File structure
An EAR file is a standard JAR file (and therefore a Zip 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.
Module
Developers can embed various artifacts within an EAR file for deployment by application servers:
- 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.
- 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. Deployed entity beans become visible to other components and, if remotely exported, to remote clients. Message Beans and Session Beans are available for remote access. - A Resource Adapter module has a .rar extension.
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 behavior is to allow complete separation between applications which use static singletons (e.g. Log4J), which would otherwise confuse the configuration between separate applications. This also enables different versions of applications and libraries to be deployed side by side.
The JBoss application servers before Version 5 were 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 in version 4.0.3 it reverted to a Unified Classloader for backwards compatibility reasons. There is now a configuration option to change this behavior. JBoss 5.x, 6.x and 7.x no longer use Unified Classloading.
The META-INF directory contains at least the application.xml deployment descriptor, known as the Java EE 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 Jakarta EE deployment descriptor there can be zero or more runtime deployment descriptors. These are used to configure implementation-specific Jakarta EE parameters.
Jakarta Enterprise Beans is one of several Java APIs for modular construction of enterprise software. EJB is a server-side software component that encapsulates business logic of an application. An EJB web container provides a runtime environment for web related software components, including computer security, Java servlet lifecycle management, transaction processing, and other web services. The EJB specification is a subset of the Java EE specification.
Jakarta EE, formerly Java Platform, Enterprise Edition and Java 2 Platform, Enterprise Edition (J2EE), is a set of specifications, extending Java SE with specifications for enterprise features such as distributed computing and web services. Jakarta EE applications are run on reference runtimes, that can be microservices or application servers, which handle transactions, security, scalability, concurrency and management of the components they are deploying.
Jakarta Server Pages is a collection of technologies that helps software developers create dynamically generated web pages based on HTML, XML, SOAP, or other document types. Released in 1999 by Sun Microsystems, JSP is similar to PHP and ASP, but uses the Java programming language.
An application server is a server that hosts applications or software that delivers a business application through a communication protocol.
A web container is the component of a web server that interacts with Jakarta Servlets. A web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet and ensuring that the URL requester has the correct access-rights. A web container handles requests to servlets, Jakarta Server Pages (JSP) files, and other types of files that include server-side code. The Web container creates servlet instances, loads and unloads servlets, creates and manages request and response objects, and performs other servlet-management tasks. A web container implements the web component contract of the Jakarta EE architecture. This architecture specifies a runtime environment for additional web components, including security, concurrency, lifecycle management, transaction, deployment, and other services.
A JAR file is a package file format typically used to aggregate many Java class files and associated metadata and resources into one file for distribution.
Jakarta Faces, formerly Jakarta Server Faces and JavaServer Faces (JSF) is a Java specification for building component-based user interfaces for web applications and was formalized as a standard through the Java Community Process being part of the Java Platform, Enterprise Edition. It is also an MVC web framework that simplifies the construction of user interfaces (UI) for server-based applications by using reusable UI components in a page.
Java Management Extensions (JMX) is a Java technology that supplies tools for managing and monitoring applications, system objects, devices and service-oriented networks. Those resources are represented by objects called MBeans. In the API, classes can be dynamically loaded and instantiated. Managing and monitoring applications can be designed and developed using the Java Dynamic Management Kit.
Apache Beehive is a discontinued Java Application Framework that was designed to simplify the development of Java EE-based applications. It makes use of various open-source projects at Apache such as XMLBeans. Apache Beehive uses Java 5, including JSR-175, a facility for annotating fields, methods, and classes so that they can be treated in special ways by runtime tools. It builds on the framework developed for BEA Systems Web logic Workshop for its 8.1 series. BEA later decided to donate the code to Apache.
Apache Axis is an open-source, XML based Web service framework. It consists of a Java and a C++ implementation of the SOAP server, and various utilities and APIs for generating and deploying Web service applications. Using Apache Axis, developers can create interoperable, distributed computing applications. Axis development takes place under the auspices of the Apache Software Foundation.
A deployment descriptor (DD) refers to a configuration file for an artifact that is deployed to some container/engine.
A Jakarta EE application is any deployable unit of Jakarta EE functionality. This can be a single Jakarta EE module or a group of modules packaged into an EAR file along with a Jakarta EE application deployment descriptor. Jakarta EE applications are typically engineered to be distributed across multiple computing tiers.
In software engineering, a WAR file is a file used to distribute a collection of JAR-files, JavaServer Pages, Java Servlets, Java classes, XML files, tag libraries, static web pages and other resources that together constitute a web application.
The Java Class Loader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine. Usually classes are only loaded on demand. The Java run time system does not need to know about files and file systems as this is delegated to the class loader.
Jakarta PersistenceAPI is a Jakarta EE application programming interface specification that describes the management of relational data in enterprise Java applications.
JBoss Tools is a set of Eclipse plugins and features designed to help JBoss and JavaEE developers develop applications. It is an umbrella project for the JBoss developed plugins that will make it into JBoss Developer Studio.
JBoss Developer Studio (JBDS) is a development environment created and currently developed by JBoss and Exadel.
EPUB is an e-book file format that uses the ".epub" file extension. The term is short for electronic publication and is sometimes styled ePub. EPUB is supported by many e-readers, and compatible software is available for most smartphones, tablets, and computers. EPUB is a technical standard published by the International Digital Publishing Forum (IDPF). It became an official standard of the IDPF in September 2007, superseding the older Open eBook (OEB) standard.
Jakarta RESTful Web Services, is a Jakarta EE API specification that provides support in creating web services according to the Representational State Transfer (REST) architectural pattern. JAX-RS uses annotations, introduced in Java SE 5, to simplify the development and deployment of web service clients and endpoints.
