EAR (file format) 3294492 221514188 2008-06-24T20:59:31Z 199.67.131.155 /* Modules */ {{Expert-portal|information technology}} An '''Enterprise ARchive''', or '''EAR''', is a file format used by [[Java EE]] for packaging one or more modules into a single archive so that the deployment of the various [[J2EE application|module]]s onto an [[application server]] happens simultaneously and coherently. It also contains [[XML]] files called ''deployment descriptors'' which describe how to deploy the modules. == File Structure == An EAR file is a standard [[JAR (file format)|JAR file]] with an .ear extension, with one or more entries representing the modules of the application, and a metadata directory called <code>META-INF</code> which contains one or more deployment descriptors. === Modules === Different artifacts can be embedded within an EAR file, artifacts which are deployed by the application server: * A Web module has a [[.war]] extension. It is a deployable unit that consists of one or more [[web component]]s, other resources, and a [[web application]] deployment descriptor. The web module is contained in a [[hierarchy]] of directories and files in a standard web application format. * [[POJO]] Java classes may be deployed in [[.jar]] files. * An [[Enterprise Java Bean]] module has a [[.jar]] extension, and contains in its own META-INF directory descriptors describing the persistent classes deployed. When deployed, entity beans are visible to other components and (if remotely exported), remote clients. Message Beans and [[Session Beans]] are available for remote access. * A [[Resource Adapter]] module has a [[Resource Adapter aRchive|.rar]] extension [[Vendors]] may support extra artifacts, with their own extensions. === Class isolation === Most application servers load classes from a deployed EAR file as an isolated tree of java [[classloader]]s, isolating the application from other applications, but sharing classes between deployed modules. For example, a deployed WAR file would be able to create instances of classes defined in a JAR file that was also included in the containing EAR file, but not necessarily those in JAR files in other EAR files. One key reason for this behaviour is to allow complete separation between applications which use static singletons (e.g. Log4J), which would otherwise mess-up the configuration between separate applications. This also enables different versions of applications and libraries to be deployed side-by-side. The [[Jboss|JBoss]] application server is notable in that it does not isolate deployed components. A web application deployed in one EAR file would have access to classes in other EAR and WAR files. This is a somewhat controversial policy. The ''Unified Classloader'' design reduces communications overhead between running applications, as class data can be shared by reference or simple copies. It also allows developers to avoid having to understand the problems that a tree of classloaders can create. However, it prevents different versions of dependent libraries from being deployed in separate applications. JBoss 4.0.2 switched to a hierarchical classloader, but as of version 4.0.3, it has reverted to a Unified Classloader for backwards compatibility reasons. There is now a configuration option to change this behavior. === META-INF directory === The <code>META-INF</code> directory contains at least the <code>application.xml</code> deployment descriptor, known as the ''J2EE Deployment Descriptor''. It contains the following XML entities: * <code>icon</code>, which specifies the locations for the images that represent the application. A subdivision is made for <code>small-icon</code> and <code>large-icon</code>. * <code>display-name</code>, which identifies the application * <code>description</code> * A <code>module</code> element for each module in the archive * Zero or more <code>security-role</code> elements for the global security roles in the application Each <code>module</code> element contains an <code>ejb</code>, <code>web</code> or <code>java</code> element which describes the individual modules within the application. [[Web module]]s also provide a <code>context-root</code> which identifies the web module by its URL. Next to the J2EE deployment descriptor one can have zero or more ''runtime deployment descriptors''. These are used to configure implementation-specific J2EE parameters. == See also == * [[WAR (file format)]] * [[JAR (file format)]] * [[JAR hell]] == Resources == * http://java.sun.com/j2ee/1.4/docs/glossary.html * http://searchsoa.techtarget.com/expert/KnowledgebaseAnswer/0,289625,sid26_gci837938,00.html [[Category:Archive formats]] [[Category:Java enterprise platform]] [[de:Enterprise Application Archive]] [[fr:EAR (format de fichier)]] [[ja:EAR]]