Enterprise JavaBeans v3.0 - 1.1. Server-Side Components

1.1. Server-Side Components

Object-oriented languages such as Java, C++, C#, Python, and Ruby are used to write software that is flexible, extensible, and reusablethe three axioms(n.[数]公理) of object-oriented development. In business systems, object-oriented languages are used to improve development of GUIs, to simplify access to data, and to encapsulate the business logic. The encapsulation of business logic into business objects is a fairly recent focus in the information-technology industry. Business is fluid, which means that a business's products, processes, and objectives evolve(v.(使)发展, (使)进展, (使)进化) over time. If the software that models the business can be encapsulated into business objects, it becomes flexible, extensible, and reusable, and therefore evolves as the business evolves.

A server-side component model may define an architecture for developing distributed business objects that combines the accessibility of distributed object systems with the fluidity of objectified business logic. Server-side component models are used on middle-tier application servers, which manage the components at runtime and make them available to remote clients. They provide a baseline of functionality that makes it easy to develop distributed business objects and assemble them into business solutions.

Server-side components can also be used to model other aspects of a business system, such as presentation and routing. A Java servlet, for example, is a server-side component that generates HTML and XML data for the presentation layer of a web application (Struts and JSF components are also examples of this type of server-side component). EJB message-driven beans, discussed later in this book, are server-side components that can be used to consume and process asynchronous messages.

Server-side components, like other components, can be bought and sold as independent pieces of executable software. They conform to a standard component model and can be executed without direct modification in a server that supports that component model. Server-side component models often support attribute-based programming, which allows the runtime behavior of the component to be modified when it is deployed, without having to change the programming code in the component. Depending on the component model, the server administrator can declare a server-side component's transactional, security, and even persistence behavior by setting these attributes to specific values.

As an organization's services, products, and operating procedures evolve, server-side components can be reassembled, modified, and extended so that the business system reflects those changes. Imagine a business system as a collection of server-side components that model concepts such as customers, products, reservations, and warehouses. Each component is like a Lego™ block that can be combined with other components to build a business solution. Products can be stored in the warehouse or delivered to a customer; a customer can make a reservation or purchase a product. You can assemble components, take them apart, use them in different combinations, and change their definitions. A business system based on server-side components is fluid because it is objectified, and it is accessible because the components can be distributed.

服务器端组件的初衷：独立的功能封装，可组装、可配置 

1.1.1. Enterprise JavaBeans Defined 

Sun Microsystems' definition of the Enterprise JavaBeans architecture is as follows: The Enterprise JavaBeans architecture is a component architecture for the development and deployment of component-based distributed business applications. Applications written using the Enterprise JavaBeans architecture are scalable, transactional, and multi-user secure. These applications may be written once, and then deployed on any server platform that supports the Enterprise JavaBeans specification.^[*]

^[*] Sun Microsystems' Enterprise JavaBeans Specification, v3.0, Copyright© 2002 by Sun Microsystems, Inc.

That's a mouthful, but it's not atypical of how Sun defines many of its Java technologieshave you ever read the definition of the Java language itself? It's about twice as long. This book offers a shorter definition of EJB:

         Enterprise JavaBeans is a standard server-side component model for distributed business applications.

This means the EJB specification offers a standard model for building server-side components that represent business processes (purchasing, inventory, and so on). Once you have built a set of components that fit the requirements of your business, you can combine them to create business applications. On top of that, as "distributed" components, they don't all have to reside on the same server. Components can reside（vi.居住） wherever it's most convenient: a TravelAgent component can "live" near the reservation database, or a Purchase business-process component can live near the user interface. You can do whatever's necessary to minimize latency, share the processing load, or maximize reliability.

       EJB是一种满足分布式商业应用的标准服务器端组件。分布式是其重要特点，可灵活部署。