barcode font vb.net Figure 1.2 EJB as a framework provides services to EJB components. in Java

Encoder DataMatrix in Java Figure 1.2 EJB as a framework provides services to EJB components.

Figure 1.2 EJB as a framework provides services to EJB components.
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What s what in EJB 3
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metadata annotations, which are property settings that mark a piece of code, such as a class or method, as having particular attributes. This is a declarative style of programming, in which the developer specifies what should be done and the system adds the code to do it. In EJB, metadata annotations dramatically simplify development and testing of applications, without having to depend on an external XML configuration file. It allows developers to declaratively add services to EJB components as and when they need. As figure 1.3 depicts, an annotation transforms a simple POJO into an EJB. As you ll learn, annotations are used extensively throughout EJB, and not only to specify services. For example, an annotation can be used to specify the type of the EJB component. Although it s sometimes easy to forget, enterprise applications have one more thing in common with a house. Both are meant to last, often much longer than anyone expects. Being able to support high-performance, fault-tolerant, scalable applications is an up-front concern for the EJB platform instead of being an afterthought. Not only will you be writing good server-side applications faster, but also you can expect your platform to grow with the success of your application. When the need to support a larger number of users becomes a reality, you won t have to rewrite your code. Thankfully these concerns are taken care of by EJB container vendors. You ll be able to count on moving your application to a distributed, clustered server farm by doing nothing more than a bit of configuration. Last, but certainly not least, with a world that s crazy about service-oriented architecture (SOA) and interoperability, EJB lets you turn your application into a web services powerhouse with ease when you need to. The EJB framework is a standard Java technology with an open specification. If it catches your fancy, you can check out the real deal on the Java Community Process (JCP) website at www.jcp.org/en/jsr/detail id=220. EJB is supported by a large number of companies and open source groups with competing but compatible implementations. On the one hand, this indicates that a large group of people will work hard to keep EJB competitive. On the other hand, the ease of portability
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Figure 1.3 EJBs are regular Java objects that may be configured using metadata annotations.
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EJB overview
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means that you get to choose what implementation suits you best, making your application portable across EJB containers from different vendors. Now that we ve provided a high-level introduction to EJB, let s turn our attention to how EJB can be used to build layered applications.
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1.1.3 Layered architectures and EJB
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Most enterprise applications contain a large number of components. Enterprise applications are designed to solve a unique type of customer problem, but they share many common characteristics. For example, most enterprise applications have some kind of user interface and they implement business processes, model a problem domain, and save data into a database. Because of these commonalities, you can a follow a common architecture or design principle for building enterprise applications known as patterns. For server-side development, the dominant pattern is layered architectures. In a layered architecture, components are grouped into tiers. Each tier in the application has a well-defined purpose, sort of like a profession but more like a section of a factory assembly line. Each section of the assembly line performs its designated task and passes the remaining work down the line. In layered architectures, each layer delegates work to a layer underneath it. EJB allows you to build applications using two different layered architectures: the traditional four-tier architecture and domain-driven design (DDD). Let s take a brief look at each of these architectures. Traditional four-tier layered architecture Figure 1.4 shows the traditional four-tier server architecture. This architecture is pretty intuitive and enjoys a wide popularity. In this architecture, the presentation layer is responsible for rendering the graphical user interface (GUI) and handling user input. The presentation layer passes down each request for application functionality to the business logic layer. The business logic layer is the heart of the application and contains workflow and processing logic. In other words, business logic layer components model distinct actions or processes the application can perform, such as billing, searching, ordering, and user account maintenance. The business logic layer retrieves data from and saves data into the database by utilizing the persistence tier. The persistence layer provides a high-level object-oriented (OO) abstraction over the database layer. The database layer typically consists of a relational database management system (RDBMS) like Oracle, DB2, or SQL Server.
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