A container is a runtime support of a system level entity [Applet, Servlet/JSP, EJB]
The four types of container that the J2EE specification defines are applet container, application-client container, web container and EJB container.
Applet Container : An applet is a java program that can be embedded into a web page.
Most of the web pages are authored in HTML. To use an applet in HTML document , the tags <APPLET> and </APPLET> are used. They are used to indicate to the browser that a java applet should be loaded. “These tags act as a container for the java applet definition. “ Applet container manages the execution of applet, and contains the web browser.
Application-client Container : The Application Client Container (ACC) is a combination of Java classes, libraries, and other files. They are used to distribute along with Java client programs that execute on their own JVM. The execution of the application client components is managed by the application-client container. The ACC can take the responsibility to collect user name and password which is treated as authentication data.
Web Container : A web container is a part of web server. It provides the run time environment to execute a web application such as a servlet, JSP. A servlet container translates the URL requests into servlet requests. The JSP implicit objects such as request, response, out, page, pageContext etc., are exposed by JSP container.
EJB Container : The EJB container, like other containers provides run-time environment to execute EJB components such as enterprise beans. An EJB Container manages transactions, state management details, multi threading, connection pooling. The applications are provided with security using EJB container. All database access required by the entity bean will be handled by the EJB container.