Introduction to J2EE APIs

Servlets

- At a high level.

- Java Servlets are the Java equivalent of CGI scripts.

- That can be used to perform processing and the servicing of client requests on a web server.

- From an implementation perspective, servlets are simply Java classes.

- That implement a predefined interface.

- One use for servlets is that they can be used to dynamically generate content.

- For presentation to the user.

- And this is achieved by embedding markup language (e.g. HTML) inside the Java code.

- As Servlets are written in Java, they have access to the rich library of features provided by Java.

- Including access to databases and other enterprise resources such as EJB.

Java Server Pages (JSP)

- JSP is another technology for presenting information to the user over the web.

- And uses a paradigm where Java code is embedded into the HTML

- The opposite of servlets, and much like Microsoft ASP.

- Pages are written as HTML files with embedded Java source code known as scriptlets.

- One of the pitfalls in using JSP is that it is very easy to build large pages.

- Containing lots of embedded Java code and business logic.

- For this reason, JSPs provide easy integration with JavaBeans and another feature called JSP tag extensions.

- These custom tags (also known as custom actions) allow re-usable functionality to be encapsulated into XML

- Like tags that can be easily used on the pages by both page developers and designers.

Enterprise JavaBeans (EJB) 

- EJB is a major part of the J2EE specification.

- And defines a model for building server-side, reusable components.

- There are three types of enterprise beans currently supported by J2EE :

    1) Session beans,
    2) Entity beans and
    3) Message-driven beans.

- Session beans can be seen as extensions to the client application.

- And are typically used to model business processes.

- There are two types of session bean :
    1) Stateful and
    2) Stateless.

- Stateful session beans are typically used to record conversational state for a single client between requests.

- Whereas stateless session beans are shared between any number of clients at any one time.

- Entity beans are typically used to model persistent business entities.

- And, in particular, data in a database.

- A common mapping is to model an entity bean on a table.

- There being one instance of that bean for every row in the table.

- There are two ways that persistence can be achieved :
    1) Container managed and
    2) Bean managed persistence.

- In container managed persistence, a mapping is defined at deployment time.

- Between the persistent properties in the bean and the columns in the table.

- With bean managed persistence, developers write the JDBC code that performs the create, read, update and delete operations.

- Finally, message-driven beans allow functionality to be executed on an asynchronous basis.

- Typically triggered by JMS messages from message-oriented middleware.

Java Message Service (JMS) 

- JMS is Java API that presents an interface into message-oriented middleware such as :
    1) IBM MQSeries,
    2) SonicMQ and
    3) so on.


- Like JDBC, JMS provides Java applications a mechanism to integrate with such systems by presenting a common programming interface.( Irrespective of the underlying messaging system).

- Functionally, JMS allows messages to be sent and received using a point-to-point or publish/subscribe paradigm.

Java Mail 

- The Java Mail API provides classes that allow we work with e – mail.

- Specifically it allows sending & receiving e – mail by using a wide variety of protocols. Such as:
    1) POP
    2) SMTP
    3) IMAP

- We can create e – mails that conform to a large number of MIME type.

- MIME – Multi Purpose Internet Mail Extension.

- Because the API uses JAF to provide support for a number of MIME types.

- JAF – Javabean Activation Framework.

- We can create HTML messages that contain embedded graphics & even have attachments.

- Most internet application requires the ability to send e – mail messages.

- We can use this API together with JAF to send from :
    1) JSP
    2) Servlets or
    3) EJB

Java Server Faces (JSF)

- This technology is a user interface framework for building web applications.

- The main component of JSF technology is as follows:

1) A GUI component framework :

    § A flexible model for rendering components in different kinds of HTML or different markup languages & technologies.

    § A Rendered (turn into) object generates the markup to render the component.

    § And converts the data that can be represented in a view.


- So, JSF does :
    1) A standard rendered kit for generating HTML markup.
    2) Input validation.
    3) Event handling.
    4) Data conversion between model objects & components.
    5) Page navigation configuration.

Java Naming Directory Interface (JNDI)

- It is an extension to the Java platform.

- And provide multiple naming & directory services.

- A naming services provides a mechanism for locating distributed objects.

- A director services organize the distributed objects & other resources. Such as:
    1) Files in to a hierarchical structures.
        - Directory services allow resources to be linked virtually.
        - So as to be located in directory service hierarchy.
        - They are different types of directory services.
        - JNDI allows the different types of directory services to be linked.
        - Thus client can any type of directory services.