What types of ejbs are there

A full example is provided in Chapter 7, "Message-Driven Beans". EJB QL defines the queries for the finder and select methods of an entity bean with container-managed persistence. The abstract schema is part of an entity bean's deployment descriptor and defines the bean's persistent fields and relationships.

The term "abstract" distinguishes this schema from the physical schema of the underlying datastore. An entity bean can be defined so as to have a relationship with other entity beans. You implement relationships differently for entity beans with bean-managed-persistence than those entity beans that utilize container-managed-persistence.

With bean-managed persistence, the code that you write implements the relationships. With container-managed persistence, the EJB container takes care of the relationships for you. For this reason, relationships in entity beans with container-managed persistence are often referred to as container-managed relationships.

Multiplicity in Container-Managed Relationships - There are four types of multiplicities all of which are supported by Oracle9 i AS: One-to-One - Each entity bean instance is related to a single instance of another entity bean.

One-to-Many - An entity bean instance is related to multiple instances of the other entity bean. Many-to-One - Multiple instances of an entity bean may be related to a single instance of the other entity bean. This multiplicity is the opposite of one-to-many. Many-to-Many - The entity bean instances may be related to multiple instances of each other. Direction in Container-Managed Relationships - The direction of a relationship may be either bi-directional or unidirectional.

In a bi-directional relationship, each entity bean has a relationship field that refers to the other bean. Through the relationship field, an entity bean's code can access its related object. If an entity bean has a relative field, then we often say that it "knows" about its related object. In a unidirectional relationship, only one entity bean has a relationship field that refers to the other.

The direction of a relationship determines whether a query can navigate from one bean to another. Oracle9 i AS Object-Relational Mapping Oracle9 i AS furnishes, out of the box, its own persistence manager for entity beans, which supplies both simple mapping and complex relationship 1:n, m:n mapping.

TopLink provides developers with the flexibility to map objects and Enterprise Java Beans to a relational database schema with minimal impact. TopLink provides EJB 2. Oracle9 i AS is also integrated with other leading O-R mapping solutions in the market.

The POA is also intended to allow persistent objects -- at least, from the client's perspective. That is, as far as the client is concerned, these objects are always alive, and maintain data values stored in them, even though physically, the server may have been restarted many times, or the implementation may be provided by many different object implementations.

Enterprise JavaBeans EJBs can be one of three types: session beans, entity beans, or message-driven beans. Stateless session beans are used for business services. They do not retain client state across calls.

Stateful session beans do maintain state across client calls. Thus, these beans manage business functions for a specific client for the life of that client. Entity beans are normally used for managing persistent data.

Message-driven beans are used for receiving messages from a JMS queue or topic. An EJB has two client interfaces:. Home interface--The home interface defines EJB life cycle methods, such as a method to create and retrieve a reference to the bean object.

The client uses both of these interfaces when invoking a method on a bean. Figure demonstrates a stateless session bean and corresponds to the following steps:. The client invokes the create method on the home interface reference home object. This creates the bean instance and returns a reference to the remote interface of the bean. The client invokes a method defined in the remote interface, which delegates the method call to the corresponding method in the bean instance through a stub.

The client can destroy the bean instance by invoking the remove method that is defined in the remote interface. Some beans, such as stateless session beans, cannot call the remove method. In this case, the container removes the bean. The home interface contains the life cycle methods, such as the create methods that specify how a bean is created. The remote interface Specifies the business methods that you implement in the bean.

The bean must also implement additional container service methods. The EJB container invokes these methods at different times in the life cycle of a bean.

The bean implementation Contains the Java code that implements the methods defined in the home interface life cycle methods , remote interface business methods , and the required container methods container callback functions. The deployment descriptor Specifies attributes of the bean for deployment. These designate configuration specifics, such as environment, interface names, transactional support, type of EJB, and persistence information.

The implementation contains logic for lifecycle methods defined in the home interface, business methods defined in the remote interface, and container callback functions defined in the SessionBean , EntityBean , or MessageDrivenBean interface. A parameter that you pass to a bean method--or a return value from a bean method--can be any Java type that is serializable. Java primitive types, such as int , double , are serializable.

Any non-remote object that implements the java. Serializable interface can be passed. A non-remote object that is passed as a parameter to a bean or returned from a bean is passed by value, not by reference. So, for example, if you call a bean method as follows:. If the bean changes the value of theNumber object on the server, this change is not reflected back to the client, because of pass-by-value semantics. If the non-remote object is complex--such as a class containing several fields--only the non-static and non-transient fields are copied.

When passing a remote object as a parameter, the stub for the remote object is passed. A remote object passed as a parameter must extend remote interfaces. The next section demonstrates parameter passing to a bean, and remote objects as return values. The EmployeeBean getEmployee method returns an EmpRecord object, so this object must be defined somewhere in the application.

The class is declared as public and must implement the java. Serializable interface so that it can be passed back to the client by value, as a serialized remote object. The declaration is as follows:. Note: The java. Serializable interface specifies no methods; it just indicates that the class is serializable. Therefore, there is no need to implement extra methods in the EmpRecord class. A session bean might contain methods that query and update data in a relational table. Session beans are often used to implement services.

For example, an application developer might implement one or several session beans that retrieve and update inventory data in a database. Session beans are transient because they do not survive a server crash or a network failure. If, after a crash, you instantiate a bean that had previously existed, the state of the previous instance is not restored. State can be restored only to entity beans. A session bean implements the javax.

SessionBean interface, which has the following definition:. At a minimum, an EJB must implement the following methods, as specified in the javax. SessionBean interface:. Stateless session beans must do nothing in this method. Stateful session beans can initiate state in this method. This method performs any required clean-up--for example, closing external resources such as file handles. The container calls this method after the bean creation. Learn RMI. Message-oriented middleware MOM.

Well, the following are some examples: In a Web-centric application, the EJBs will provide the business logic that sits behind the Web-oriented components, such as servlets and JSPs. Links Home. Introduction to EJBs.

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