The client calls a business method on the created bean. When finished the client destroys the bean by calling remove on the home interface. Home Bean instance 1 4 4 2 35 9. Arguments are passed-by-value in calls made by the remote client and passed-by-reference in calls made by the local client. Session bean EJB 1.
Entity bean EJB1. An entity bean is identified by its primary key. Types of persistence: a. Clients do not have direct access to MDBs through home or component interfaces. MDBs are similar to stateless session beans in that they may receive messages from multiple clients. MDBs allow asynchronous interaction between client and server which reduces the coupling between them.
MDBs may be used as an asynchronous interface to a server application. The MDB receives messages and converts these to method calls on session and entity beans in the bean container.
Stateless session bean EJB 1. They either do not exist or are ready for receiving method invocations. Lifecycle steps: 1. The client obtains a reference to a stateless session bean JNDI lookup. The bean goes into the state Ready. The client invokes business methods on the bean.
When the client reference goes out of scope, the lifecycle of the bean ends. Create bean 2. Dependency injection if any 3. PostConstruct callbacks if any Does not exist Ready 5. PreDestroy callbacks if any Statefull session bean EJB 1. The EJB container may move evict unused beans to secondary storage, e. When a client invokes a method on a passivated object, the container resurrects a bean of the requested type, loads it with state data that it had before passivation state data is stored separately on disk and then performs the method invocation.
Lifecycle steps: The bean creation process steps 1. The server may, when it detects that the bean is not invoked for some time, passivate the bean. Before passivating the bean, the container calls the PrePassivate callback. When a new client invocation arrives, the EJB container retrieves a bean of the requested type, fills it with state data the bean had before passivation, calls the PostActive annotation methods and then the called business method.
The actions for bean destruction are the same as for stateless beans. PostConstruct callbacks if any Does not exist Ready 7. PreDestroy callbacks if any Passive 5. PrePassivate callbacks 6. PostActivate callbacks Entity bean EJB 1.
The most common examples are graphical in nature. For example, a GUI pushbutton widget can be developed as a bean. Any compliant development tool can examine the button bean and determine its properties e. A specialization of the button might be a specific instance of it on a specific panel with a label of "Exit.
The purpose of EJB is to provide a server-side encapsulation of application logic for security and transactional purposes.
It does not have the development tool semantics of JavaBeans. An example of an EJB might be a business entity such as an invoice.
Access to the invoice is controlled by an access control list associated with the bean, and updates to the invoice or any of its line items must done as an atomic transaction. Each has its place in the application architecture, and each is well supported by the WebLogic framework. The Enterprise JavaBean specification java. Summary This paper has described how WebLogic EJB, while offering the developer the flexibility and universality of the JavaSoft standard, provides the power and performance necessary for a production environment.
Because WebLogic's EJB implementation depends upon and takes advantage of the sophisticated infrastructure of WebLogic, WebLogic EJB is fast and scalable, with many additional features that support real-world use of EJB for building complex, distributed systems that depend on security, transactions and state management.
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Interview Questions. The new version of the specification, 3. For a presentation layer based in desktop, however, the offer of open-source frameworks is quite reduced. Basically, we have knowledge of the MVC framework JForms, which, however, did not pass from version 0. Besides making possible a very richer interactivity with the user, since several interface functionalities are quite difficult - if not impossible - to be implemented in a browser, there are several reasons to develop desktop applications: visual handling of graphic elements - diagrams, graphics, images etc; distributed processing, in the cases in which it is desirable to have code running in the clients; local cache of information, to increase access performance, and others.
Even traditional disadvantages, like the necessity of installing software locally in several client machines, have been significantly minimized by the use of technologies like Java Web Start. The proposed model is an adaptation of the Model-View-Controller architectural pattern [2].
In this pattern, the View layer contains only the code for the assembly of the user interface, the Controller contains the logic for the treatment of events occurring in the system, and Model is the layer responsible for the treating and supplying data for the other application layers.
Besides providing the construction of visions and controllers ad hoc, the ftinctionalities offered by the presentation layer offer interfaces with almost functional specific characteristics, and the framework user will only extend some few hot spots. However, we think this is a quality, since it gives the programmer the possibility to build virtually anything in the user interface, in a strictly object oriented way.
To avoid dealing directly with Swing API complexities, the framework offers classes that work as Fa9ades [4], hiding complexity and supplying a lighter and dry API. Therefore, one of the main requisites of the presentation layer is to use the power and flexibility of the API and, at the same time, create facilities so that the user productivity is maximized.
Thus, there is, for each item of interface, a controller that assumes the function of helper ui the foregoing standard, assuming the tasks of process data for visualization, storing intermediary data models, and working as a business data adapter.
Besides that, controllers and viewers abstracts classes automate, by default, repetitive tasks like reading and writing component data, allowing very little codification for the hot spot implementation in most cases. A visualization screen, however, has not only the customized viewer, but a series of other supportive visual components, like menus, toolbars, navigators, search bars and others, that have fixed or little customized appearance and behavior.
Such components are considered frozen spots, though they could be eventually extended without any type of impediment related to couplings. The communication between several visual classes that compose the frozen structure of the presentation layer is implemented by the use of the Mediator design pattern [4], making possible a decoupled communication between the components and giving, even for the frozen spots, great flexibility and extensibility.
Presentation layer schema The Controller also has another responsibility in the framework architecture, which is to carry out the communication between the hot spots and the DataFrameDu-ector the entity that does the mediation between the participant colleagues of the Mediator design pattern that manages the standard components.
This communication also is done in completely automated way, so that the developer of the hot spots will be able to ignore even the existence of the frozen spots. The described schema is shown at Figure 1. The separation of business layer from other layers brings several benefits for the application, for instance, the possibility of diverse systems using different graphical interfaces desktop and web share the same business logic resident on a centralized or distributed repository, like an application server.
But it is not only the detachment of implementation of the business layer from the graphic interface that became this kind of solution so popular and efficient in the modem object- oriented software development. The clear separation of different concerns obtained from software partition makes technical issues concerning one layer be transparent to others. For instance, in the business layer, problems as data security and concurrent transaction management must be considered by solution developers, but ignored or, at least, minimized by user interface designers.
Design Issues The primary approach of the framework is oriented to reduce the complexity of J2EE architecture's distributed components development, separating the business logic implementation itself, isolating it from technological low level details. The frinction of the framework is to automate the codification of the underlying server application management and the EJB component that will be installed in the server. For this goal, the framework uses a set of classes that hide from the developer all details concerning the access, from client, to EJB components and hiding the EJB classes that connect with data services to provide persistence capabilities, by example.
An immediate consequence is to transform JNDI into a transparent service for the user. Some design patterns are used for achieve these facilities. One of the patterns used is EJBHomeFactory [9], whose objective is to provide a factory that will search and obtain the home object object responsible by the EJB component life cycle management in the container, e.
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