J2EE

What is J2EE?
J2EE is an environment for developing and deploying enterprise applications. The J2EE platform consists of a set of services, application programming interfaces (APIs), and protocols that provide the functionality for developing multitier, web-based applications.

What is the J2EE module?
A J2EE module consists of one or more J2EE components for the same container type and one component deployment descriptor of that type.

What are the components of J2EE application?
A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its related classes and files and communicates with other components. The J2EE specification defines the following J2EE components:
Application clients and applets are client components.
Java Servlet and JavaServer PagesTM (JSPTM) technology components are web components.
Enterprise JavaBeansTM (EJBTM) components (enterprise beans) are business components.
Resource adapter components provided by EIS and tool vendors.

What are the four types of J2EE modules?

1. Application client module
2. Web module
3. Enterprise JavaBeans module
4. Resource adapter module

What does application client module contain?
The application client module contains:

–class files,
–an application client deployment descriptor.

Application client modules are packaged as JAR files with a .jar extension.

What does web module contain?
The web module contains:

–JSP files,
–class files for servlets,
–GIF and HTML files, and
–a Web deployment descriptor.

Web modules are packaged as JAR files with a .war (Web ARchive) extension.

What are the differences between Ear, Jar and War files? Under what circumstances should we use each one?

There are no structural differences between the files; they are all archived using zip-jar compression. However, they are intended for different purposes.

–Jar files (files with a .jar extension) are intended to hold generic libraries of Java classes, resources, auxiliary files, etc.
–War files (files with a .war extension) are intended to contain complete Web applications. In this context, a Web application is defined as a single group of files, classes, resources, .jar files that can be packaged and accessed as one servlet context.
–Ear files (files with a .ear extension) are intended to contain complete enterprise applications. In this context, an enterprise application is defined as a collection of .jar files, resources, classes, and multiple Web applications.

Each type of file (.jar, .war, .ear) is processed uniquely by application servers, servlet containers, EJB containers, etc.

What is the difference between Session bean and Entity bean ?
The Session bean and Entity bean are two main parts of EJB container.
Session Bean

–represents a workflow on behalf of a client
–one-to-one logical mapping to a client.
–created and destroyed by a client
–not permanent objects
–lives its EJB container(generally) does not survive system shut down
–two types: stateless and stateful beans
Entity Bean
–represents persistent data and behavior of this data
–can be shared among multiple clients
–persists across multiple invocations
–findable permanent objects
–outlives its EJB container, survives system shutdown
–two types: container managed persistence(CMP) and bean managed persistence(BMP)

What is “applet” ?
A J2EE component that typically executes in a Web browser but can execute in a variety of other applications or devices that support the applet programming model.

What is “applet container” ?
A container that includes support for the applet programming model.

What is “application assembler” ?
A person who combines J2EE components and modules into deployable application units.