6.3 Application Deployment

Application deployment is the process of making a J2EE application available to end users on an application server. OC4J can deploy a J2EE application in an EAR file or a WAR (Web Application aRchive) file. When deploying a WAR file, OC4J automatically wraps a WAR file in an EAR file. OC4J provides a mapping mechanism so that differences in deployment environments can be resolved during the deployment process. OC4J also supports application development by accessing servlets and EJBs from an operating system directory without requiring OC4J-specific deployment descriptors. The remainder of this section takes a closer look at how OC4J handles application deployment, starting with standard J2EE deployment.

6.3.1 J2EE Deployment

J2EE applications are typically composed of JSPs, servlets, and EJBs. In a well-written J2EE application, each program unit acts as a separate and distinct component. To facilitate the composition of components into a working application, J2EE components have a descriptor file. A descriptor file exposes any configuration parameters, such as database resources, so that they can be specified declaratively ”that is, without changing a line of Java code.

EJBs are placed into a Java ARchive (JAR) file that contains the EJB, with any supporting custom classes, and a deployment descriptor file named ejb-jar.xml . There is typically one JAR file for each session or message-driven EJB, whereas a JAR file usually contains multiple entity EJBs because related entity EJBs must be co-located.

All servlets under a particular application context directory in a servlet container use the same deployment descriptor file named web.xml ; so do any JSP files that reside in the same application context directory because the JSP translator automatically compiles them into servlets. The servlets, JSPs, any supporting custom class libraries, JSP tag libraries, and the web.xml file are all placed into a WAR file; such a file facilitates deployment of a web application to another web container.

To create an application that uses both EJBs and servlets, WAR files and any required EJB JAR files are placed into a EAR file, along with an application deployment descriptor file appropriately named application.xml .

Applications are usually developed on one server, deployed on a second for quality assurance testing, and then deployed on a third to put them into production. If the environments aren't exactly the same, how do you resolve the differences? It is possible to make the appropriate changes to the individual J2EE descriptor files for the target environment before archiving the files into a WAR or EAR file, but this is at best a deployment nightmare. Another solution is for the host application server to provide a mapping mechanism during deployment. That is how OC4J solves this deployment problem.

6.3.2 OC4J Deployment

In OC4J, each J2EE deployment descriptor has an OC4J-specific counterpart that can map a J2EE deployment resource name to one that is available in the OC4J. Let's look at an example. Suppose that a data source defined on a development server is named DefaultDS and that the appropriate entry in the web application's deployment descriptor file, web.xml , references that name, yet on the production server, a dedicated data source named MyAppDS is defined for the web application. You can map the J2EE deployment descriptor reference DefaultDS to the dedicated data source name by specifying the mapping in the appropriate OC4J-specific deployment descriptor file orion-web .xml . Table 6-2 shows the relationships between the J2EE and OC4J specific deployment descriptor files.

Table 6-2. Deployment descriptor relationships

Type	J2EE	OC4J
EJB	ejb-jar.xml	orion-ejb-jar .xml
Web (servlet and JSP)	web.xml	orion-web.xml
Application	application.xml	orion-application.xml
Resource adapter	ra.xm l [2]	oc4j-ra.xml oc4j-connectors.xml

^[2] Supplied with the resource adapter

Using this mechanism, OC4J can deploy any J2EE-compliant application without having to modify its deployment descriptors. Deployment mappings are specified during deployment using Application Server Control.

6.3.3 OC4J Development

You can use OC4J as a development environment to compile EJBs, servlets, and so on, in the server directory tree. Doing so requires you to create a standard deployment directory structure and the appropriate J2EE deployment descriptor files, and make a set of manual configuration changes through Application Server Control.

If you create a separate development instance for each development project, you can restart OC4J quickly, without incurring the wrath of developers who may be working on other projects. When using OC4J as a development environment, your application's response times will be slower because nonarchived classes are loaded as-needed, whereas archived classes (that is, classes in JAR files) are cached in memory.