What are ear, war and jar files? What are J2EE Deployment Descriptors?

ear, war and jar are standard application deployment archive files. Since they are a standard, any application
server (at least in theory) will know how to unpack and deploy them.

An EAR file is a standard JAR file with an “.ear” extension, named from Enterprise ARchive file. A J2EE
application with all of its modules is delivered in EAR file. JAR files can’t have other JAR files. But EAR and WAR (Web ARchive) files can have JAR files.

An EAR file contains all the JARs and WARs belonging to an application. JAR files contain the EJB classes and WAR files contain the Web components (JSPs, static content (HTML, CSS, GIF etc), Servlets etc.).

The J2EE application client's class files are also stored in a JAR file. EARs, JARs, and WARs all contain an XML-based

Deployment Descriptors

A deployment descriptor is an XML based text file with a “.xml” extension that describes a component's
deployment settings. A J2EE application and each of its modules has its own deployment descriptor. Pay attention to elements marked in bold in the sample deployment descriptor files shown below.
􀂃 application.xml: is a standard J2EE deployment descriptor, which includes the following structural
information: EJB jar modules, WEB war modules, etc. Also since EJB jar modules are
packaged as jars the same way dependency libraries like log4j.jar, commonUtil.jar etc are packaged, the
application.xml descriptor will distinguish between these two jar files by explicitly specifying the EJB jar
modules.