This section describes the need to migrate J2EE applications and the particular files that must be migrated. Following successful migration, a J2EE application is redeployed to the Application Server.
Redeployment is also described in this section.
The following topics are addressed:
Although J2EE specifications broadly cover requirements for applications, they are nonetheless evolving standards. They either do not cover some aspects of applications or leave implementation details to the application providers.
This leads to different implementations of the application servers, also well as difference in the deployment of J2EE components on application servers. The array of available configuration and deployment tools for use with any particular application server product also contributes to the product implementation differences.
The evolutionary nature of the specifications itself presents challenges to application providers. Each of the component APIs are also evolving. This leads to a varying degree of conformance by products. In particular, an emerging product, such as the Application Server, has to contend with differences in J2EE application components, modules, and files deployed on other established application server platforms. Such differences require mappings between earlier implementation details of the J2EE standard, such as file naming conventions, messaging syntax, and so forth.
Moreover, product providers usually bundle additional features and services with their products. These features are available as custom JSP tags or proprietary Java API libraries. Unfortunately, using these proprietary features renders these applications non-portable.
For migration purposes, the J2EE application consists of the following file categories:
Deployment descriptors (XML files)
JSP source files that contain Proprietary APIs
Java source files that contain Proprietary APIs
Deployment is accomplished by specifying deployment descriptors (DDs) for standalone enterprise beans (EJB JAR files), front-end Web components (WAR files) and enterprise applications (EAR files). Deployment descriptors are used to resolve all external dependencies of the J2EE components/applications. The J2EE specification for DDs is common across all application server products. However, the specification leaves several deployment aspects of components pertaining to an application dependent on product-implementation.
J2EE specifies how to extend JSP by adding extra custom tags. Product vendors include some custom JSP extensions in their products, simplifying some tasks for developers. However, usage of these proprietary custom tags results in non-portability of JSP files. Additionally, JSP can invoke methods defined in other Java source files as well. The JSPs containing proprietary APIs needs to be rewritten before they can be migrated.
The Java source files can be EJBs, servlets, or other helper classes. The EJBs and servlets can invoke standard J2EE services directly. They can also invoke methods defined in helper classes. Java source files are used to encode the business layer of applications, such as EJBs.Vendors bundle several services and proprietary Java API with their products. The use of proprietary Java APIs is a major source of non-portability in applications. Since J2EE is an evolving standard, different products can support different versions of J2EE component APIs. This is another aspect that migration addresses.
Deployment refers to deploying a migrated application that was previously deployed on an earlier version of Sun’s Application Server, or any third party application server platforms.
Deploying a migrated application is described in Chapter 3, Assembling and Deploying Applications, in Sun Java System Application Server Platform Edition 8.1 2005Q2 Update 2 Developer’s Guide. However, when migration activities are performed with automated tools, such as the Migration Tool for Sun Java System Application Server 8.1 (for J2EE applications) or the Sun ONE Migration Toolbox (for Netscape Application Servers), there might be post-migration or pre-deployment tasks that are needed (and defined) prior to deploying the migrated application.
See Chapter 8, Migration Tools and Resources for more information about migration tools that are available.