Packaging EJB 3 Applications
The real success of Java EE applications lies in assembly and deployment, as this is the key to delivering on Java's promise of write once, run anywhere (WORA). If you fail to fully grasp this step, your application may not realize this level of portability.
|Java platform roles: it's all about juggling hats|
The Java EE platform defines different roles and responsibilities relating to development, assembly, and deployment of Java EE applications. In this article we are mainly interested in the Developer, Assembler, and Deployer roles, but we introduce you to all the roles so that you can be familiar with them. The roles defined by the specifications are
The database administrator is not one of the defined Java EE roles. The database administrator may not even understand a line of Java code. However, the importance of this role cannot be overlooked, especially in large corporations where relational databases are outside the control of the application developers. Developers, Assemblers, and Deployers may need to work with the DBAs in order to successfully build and release Java EE applications.
It's all about the division of labor. Many believe that the difficulties of earlier EJB practices were a result of the division of the EJB roles. In reality, the previous EJB specifications were not the real culprit—the source of all the confusion is the Java EE specification. While the Java EE and EJB specifications define seven roles, the problem is that many project teams do not even have seven people—how can a two- or three-person team wear that many hats?
A typical application has a handful of Java classes, and maintenance can be a nightmare if you are shipping your applications from one environment to another. To simplify maintenance you can create a Java archive (JAR) file. Typically, a JAR file is a file in zip format that contains classes. However, enterprise Java applications are packaged as specialized versions of JAR files—EAR, WAR, and EJB-JAR modules—before they can be deployed to a Java EE.compliant application server.
In this article we begin with a discussion of application packaging and deployment. The article also provides critical information on class loading, so that you can appreciate why the archives are packaged as they are. This is intended to provide you a better understanding of the packaging requirements for EJB that include entities. We explain the need for deployment descriptors, and look at how to use them. Finally, we look at a persistence unit and how to per form object-relational (O/R) mapping using XML.
Packaging your applications
A typical enterprise Java application may contain several Java classes of different types, such as EJBs, servlets, JavaServer Faces (JSF) classes, as well as static files such as JSPs and HTML files. EJBs run in the EJB container whereas web applications such as servlets and JSF managed beans run in the web container. To run your application you have to make it available to the Java EE application server. This is known as deployment. Since EJB is a core part of the Java EE specification, you have to follow the Java EE standard for deployment.
To understand packaging, you must consider how it fits into the bigger picture of Java EE packaging and know what constitutes a complete enterprise Java application. Up to this point we have focused on using EJB components such as session beans and MDBs to build business logic and JPA entities to implement your persistence code. However, your application will not be complete without a presentation tier that accesses the business logic you built with EJBs. For example, the EJBs we built for ActionBazaar do not make sense unless we have a client application accessing them. Most likely, you've used standard technologies such as JSP or JSF to build the web tier of your applications. These web applications, together with EJBs, constitute an enterprise application that you can deploy to an application server.
To deploy and run an application, you have to package the complete application together—the web module and EJBs—and deploy to an application server. Usually you will group similar pieces of the application together in modules. Java EE defines a standard way of packaging these modules in JAR files, and specifies the formats for these JARs. One of the advantages of having these formats defined as part of the specification is that they are portable across application servers.
Table 1 lists the archives or modules supported by Java EE 5 and their contents. Note that each archive type is used for packaging a specific type of module, such as EJB or web. For instance, a WAR is used to package a web-tier application module, and the EAR file is intended to be the |über archive containing all the other archives so that in the end, you're only deploying one file. The application server will scan the contents of the EAR and deploy it. We discuss how an EAR is loaded by the server in section 1.2.
