Java EE consists of a set of specifications which define:
- The services and functionality that the platform provides to applications
- The APIs that applications use to access these features
- How application servers must implement these features
The Java EE platform defines services and functionality that most web and enterprise applications need; it would not make sense for each application to reinvent the wheel. For example, a social networking site and an online store both need a web server, an authentication framework, a database connectivity engine, and so on.
An application server is a Java program that implements the services and functionality defined by the Java EE specifications. There are many Java EE application servers on the market, some of them available for free. Although some application servers provide additional proprietary functionality, a Java EE application that uses standard features should run in any application server with minor or no modifications.
An important distinction between a Java SE application and a Java EE application is the following: a Java SE application controls the main loop of execution, while a Java EE application does not.
Java EE applications do not have a main() method and can only run within an application server, which controls the main loop of execution and invokes the application as required.
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