The J2EE standard represents a  set of application component framework to build almost all kinds of business applications. These applications can be a simple web app or a complex enterprise-level application. If you are going forth with custom J2EE development for developing a web application, then you need to understand its components first. J2EE applications comprise of various components. A J2EE component is a self-contained software module that is assembled into a J2EE application with its relevant classes and files that interacts with other components. 

The J2EE standard describes various types of components that are created by a developer. Each component is a modular software unit that is used within the application server and communicates with its host environment and other components through J2EE APIs that are accessible to it. J2EE applications are made up of JavaServer Pages (JSP), Java servlets, and Enterprise JavaBeans (EJB) modules. J2EE applications are packaged in Java Archive (JAR) files, which are later distributed to production sites. Administrators install J2EE applications on the application server by extracting these J2EE JAR files onto one or more server instances. These components are written in the Java programming language and compiled in the same way as other programs.  

The J2EE application components are as follows:

Applet 

These are client-side GUI components that run inside a browser and are hosted by an applet container. They are embedded in the webpage to generate the dynamic web design and rich user interface to an enterprise application.  The main advantage of the applet is that it is secured and has a low response time as it works on the client-side. 

Application client 

The application client is a Java-based independent program that runs on the client machine within a supported JVM. They are used to perform tasks, such as system or application administration. They offer a user interface similar to the existing ones on the native client while accessing the J2EE business tier features. It can also set up an HTTP connection while communicating with a servlet.  

Advantages:  

• An application client can run in both web browsers and application containers.
• Because of having necessary libraries it can access all J2EE services. 

Web component

Web component is a server-side object that can respond to HTTP requests. They are used by web-based clients(browsers) to interact with J2EE applications. They contain Java servlets, JSP pages, filters, and web event listeners. Java servlets are used to process requests and construct responses while JSP pages are used to create dynamic web pages and server independent web-based applications. There are four steps involved in the web component development:

1. Writing web component code
2. Packaging the web component and its resources
3. Installing a web component in the application container
4. Accessing the link of the web component

Servlets

Servlet is a web component that is integrated on the web-server to create dynamic web pages. It extends a web server's capability to respond to incoming requests and to support the dynamic processing of the application. They can be used in any request or response programming model and often used with the HTTP method. In this method, they use the data embedded within HTTP requests as input and provide output using the HTTP response. Servlet also acts as API to provide several interfaces and classes, including documentation.

JavaServer Pages

JSP is an extended version of the servlet, JavaServer Pages offers dynamic web application capability to the J2EE Platform. They are a server-side technology that performs this task with the help of templating functionality. A JSP page contains HTML elements that remain static throughout the page to form the base template. After that, dynamic elements are inserted through the embedded Java code within the HTML page. JSP is easy to maintain and has access to the entire API of JAVA.

Note: JavaServer Pages are converted to servlets before running, but the complexities of this process are managed by the web container.

Filters 

These components provide the ability to dynamically intercept requests by clients for using the information contained in the request. They also intercept the response provided to these requests by the web container to transform or use the information in request or response communication. Filters do not create responses by themselves, instead, they provide universal functions that can be "attached" to servlets or JSP pages. 

They provide and recurring and reusable mechanism to handle tedious tasks such as authentication, auditing, logging, etc. Some of the applications of filters are as follows: image conversion, data compression, localization, XSL/T transformations of XML content. 

Web event listeners

Web event listeners are created to perform a specific function when a relevant event like creation or invalidation occurs within the web container. This functionality provides efficient resource management and automated processing based on event status. 

Enterprise JavaBeans 

Enterprise Java Beans (EJB) is a Java API for standard manufacture of enterprise software. They are server-side software components that summarise and encapsulate the business logic of an enterprise application. Enterprise Java Beans web repository generates a runtime domain for web-related software services such as computer reliability, Java Servlet Lifecycle (JSL) management, and transaction procedure. The EJB enumeration, a subset of the Java EE enumeration, provides a convenient way to implement the server-side business software usually seen in enterprise applications. It is responsible for performing lifecycle management, security, transaction management, and object pooling. There are three types of Enterprise Java Beans categorized by the specification.

• Session beans- Session bean contains business logic for the application that can be requested by local, remote or service clients.

• Entity beans- These are an enclosed representation of enterprise objects made available with a persistence mechanism.

• Message-Driven beans- Similar to the session bean, it also contains business logic that can be requested by passing message.

Evon’s experience will help you implement them 

Evon Technologies has extensive experience in Java web development and we have been building enterprise-scale J2EE applications for years with that expertise. We have helped our clients in building scalable J2EE web apps and device-independent web portals with our architecture and W3C compliance. We can help you build Enterprise Information portals to handle your enterprise front-ending as well as internal employee collaboration and communication.  Our team of developers is proficient in Apache Camel, RabbitMQ & ActiveMQ and our broad knowledge of WSDL, UDDI, XML / XML Schema helps us integrate your diverse system. Additionally, We have experience working on reporting frameworks like Jasper Reports and Pentaho which helps in building most advanced reports and Business Intelligence applications. 

Contact us to hire Java developers and get the best of the latest J2EE web applications.