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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 modules onto an application server happens simultaneously
and coherently. It also contains XML files called deployment descriptors which describe how
to deploy the modules on an application server.
File Structure
An EAR file is a standard JAR file with an .ear extension, with one or more entries
representing the modules of the application, and a metadata directory called META-INF 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 components, 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.
- Arbitrary 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 .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 classloaders, 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 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 META-INF directory contains at least the application.xml deployment descriptor, known as the
J2EE Deployment Descriptor. It contains the following XML entities:
icon, which specifies the locations for the images that represent the application. A subdivision is made for
small-icon and large-icon.display-name, which identifies the applicationdescription- A
module element for each module in the archive
- Zero or more
security-role elements for the global security roles in the application
Each module element contains an ejb, web or java element which describes
the individual modules within the application. Web modules also provide a
context-root 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
Resources
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