Ejb what is

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When use Enterprise Java Bean? Application needs Remote Access. In other words, it is distributed. Application needs to be scalable. EJB applications supports load balancing, clustering and fail-over. The onMessage method can call helper methods or can invoke a session bean to process the information in the message. A message can be delivered to a message-driven bean within a transaction context, so all operations within the onMessage method are part of a single transaction.

If message processing is rolled back, the message will be redelivered. To invoke the methods of an EJB locally, the bean can be injected in any managed class running in the container — say a Servlet:.

Invoking the method from a remote JVM is trickier and requires a bit more code. As a prerequisite, EJB must implement a remote interface to enable remoting capabilities.

You will need to write an EJB client which will perform a lookup over the network. The interface is annotated with Remote :. First, we created a Context with properties referring to the remote JVM. Once we get the remote EJB instance, we were able to invoke the method. As it happens, the Maven EJB plugin will generate a client jar file which will only have all the remote interfaces.

You just need to configure the plugin:. In case of Stateful beans, a new instance of the bean is returned every time a client performs a lookup. In case of Stateless beans, any one bean from the pool is returned. With both Stateless and Stateful enterprise beans, methods can be concurrently invoked by multiple clients or by multiple threads from the same client. However, in case of Singleton enterprise beans, the default mode is LockType.

This means that only one thread is allowed to invoke the method at once. That can be changed by adding the Lock annotation over a method and setting to LockType. READ :.

This fine-grained concurrency management over method level allows developers to build robust multi-threaded applications without having to deal with actual threads. Most clients read from the Map but a few do put elements into it.

Marking the get method as lock type read and put method as lock type write would make up for a perfect implementation:. A write-lock locks the whole class, so when the map is being updated in the addElement method, all the threads trying to access getElement will also be blocked. Running scheduled jobs in EJB is simplified to the maximum possible level i. The container creates bean instances at deployment time, adding and removing instances during operation based on message traffic.

Example: In an on-line shopping application, where the process of taking an order from a customer results in a process that issues a purchase order to a supplier, the supplier ordering process could be implemented by a message-driven bean.

While taking the customer order always results in placing a supplier order, the steps are loosely coupled because it is not necessary to generate the supplier order before confirming the customer order. It is acceptable or beneficial for customer orders to "stack up" before the associated supplier orders are issued. These sections briefly describe classes required for each bean type, the EJB run-time environment, and the deployment descriptor files that govern a bean's run-time behavior.

The composition of a bean varies by bean type. Table defines the classes that make up each type of EJB, and defines the purpose of the class type. The EJB 2.

These interfaces remain available for use with EJB 3. Table Components of EJB 3. The remote business interface exposes business logic to remote clients—clients running in a separate application from the EJB. It defines the business methods a remote client can call. The local business interface exposes business logic to local clients—those running in the same application as the EJB. It defines the business methods a local client can call.

The no-interface view a variation of the Local view that exposes the public methods of the bean class without the use of a separate business interface. An EJB container is a run-time container for beans that are deployed to an application server. The container is automatically created when the application server starts up, and serves as an interface between a bean and run-time services such as:. Unlike traditional Java EE server-based execution, embeddable usage allows client code and its corresponding enterprise beans to run within the same virtual machine and class loader.

This provides better support for testing, offline processing for example, batch jobs , and the use of the EJB programming model in desktop applications. Most of the services present in the enterprise bean container in a Java EE server are available in the embedded enterprise bean container, including injection, container-managed transactions, and security.

Enterprise bean components execute similarly in both embedded and Java EE environments, and therefore the same enterprise bean can be easily reused in both standalone and networked applications. As of EJB 3. However, you can still to use XML deployment descriptors if you want. In the case of conflicts, the deployment descriptor value overrides the annotation value.

All beans must be specified in an ejb-jar. An ejb-jar. This file is required if your beans take advantage of WebLogic Server-specific features. Like ejb-jar. Entity beans that use container-managed persistence must be specified in a weblogic-cmp-jar. An EJB can be accessed by server-side or client-side objects such as servlets, Java client applications, other EJBs, web services, and non-Java clients.

Any client of an EJB, whether in the same or a different application, accesses it in a similar fashion. WebLogic Server automatically creates implementations of an EJB's home and business interfaces that can function remotely, unless the bean has only a local interface. Clients access enterprise beans either through a no-interface view or through a business interface. A no-interface view of an enterprise bean exposes the public methods of the enterprise bean implementation class to clients.

Clients using the no-interface view of an enterprise bean may invoke any public methods in the enterprise bean implementation class or any superclasses of the implementation class. A business interface is a standard Java programming language interface that contains the business methods of the enterprise bean. The client of an enterprise bean obtains a reference to an instance of an enterprise bean through either dependency injection, using Java programming language annotations, or JNDI lookup, using the Java Naming and Directory Interface syntax to find the enterprise bean instance.

Dependency injection is the simplest way of obtaining an enterprise bean reference. EJB annotation. Applications that run outside a Java EE server-managed environment, such as Java SE applications, must perform an explicit lookup. Because of network overhead, it is more efficient to access beans from a client on the same machine than from a remote client, and even more efficient if the client is in the same application.

T3—To communicate with remote objects. RMI—To communicate with remote objects. RMI enables an application to obtain a reference to an object located elsewhere in the network, and to invoke methods on that object as though it were co-located with the client on the same JVM locally in the client's virtual machine. An EJB's remote interface extends java. URL resource connection factory.