As part of our Java Tutorials program, we will list a number of interview questions to aid in a better understaing of Java and J2EE and, hopefully, provide a greater likelihood of getting a job.  Let us begin with the basics:

1. What is meant by J2EE?

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J2EE is an abreviation for Java 2 Platform Enterprise Edition

2.  What is the purpose of J2EE?

The purpose of J2EE is to provide a component based platform in a multitier application model with transaction management, web services and reusable component support.

3.  What is the tier structure of the typical J2EE application?

A typical J2EE application consists of the following tiers/machines:  the client machine (browser/non-browser application), the J2EE server (an application server such as Oracle, JBoss, GlassFish, Tomcat) and a database.

4.  What are J2EE components?

Components are self-contained software units written in java that, when combined, are part of a larger application.  A common misconception among novice developers is that these components are written by one institution.  It is quite common to purchase components or software applications and then integrate them into the application server environment.  Examples of a component are as follows:

• Client components - application/applets
• Web component - servlets/JSPs
• Business component - enterprise java bean
• Resource adapters - provided by tool vendors/EIS

5.  What are J2EE clients?

J2EE clients are either web clients or application clients.  The web client connects to the server by way of Transmission Control Protocol, TCP, with non-persistent connections and handles all communication through markup languages i.e. XML or HTML.  Application clients connect to the server via RMI with persistent connections and typically provide the user with a richer interface for handling more sophisticated tasks.  Such tasks may include handling user generated sql calls, testing electrical grids or any of a number of other actions that are not appropriate or possible on the net.

6.  What are Servlets?

Servlets are a library of Java classes created to handle and respond to incoming requests.  Most often, these are http requests but can be used for other client-server protocols. This technology includes, but is not limited to, Java Server Pages, JSTL, Scriplets and much more.  They are the basis of many frameworks such as Wicket, Java Server Faces (JSF), Struts 1 & 2 and others.

7.  Explain the different types of enterprise beans.

Session Bean - a software component, created for whatever reason, whose lifetime is dependent upon the length of the conversation it has with the client.  It deletes itself after it completes the executable task.

Entity Bean - unlike a session bean, an entity bean persists data to a row in a database so that when a server is shut down, the information can be retrieved.

Message Driven Bean - uses asynchronous message handling to receive Java Message Service messages.  Depending upon how the developer configures this bean and the incoming messages, messages may be persisted to a database or, like a session bean, lost enroute in the event of a server crash.

8.  What is a J2EE container?

A container is an interface between a component and low-level platform specific functionality which supports the component i.e. how components communicate with one another, how they are defined and accessed, multi-threading ...

9.  What is meant by J2EE assembly process?

The process of specifying the container and the application settings.

10.  What does it mean to pack a J2EE application?

Compressing files in java means to place them into a Java Archive file (JAR) which is akin to a zip file.  In a J2EE application, such a file in referred to as an Enterprise Archive file (EAR) or a JAR file with a .ear extension.  Another compressed file in java is a Web Archive file (WAR) which is a compressed collection of java web files.  Like the EAR file, it, too, has a unqiue extenstion:  .war.  The reason for compression lies in the need to ensure that files are not lost and that the application footprint (the size of the applcation), is not exceedingly large.

11.  What is a J2EE deployment descriptor?

Deployment descriptors are XML files with a .xml extension that describe the deployment settings of an application.  There are two types of deployment descriptors in J2EE:  J2EE and runtime.  The J2EE descriptor is used to configure the deployment descriptor settings; the runtime descriptor is used to configure the J2EE implementation parameters.