beans associated with a page also act as controllers into the business logic of the application. The situation is unfortunately not as straightforward with JSPs. JSP pages cannot be the target of resource injection because all the information represented by these injection annotations needs to be known at deployment time. If an annotation is included in a JSP page, it won't be seen at deployment time. [actually, it would if the page had been precompiled, but one cannot assume that all pages are precompiled]. A possible solution is to use a tag library since tag handlers can be the target of resource injections. See Sahoo's blog for an example. The problem though is that this promotes the use of scriptlets in a JSP page, which should ideally be avoided. Ideally, we'd want a bean that is declared in a JSP page via <jsp:useBean> to act as a controller and be "injectable", just as is the case with managed beans in JSF. Since this is not possible, a solution is to allow this controller bean to access resources that have been injected elsewhere. That's what we're planning to do in Duke's bookstore; simply use a ServletContextListener to save global instances of the injected resources. These resources can then be accessed from the controller bean. Confused? In a forthcoming blog I'll describe how JNDI and JDBC are used in the J2EE 1.4 version of Duke's bookstore, and how we're planning to modify the different implementations of the webapp (servlets, JSP, and JSF) to take advantage of Resource Injection and the Java Persistence API introduced in Java EE 5. And if you have comments on the topic, please share them with us all... Thanks to Jennifer, Sahoo, and Marina for their contributions in helping clarify the migration path to Resource Injection and the Java Persistence API. Bookmark blog post:  del.icio.us  Digg  DZone  Furl  Reddit
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