Table 1 Enterprise Java applications need to be assembled into specific types of JAR files before they can be deployed to an application server. These are the available module types as specified by Java EE.
|CAR||Client application archives||application-client .xml||Thick Java client for EJBs|
|EAR||Enterprise application archive||application.xml||Other Java EE modules such as EJB-JARs.|
|EJB-JAR||EJB Java archive||ejb-jar.xml||Session beans, message-driven beans, and optionally entities. Needs a persistence.xml if entities are packaged.|
|RAR||Resource adapter archives||ra.xml||Resource adapters.|
|WAR||Web application archives||web.xml||Web application artifacts such as servlets, JSPs, JSF, static files, etc. Entities can also be packaged in this module. Needs a persistence.xml if entities are packaged.|
To create these files, you can use the jar utility that comes with JDK. The final step is to assemble all the JAR files into one EAR file for deployment. In 3.1 we show you a build script that creates a JAR file. Each of these JAR types contains an optional deployment descriptor that describes the archive. As we have been discussing throughout this article, you can use metadata annotations instead of a deployment descriptor.
In this article, we focus primarily on the EAR file and the EJB-JAR file, which contains the session and message-driven beans, as well as entities.
It's worth mentioning that entities can be packaged in most archive types. For example, the ability to package entities in WARs allows you to use the EJB 3 JPA in simple web applications or with lightweight frameworks such as Spring. Note that entities are not supported in RAR modules. This statement, however, begs the question of why Java does not have a different archive type to package EJB 3 entities, just as JBoss has the Hibernate Archive (HAR) to package persistence objects with Hibernate's O/R framework.
You may know the answer to this question if you have followed the evolution of the EJB 3 specification. For those who haven't, we now regale you with Tales from the Expert Group (cue spooky music)...
During the evolution of the EJB 3 Public Draft, the PAR (Persistence Archive) was introduced, which mysteriously vanished in the Proposed Final Draft. A huge, emotional battle was fought in the EJB and Java EE expert groups over whether to introduce a module type for a persistence module at the Java EE level, and suggestions were sought from the community at large, as well as from various developer forums. Many developers think a separate persistence module is a bad idea because entities are supported both outside and inside the container. Considering that persistence is inherently a part of any enterprise application, it makes sense to support packaging entities with most module types, instead of introducing a new module type specialized for packaging entities.
Now that you know what modules are supported and a little about how they were arrived at, shall we take a quick peek under the hood of an EAR module?
Dissecting the EAR file
To understand how deployment works, let's take a closer look at the EAR file, the top-level archive file that contains other Java EE archives when it is deployed to the application server. For instance, the ActionBazaar application contains an EJB module, a web module, a JAR containing helper classes, and an application client module. The file structure of the EAR file that ActionBazaar uses looks like this:
META-INF/application.xml actionBazaar-ejb. jar actionBazaar .war actionBazaar-client.jar lib/actionBazaar-commons.jar
application .xml is the deployment descriptor that describes the standard Java EE modules packaged in each EAR file. The contents of application.xml look something like listing 1.
Listing 1: Deployment descriptor for the ActionBazaar EAR module
If you review the EAR file descriptor in listing 1, you'll see that it explicitly identifies each of the artifacts as a specific type of module. When you deploy this EAR to an application server, the application server uses the information in the deployment descriptor to deploy each of the module types.
Java EE 5 made the deployment descriptor optional, even in the EAR. This is a departure from previous versions of Java EE, where it was mandatory. The Java EE 5.0.compliant application servers deploy by performing automatic detection based on a standard naming convention or reading the content of archives; see http://java.sun.com/blueprints/code/namingconventions.html.
Next, let's take a look at how application servers deploy an EAR module.
Loading the EAR module
During the deployment process, the application server determines the module types, validates them, and takes appropriate steps so that the application is available to users. Although all application servers have to accomplish these goals, it's up to the individual vendor exactly how to implement it. One area where server implementations stand out is in how fast they can deploy the archives.
While vendors are free to optimize their specific implementation, they all follow the specification's rules when it comes to what is required to be supported and in what order the loading occurs. This means that your application server will use the algorithm from figure 1 when attempting to load the EAR file that contains modules or archives from table 1.
Before we delve into how EJB components and entities are packaged, let's briefly discuss what class loading is and how it works in the Java EE environment.
Figure 1: Rules followed by application servers to deploy an EAR module. Java EE 5 does not require a deployment descriptor in the EAR module that identifies the type of modules packaged. It is the responsibility of Java EE container to determine the type of module based on its name (extension) and its content. It does so by following this algorithm.
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