Application server

An Application Server is any server that supplies additional functionality related to enterprise computing -- for instance, load balancing, database access classes, transaction processing, messaging, and so on.

As for the application server, according to our definition, an application server exposes business logic to client applications through various protocols, possibly including HTTP. While a Web server mainly deals with sending HTML for display in a Web browser, an application server provides access to business logic for use by client application programs. The application program can use this logic just as it would call a method on an object (or a function in the procedural world).

Such application server clients can include GUIs (graphical user interface) running on a PC, a Web server, or even other application servers. The information traveling back and forth between an application server and its client is not restricted to simple display markup. Instead, the information is program logic. Since the logic takes the form of data and method calls and not static HTML, the client can employ the exposed business logic however it wants.

In most cases, the server exposes this business logic through a component API, such as the EJB (Enterprise JavaBeans) component model found on J2EE (Java 2 Platform, Enterprise Edition) application servers. Moreover, the application server manages its own resources. Such gate-keeping duties include security, transaction processing, and resource pooling, and messaging. Like a Web server, an application server may also employ various scalability and fault-tolerance techniques.

The web server may need to execute an application in response to the user’s request. It may be generating a list of news items, or handling a form submission to a guest book. If the server application is written as a Java Servlet, it will need a place to execute, and this place is typically called a Servlet Engine

EJB Application Servers provide an EJB container, which is the environment that beans will execute in, and this container will manage transactions, thread pools, and other issues as necessary. These application servers are usually stand-alone products, and developers would tie their servlets/JSP pages to the EJB components via remote object access APIs. Depending on the application server, programmers may use CORBA or RMI to talk to their beans, but the baseline standard is to use JNDI to locate and create EJB references as necessary.

WebLogic contains a web server, servlet engine, JSP processor, JMS facility, as well as an EJB container.

An Application Server = Web Server + EJB Container + middle ware services(resources Such gate-keeping duties include security, transaction processing, and resource pooling, and messaging)

Some examples of application server

The Java Enterprise Edition Application servers are:

BEA WebLogic server,Red Hat JBoss, IBM WebSphere Application server, Glassfish Application server.