Most Java EE applications are deployed as EAR archives. An EAR archive is a JAR file that typically contains a WAR archive for the web pages, servlets, and other web-related components, one or several EJB3 JARs that provide services (e.g., data access and transaction) to the WAR components, and some other support library JARs required by the application. An EAR file also have deployment descriptors such as
jboss-app.xml. Below is the basic structure of a typical EAR application.
│ ├── application.xml
│ └── jboss-app.xml
│ ├── web pages and JSP /JSF pages
│ └── WEB-INF
│ ├── web.xml
│ ├── jboss-web.xml
│ ├── faces-config.xml
│ ├── ...
│ ├── lib/
│ │ └── tag library JARs
│ └── classes/
│ └── servlets and other classes used by web pages
│ ├── EJB3 bean classes
│ └── META-INF/
│ ├── ejb-jar.xml
│ └── persistence.xml
└──Library JARs for the EAR
Notice that in JBoss Enterprise Application Platform, unlike in many other application servers, you do not need to declare EJB references in the web.xml file in order for the components in the WAR file to access EJB3 services. You can obtain the references directly via JNDI as we discussed earlier in the chapter.
A typical application.xml file is as follows. It declares the WAR and EJB3 JAR archives in the EAR, and defines the web content root for the application. Of course, you can have multiple EJB3 modules in the same EAR application. The application.xml file could also optionally define a shared classpath for JAR files used in this application. The JAR file location defaults to lib in JBoss Enterprise Application Platform -- but it might be different in other application servers.
The jboss-app.xml file provides JBoss-specific deployment configuration for the EAR application. For instance, it can specify the deployment order of modules in the EAR, deploy JBoss-specific application modules in the EAR, such as SARs (Service ARchive for MBeans) and HARs (Hibernate ARchive for Hibernate objects), provide security domain and JMX MBeans that can be used with this application, etc. You can learn more about the possible attributes in jboss-app.xml in its DTD: http://www.jboss.org/j2ee/dtd/jboss-app_4_2.dtd.
A common use case for jboss-app.xml is to configure whether this EAR file should be deployed in its own scoped classloader to avoid naming conflicts with other applications. If your EAR application is deployed in its own scoped classloader and it only has one persistence-unit defined in its EJB3 JARs, you will be able to use @PersistenceContext EntityManager to inject EntityManager to session beans without worrying about passing the persistence unit name to the @PersistenceContext annotation. The following jboss-app.xml specifies a scoped classloader myapp:archive=myapp.ear for the EAR application.
The EAR deployment is configured by the
<JBOSS_HOME>/server/<PROFILE>/deploy/ear-deploy.xml file. This file contains three attributes as follows.
A flag indicating if ear deployments should
have their own scoped class loader to isolate
their classes from other deployments.
A flag indicating if the ear components should
have in VM call optimization disabled.
A flag the enables the default behavior of
the ee5 library-directory. If true, the lib
contents of an ear are assumed to be the default
value for library-directory in the absence of
an explicit library-directory. If false, there
must be an explicit library-directory.
If you set the Isolated parameter to true, all EAR deployment will have scoped classloaders by default. There will be no need to define the classloader in jboss-app.xml. The CallByValue attribute specifies whether we should treat all EJB calls as remote calls. Remote calls have a large additional performance penalty compared with local call-by-reference calls, because objects involved in remote calls have to be serialized and de-serialized. For most of our applications, the WAR and EJB3 JARs are deployed on the same server, hence this value should be default to false and the server uses local call-by-reference calls to invoke EJB methods in the same JVM. The EnablelibDirectoryByDefault attribute specifies whether the lib directory in the EAR archive should be the default location for shared library JARs.