JVM

All WebSphere Application Servers are essentially Java Virtual Machines (JVMs). IBM has implemented the JEE application server model in a way that maximizes the JEE specification, and also provides many enhancements creating specific features for WAS. JEE applications are deployed to an Application Server.

Web container

A common type of business application is a web application. The WAS web container is essentially a Java-based web server contained within an application server's JVM, which serves the web component of an application to the client browser.

EJB container

Applications need not only comprise of web components. In a more complex enterprise-based application, business objects are created to provide a layer of abstraction between a web application and the underlying data. The EJB container provides the services required to manage the business components as implanted with EJBs.

Virtual hosts

A virtual host is a configuration element that is required for the web container to receive HTTP requests. As in most web server technologies, a single machine may be required to host multiple applications and appear to the outside world as multiple machines. Resources that are associated with a particular virtual host are designed not to share data with resources belonging to another virtual host, even if the virtual hosts share the same physical machine. Each virtual host is given a logical name and assigned one or more DNS aliases by which it is known. A DNS alias is the TCP/host name and port number that is used to request a web resource, for example,<hostname>:9080/<servlet>.

By default, two virtual host aliases are created during installation. One for the administration console called admin_host and another called default_host, which is assigned as the default virtual host alias for all application deployments, unless overridden during the deployment phase. All web applications must be mapped to a virtual host, otherwise web browser clients cannot access the application that is being served by the web container.

Environment settings

WebSphere uses Java environment variables to control settings and properties related to the server environment. WAS variables are used to configure product path names, such as the location of a database driver, for example, ORACLE_JDBC_DRIVER_PATH, and environmental values required by internal WAS services and/or applications.

Resources

Configuration data is stored in XML files in the underlying configuration repository of the WebSphere Application Server. Resource definitions are a fundamental part of J2EE administration. Application logic can vary depending on individual business requirements, and there are several resource types that can be used by an application. The following table shows a list of some of the most commonly used resource types:

JNDI

The Java Naming and Directory Interface (JNDI) is employed to make applications more portable. JNDI is essentially an API for a directory service, which allows Java applications to look up data and objects via a name. Naming operations, such as lookups and binds, are performed on contexts. All naming operations begin with obtaining an initial context. You can view the initial context as a starting point in the namespace. Applications use JNDI lookups to find a resource using a known naming convention. You can override the resource the application is actually connecting to without requiring a reconfiguration or code change in the application. This level of abstraction using JNDI is fundamental and required for the proper use of WAS by applications.

Application file types

There are four main file types we work with in Java applications. An explanation of these file types is shown in the following table: