qr code generator java program Figure 10-5: An event adapter acting as a transaction coordinator in Java

Generating QR Code in Java Figure 10-5: An event adapter acting as a transaction coordinator

Figure 10-5: An event adapter acting as a transaction coordinator
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Customization Property editors are useful for simple beans For complex beans, or for beans with complex data types as properties, the default property editors may not be enough For such cases, JavaBeans defines the notion of a customizer A customizer is a class that provides a way for a user of a bean to provide values for the properties of the bean at design time A customizer can be a wizard, a specialized user interface, or an editor that allows complex data types to be entered A customizer needs to inherit (directly or indirectly) from javaawtComponent because it is used by the bean IDE as a GUI It must also implement the javabeansCustomizer interface The methods in this interface are used for managing the interaction, with the rest of the IDE using property change events Attaching a customizer to a certain bean involves going through the BeanInfo object The customizer for a bean is retrieved by a call to the getCustomizerClass() method, by the getBeanDescriptor() method after the bean description is retrieved from the BeanInfo object
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Persistence When using a bean within an IDE, the user typically customizes it by setting properties, attaching events, and so on This information must be externalized to some persistent store in order for the system to function correctly at runtime JavaBeans use Java object serialization as the mechanism for storing out and reading in beans While the specification defines other alternatives for making a bean persistent, including allowing the tool to control the process and allowing specialized formats to be used, the current state of the market is such that you should use serialization and externalization only
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Summary After reading this chapter, you should have a good grasp of the JavaBeans component model and the preferred way to build your Java classes By following the conventions defined in this model, you will ensure not only that your code and metadata can be used within the context of other tools and frameworks, but also (and probably this is 174
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more important) that your code can be easily read and understood Following the conventions defined in the JavaBeans model is especially important when you are writing visual components and components that are to be used in a visual composition IDE; still, our opinion is that it is always important In the next chapter, we will go one step further and describe the Enterprise JavaBeans (EJB) component model This model is much more sophisticated and provides many more services
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11: Enterprise JavaBeans Overview Now that you have a pretty good handle on JavaBeans, let's move on to the most important component model for building server-side functionality in the Java world: Enterprise JavaBeans (EJB) This chapter describes the EJB architecture and the many components that comprise this architecture It describes what code needs to be built, as well as who builds it The bean, container, and server work together in supporting the client; this chapter provides details on the lifecycle of both entity beans and session beans You should not underestimate the difficulty of mastering EJBs The model is complex, and it takes time for you to understand how to make everything work together This chapter is dedicated to describing the EJB model from the ground up, and the following chapters will provide examples of EJBs running within WebSphere Server-Side Components EJB is a server-side component architecture that enables and simplifies the process of building enterprise-level distributed applications in Java What this long definition means is that EJB provides a framework that allows you to implement a distributed Java application without writing code to handle the complex issues of distributed programming, such as communication and synchronization The EJB architecture also manages scalability, security, and reliability issues well All in all, EJB allows enterprise programmers to focus on the specifics of their applications, without worrying about the infrastructure of the application The EJB server provides all the infrastructure you will need You can choose to use any vendor's EJB server, and the architecture code is portable between servers This book focuses on the WebSphere Application Server, but it is important for you to keep in mind that one of the advantages of EJBs is their portability This portability, of course, stems from the fact that Java is portable and is shared by all Java 2 Platform Enterprise Edition (J2EE) mechanisms, such as JavaServer Pages (JSP) and servlets The EJB architecture was introduced by Sun Microsystems as the cornerstone of its strategy of pushing Java onto the server side The EJB architecture aims at separating system-level issues such as communication, distribution, transactions, and security from actual business logic and domain-specific issues EJB is all about letting developers focus on building domain functionality The use of a standard application programming interface (API) and architecture allows developers to rely on off-the-shelf solutions for system and architecture problems, without being tied to a specific vendor The goals of the EJB architecture, as defined in the EJB specification, are as follows: To define a component architecture for the development of Java-based distributed business applications To enable the mixing of components built by different vendors in providing an enterprise-level solution To facilitate scenarios in which experts provide solutions for transactions, security, distribution, multithreading, and other system-level issues in a way that they can be used easily by domain experts building business applications To address the entire software lifecycle and provide definitions by which EJB components can be used within tools during development time (much like JavaBeans address both the runtime and the design time issues) To define aspects of interoperability Specifically, EJB relies on the Common Object Request Broker Architecture (CORBA) for distributed object 176
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interoperability; this does not mean that EJB interoperability is only related to CORBA, as many other API mappings and transports are also possible Enterprise JavaBeans Architecture The EJB architecture attempts to take the idea of JavaBeans, discussed in the last chapter, one step further Instead of simply providing a framework for building reusable software components, it provides a framework for building distributed, transactionoriented components This distinction has immense consequences One of them is that while JavaBeans are merely development components for programmers, EJBs are deployable components that are ready for use In a real-world distributed setting, the placement of objects on physical machines is usually determined at deployment time, not at development time Furthermore, because this placement can change over time, it obviously is preferable that the placement can be modified easily without making software modifications Being deployable objects, EJBs must have a standard and we ll-defined deployment environment To this end, the EJB architecture defines the concepts of containers and servers Containers hold (or contain) EJB instances The specification defines the mutual responsibilities of containers and EJB components, as well as their interactions EJB containers are, in turn, contained in EJB servers The original EJB specification did not determine the contract between the EJB server and the EJB container; this was left to the discretion of the vendors As a result, it was not quite clear where the container ended and where the server began In fact, if the server has only one container, there is no real difference between the server and the container This problem was resolved with the introduction of the J2EE platform, discussed in 15 With J2EE, the concept of a container was extended, and the distinction between containers and servers was clarified An interesting note about containers: WebSphere 35x allows multiple containers to be created, but this is not advised The best practice in WebSphere 35x is to have one container per application server WebSphere 40 ends this limitation, thereby providing for more robust application deployments An enterprise bean can exist only within a container The enterprise bean is never accessed directly, nor does instance creation take place directly The container manages access to enterprise bean functionality, as well as instance creation and destruction Because EJBs are distributed objects that are accessed remotely, the actual access is made via special proxy objects called remote EJB objects These allow the bean developer to delegate the responsibility of managing transactions, persistence, communication, and security to the container Figure 11-1 provides a schematic depiction of the EJB architecture
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Figure 11-1: Overview of EJB architecture While the relationship between an enterprise bean and the container holding it is a close one, the EJB specification requires that EJBs be portable among different containers In other words, EJBs and containers are required to be fully decoupled This 177
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means that all services provided to the enterprise bean by the container are well defined in the EJB specification In its quest to define a framework for distributed object-oriented systems, the EJB architecture must create the right balance between various contradictory requirements On one hand, the framework must be simple so that it is easy to understand and use On the other hand, EJBs target enterprise-level systems very large, typically missioncritical, systems Specifically, enterprise systems have strict performance and failureresilience requirements, as well as other specialized requirements To achieve the correct balance between the various requirements and manage the inherent complexities of the issues at hand, the EJB specification takes a "divide-andconquer" approach The tasks and responsibilities relating to the EJB lifecycle are divided between several parties Unlike JavaBeans, where only two parties (the bean developer and the bean user) are involved, in EJB there are six (!) different parties: the bean provider (or developer), the container provider, the server provider, the application developer, the application deployer, and the system administrator These parties are referred to as roles, and some of the roles might be combined As examples, the application deployer and the system administrator are sometimes the same person; and the same vendor usually provides the EJB container and the EJB server The next section details the responsibilities of each role To allow the different parties to communicate with each other, an enterprise bean defines sets of properties that affect its runtime behavior EJB properties are separated as metadata, or descriptors, to allow customization at setup and deployment time, similar to the way in which JavaBeans define the BeanInfo object and provide customization However, there are differences in the nature of the properties JavaBeans properties typically are related to user interfaces, whereas EJB properties involve control over issues related to distribution, such as transactions, security, and persistence EJB Roles The EJB specification provides a clear-cut definition of the responsibilities of each role By clearly defining the roles of each party, EJB facilitates the development of distributed and highly scalable applications in which products from different vendors can interoperate The remainder of this section explains the different EJB roles The Bean Provider The bean provider's role is to develop the EJB This provider could be an in-house developer or a vendor of reusable software components The most important role of the bean provider is to design the bean in a reusable manner While the EJB architecture is designed to promote reusability, that is not to say that any EJB is reusable by definition EJBs are truly reusable if they are designed properly The main design choice for the bean developer is in the definition of "business" methods (methods in the application domain, as opposed to methods involving more prosaic issues such as persistence, transactions, and so on) EJB methods form the contract between the enterprise bean and users of the bean The methods are grouped into an interface published by the EJB developer, and this interface must be applied by the bean implementation The bean developer must provide yet another interface called the home interface, which is defined for every EJB type EJB home interfaces implement the commonly used "factory" design pattern The factory is responsible for instantiating and destroying objects in this case EJB objects Another responsibility of the home interface is bean lookup Bean lookup is used for acquiring a reference to a bean This is not possible for all types of beans, only for entity beans persistent beans that have an application178
