deployment descriptor

A deployment descriptor is a file for configuringsoftware based on XML code. It is also referred to as a deployment descriptor having generic functionality and describing the declarative property of an application. In the context of Enterprise JavaBeans( EJB), a Deployment Descriptor is responsible for the central definition of runtime and control information. However, as of version 3.0 of EJB, annotations are also permitted as an alternative means of description. Similarly, for example, for a servlet-basedweb application to be installed in a servlet container, a corresponding installation statement must be stored in an XML file. In this example, the deployment descriptor specifies the parameters to be used for initialization and necessary security settings.

However, the definition and provision of deployment descriptors often contributes to higher development efforts. This is mostly connected with the high complexity of the corresponding XML files, the programming that is not so convenient in this respect, and the vulnerability with regard to possible errors during creation and maintenance. Another aspect of the use of deployment descriptors is that incorrect or missing information is initially only noticed during the deployment process, or error messages in this regard are even generated at runtime.

To avoid shortcomings of this kind, generators such as XDoclet are also used to generate deployment descriptors. Thus it is possible to describe the specific parameters of Enterprise JavaBeans in older versions (2.x and earlier) directly in the source code in the form of JavaDoc comments.

If at the time of implementation the characteristics of an application are not yet known or are dependent on the deployment environment, the use of a deployment descriptor makes sense. If annotations are available as an alternative, as for example with EJB's, an order of priority must generally be observed - the values of the annotations are overwritten by the values in the Deployment Descriptor.