An application server is a component-based product that resides in the middle-tier of a server centric architecture. It provides middleware services for security and state maintenance, along with data access and persistence.
Java application servers are based on the Java™ 2 Platform, Enterprise Edition (J2EE™). J2EE uses a multi-tier distributed model. This model generally includes a Client Tier, a Middle Tier, and an EIS Tier. The Client Tier can be one or more applications or browsers. The J2EE Platform is in the Middle Tier and consists of a Web Server and an EJB Server. (These servers are also called "containers.") There can be additional sub-tiers in the middle tier. The Enterprise Information System (EIS) tier has the existing applications, files, and databases.
For the storage of business data, the J2EE platform requires a database that is accessible through the JDBC, SQLJ, or JDO API. The database may be accessible from web components, enterprise beans, and application client components. The database need not be accessible from applets.
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