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level identity From the perspective of client code, object lookup involves two stages: locating the home interface and then using it to locate a specific object As part of the inherent support for distributed programming, client software never makes direct use of the enterprise bean Instead of using object references, it holds references to remote "proxies" or "stubs" that, in turn, communicate with the container The container provides the execution environment for the EJB Because the container is responsible for managing system-level issues, it must be aware of any method invocations EJB Server Provider and EJB Container Provider Theoretically, the roles of providing the server and the container are distinct, but in practice, the server is provided with the container implementation However, the J2EE architecture allows for the introduction of new containers into a J2EE server The EJB container provides the execution environment for EJB It provides all the required services for large, distributed applications such as security services, transaction services, communication services, and so on The container handles all aspects of the bean lifecycle, as well as method invocation From the application developer's standpoint, the fact that all these services are provided by the container has immense benefits When relying on these services, EJB developers can ignore issues of security, transactions, resource management, lifecycle management, persistence, and multithreading The container handles everything, and it is supposed to do so in an efficient manner To grant vendors some flexibility, the EJB specification allows "extended services" to be provided by the server or the container; these services are beyond the EJB specification Such services might be aimed at advanced performance and reliability via load balancing and transparent failover, or support for legacy systems via EJB connectors WebSphere offers connectors to IBM legacy systems such as Customer Information Control System (CICS), Encina, Information Management System (IMS), DB2, and MQSeries Obviously, EJBs that rely on such extended services might be limited in the range of EJB containers and servers in which they can run Note that some of these services were replaced by J2EE Connectors (see 15) The Application Developer The application developer is a person who utilizes available EJBs to construct an enterprise application The available EJBs might be taken out of Java libraries provided by commercial vendors or developed in-house While the available EJBs provide some functionality, they are usually implemented in a generic fashion, and if EJBs are taken from multiple sources, they might not be immediately interoperable The application developer should provide the "glue" for connecting the different components As EJBs are middle-tier software, they do not provide any user interface The application developer should implement some user interface, which could be any Java interface such as JFC/Swing; in the J2EE model, it wo uld typically be implemented using JSP and servlets User interface code would invoke the EJBs In addition, the application developer might be required to develop some EJBs that are needed for solving domainspecific problems or for connecting the different components The Application Deployer The application deployer bridges the gap between the container and server provider and the application developer The application deployer physically places the EJBs onto servers and containers The deployer has the freedom to deploy the beans in any manner so that he or she can customize the installation to fit the available resources 179
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and their configuration Typical considerations would be performance issues The deployer attempts to place the beans so that there is some "locality of reference" (in other words, so that the clients invoking the beans will be as close as possible to the beans), but at the same time attempts to balance the load among different servers Other considerations might be technical concerns, such as communication bandwidth between nodes or the existence of firewalls Another important responsibility of the deployer is security settings The deployer can assign access rights to different EJB methods, as well as to Web resources These rights should be assigned in a way that complies with the security policy of the organization and does not compromise the safety of the data stored in the system The System Administrator The system administrator is in charge of troubleshooting any problems when the deployed application is being used Various monitoring tools are at the administrator's disposal; in WebSphere, the Administrative Console provides such tools Problems in execution could arise in various areas, such as in the hardware, the operating sys tem, the Web server, the application server, and of course the application itself The system administrator should be able to pinpoint the problem as much as possible, and then fix it or forward it back to the deployer or the application developer as required EJB Components The EJB architecture provides a framework for building distributed transaction-oriented components Like JavaBeans, EJB is a component model; as such, it defines the structure of an EJB component and describes the world in which it lives an extremely important concept A well-known thesis in philosophy is that to fully describe an entity, one must describe its surroundings and how it interacts with its environment This is the approach taken by the EJB architecture An EJB component is a Java component that implements business logic As a component living within the EJB world, the EJB component must assume a well-defined environment and a set of interactions in which to participate For example, the EJB specification defines the responsibilities of containers and the responsibilities of EJB components being managed by a container It also fully defines the interaction between the container and the EJB component, thereby defining a structured world upon which an EJB builder can rely This brings the task of building EJB components and EJB-based systems a little closer to being an engineering discipline than an art; that is the ultimate purpose of the computer science field known as software engineering The fundamentals of an EJB component's environment are defined as follows: An enterprise bean is developed to implement business functionality Session beans typically handle the domain information and perform business functions, whereas entity beans store the information An enterprise bean is not an island of its own It lies within a container deployed within a server It is never accessed directly; the container manages access to its functionality This allows the responsibility of managing things like transactions and security to be moved from the enterprise bean to the container and the server For example, the container exclusively performs instance creation of the enterprise bean type This allows the container to manage lifecycle and transaction issues for the class Enterprise beans and containers are fully decoupled, which means that multiple EJB-compliant containers can be used to manage instances of an enterprise bean All services provided to the enterprise bean by the container are well defined in the EJB specification 180
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