COMPONENT TECHNOLOGIES
These components will be dependent upon the container for many services, such as life cycle management, threading, and security. These allows us to concentrate on providing the requisite business functionality without getting into details of low-level (container-level) semantics.
Web components
These can be categorized as any component that response to an HTTP request. A further distinction can be drawn is based on the hosting container for the application components.
Servlets
Servlets are server side programs that allow application logic to be embedded in the HTTP request-response process. Servlets provide a means to extends the functionality of the web server to enable dynamic content in HTML, XML, or other web languages.
JavaServer Pages
JavaServer Pages (JSP) provides a way to embed components in a page, and to have them do their work to generate the page is eventually send to the client. A JSP page can contain HTML, Java code, and JavaBean components. JSP pages are infact an extension of the servlets programming model. When a user requests a JSP page, a web container compiles the JSP page into a servlets. The web container then invokes the servlets and returns the resulting content to the web browser. Thus, JSP pages provide a powerful and dynamic page assembly mechanism that benefits from the many advantages of the Java platform.
Enterprise JavaBean components
Enterprise JavaBeans (EJB) is a distributed component model for developing secure, scalable, transactional, and multi-user components. To put it simply, EJBs are reusable software units containing business logic. Just as JSP pages allow the separation of application and presentation logic, EJBs allow separation of application logic from system-level services thus allowing the developer to concentrate on the business domain issues and not system programming. These enterprise bean business objects take three basic forms – again, It is not
necessary to implement them all – session beans, entity beans, and message driven beans.
Session Beans
Session beans themselves come in two types. A stateful session bean is a transient object used to represent a clients interaction with the system – it performs the clients request in the application, accessing a database etc., and when the clients operations are compleate and it is destroyed. The alternative, a stateless session bean, maintains no state between client requests. Generally this type of session bean is used to implement a specific service that does not require client state.
Entity Beans
An entity bean on the other hand is a persistent object that models the data held within the data store , that is, it is an object wrapper for the data. Compared to session beans that can be used by any client, entity beans can be accessed concurrently by many clients but must maintain a unique identity through a primary key.
Message-Driven Beans
Message-driven beans are a special class of EJBs that are not meant for direct client invocation.
The purpose of message-driven beans is to process messages received via JMS. Message-driven beans complement the asynchronous nature of JMS by providing a means of processing messages within the EJB container. When an application client or an application sends a message via JMS, the container invokes the appropriate message-driven bean to process the message.
These can be categorized as any component that response to an HTTP request. A further distinction can be drawn is based on the hosting container for the application components.
Servlets
Servlets are server side programs that allow application logic to be embedded in the HTTP request-response process. Servlets provide a means to extends the functionality of the web server to enable dynamic content in HTML, XML, or other web languages.
JavaServer Pages
JavaServer Pages (JSP) provides a way to embed components in a page, and to have them do their work to generate the page is eventually send to the client. A JSP page can contain HTML, Java code, and JavaBean components. JSP pages are infact an extension of the servlets programming model. When a user requests a JSP page, a web container compiles the JSP page into a servlets. The web container then invokes the servlets and returns the resulting content to the web browser. Thus, JSP pages provide a powerful and dynamic page assembly mechanism that benefits from the many advantages of the Java platform.
Enterprise JavaBean components
Enterprise JavaBeans (EJB) is a distributed component model for developing secure, scalable, transactional, and multi-user components. To put it simply, EJBs are reusable software units containing business logic. Just as JSP pages allow the separation of application and presentation logic, EJBs allow separation of application logic from system-level services thus allowing the developer to concentrate on the business domain issues and not system programming. These enterprise bean business objects take three basic forms – again, It is not
necessary to implement them all – session beans, entity beans, and message driven beans.
Session Beans
Session beans themselves come in two types. A stateful session bean is a transient object used to represent a clients interaction with the system – it performs the clients request in the application, accessing a database etc., and when the clients operations are compleate and it is destroyed. The alternative, a stateless session bean, maintains no state between client requests. Generally this type of session bean is used to implement a specific service that does not require client state.
Entity Beans
An entity bean on the other hand is a persistent object that models the data held within the data store , that is, it is an object wrapper for the data. Compared to session beans that can be used by any client, entity beans can be accessed concurrently by many clients but must maintain a unique identity through a primary key.
Message-Driven Beans
Message-driven beans are a special class of EJBs that are not meant for direct client invocation.
The purpose of message-driven beans is to process messages received via JMS. Message-driven beans complement the asynchronous nature of JMS by providing a means of processing messages within the EJB container. When an application client or an application sends a message via JMS, the container invokes the appropriate message-driven bean to process the message.
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