SAR (Service Archive)

SAR file is created with the .sar extenstion same like how other archive files (JAR, WAR, EAR) created. But the purpose of the SAR file is differenet from the other archive files. It is used for deploying a service component in the application server without dependent on other componenets. You can create a seperate componenet as a SAR file and deploy it in the server. When application server starts, the component will be deployed and started running independently. Most of the times these SAR files are used for writing thread related components which has to be running independently.

JBoss's service archive architecture is based on the Java Extension Management (JMX). During the server bootup process, the service archive deployer (SARDeployer) instantiates the JBoss service classes and exposes them as manageable beans through JMX. You can view all of registered JBoss service components in the JBoss JMX Console web application.

Use of persistence.xml file

The EntityManager API is great, but how does the server/ application know which database it is supposed to save / update / query the entity objects?

How do we configure the underlying object-relational-mapping engine and cache for better performance and trouble shooting? The persistence.xml file gives you complete flexibility to configure the EntityManager.

• The persistence.xml file is a standard configuration file in JPA.
• It has to be included in the META-INF directory inside the JAR file that contains the entity beans.
• The persistence.xml file must define a persistence-unit with a unique name in the current scoped classloader.
• The provider attribute specifies the underlying implementation of the JPA EntityManager. 
• In JBoss AS, the default and only supported / recommended JPA provider is Hibernate.
• The jta-data-source points to the JNDI name of the database this persistence unit maps to e.g. java:/DefaultDS.