J2EE Application

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J2EE stands for Java Enterprise edition.J2ee applications are made up of components such as JavaServer Pages (JSP), Java servlets, and Enterprise JavaBeans (EJB) modules. These components enable software developers to build large-scale, distributed applications. Developers package J2EE applications in Java Archive (JAR) files (similar to zip files), which can be distributed to production sites. Administrators install J2EE applications onto the Application Server by deploying J2EE JAR files onto one or more server instances (or clusters of instances).

Containers

The J2EE application model divides enterprise applications into three fundamental parts: components, containers, and connectors. Components are the key focus of application developers, while system vendors implement containers and connectors to conceal complexity and promote portability. Containers intercede between clients and components, providing services transparently to both, including transaction support and resource pooling. Container mediation allows many component behaviors to be specified at deployment time, rather than in program code.

In the Application Server, each server instance includes two containers: web and EJB. A container is a runtime environment that provides services such as security and transaction management to J2EE components. Web components, such as Java Server Pages and servlets, run within the web container. Enterprise JavaBeans run within the EJB container.

J2EE Services

The J2EE platform services simplify application programming and allow components and applications to be customized at deployment time to use resources available in the deployment environment. This section gives a brief overview of the J2EE platform naming, deployment, transaction, and security services.The J2EE platform provides services for applications, including:

Naming -A naming and directory service binds objects to names. A J2EE application can locate an object by looking up its Java Naming and Directory Interface (JNDI) name.
Security - The Java Authorization Contract for Containers (JACC) is a set of security contracts defined for the J2EE containers. Based on the client’s identity, containers can restrict access to the container’s resources and services.
Transaction management - A transaction is an indivisible unit of work. For example, transferring funds between bank accounts is a transaction. A transaction management service ensures that a transaction is either completed, or is rolled back.
Message Service - Applications hosted on separate systems can communicate with each other by exchanging messages using the Java™ Message Service (JMS). JMS is an integral part of the J2EE platform and simplifies the task of integrating heterogeneous enterprise applications.
Web Services
Clients can access a J2EE 1.4 application as a remote web service in addition to accessing it through HTTP, RMI/IIOP, and JMS. Web services are implemented using the Java API for XML-based RPC (JAX-RPC). A J2EE application can also act as a client to web services, which would be typical in network applications.

Web Services Description Language (WSDL) is an XML format that describes web service interfaces. Web service consumers can dynamically parse a WSDL document to determine the operations a web service provides and how to execute them. The Application Server distributes web services interface descriptions using a registry that other applications can access through the Java API for XML Registries (JAXR).

Client Access

Clients can access J2EE applications in several ways. Browser clients access web applications using hypertext transfer protocol (HTTP). For secure communication, browsers use the HTTP secure (HTTPS) protocol that uses secure sockets layer (SSL).

Rich client applications running in the Application Client Container can directly lookup and access Enterprise JavaBeans using an Object Request Broker (ORB), Remote Method Invocation (RMI) and the internet inter-ORB protocol (IIOP), or IIOP/SSL (secure IIOP). They can access applications and web services using HTTP/HTTPS, JMS, and JAX-RPC. They can use JMS to send messages to and receive messages from applications and message-driven beans.

Clients that conform to the Web Services-Interoperability (WS-I) Basic Profile can access J2EE web services. WS-I is an integral part of the J2EE standard and defines interoperable web services. It enables clients written in any supporting language to access web services deployed to the Application Server.

The best access mechanism depends on the specific application and the anticipated volume of traffic. The Application Server supports separately configurable listeners for HTTP, HTTPS, JMS, IIOP, and IIOP/SSL. You can set up multiple listeners for each protocol for increased scalability and reliability.


J2EE applications can also act as clients of J2EE components such as Enterprise JavaBeans modules deployed on other servers, and can use any of these access mechanisms.