As a downside, owing to the shared nature of the bean, developers are responsible to ensure that they are thread safe. A Singleton session bean is instantiated once per application and exists for the lifecycle of the application. Singleton session beans are designed for circumstances in which state must be shared across all clients.
Similar to Stateless beans, developers must ensure that singletons thread safe. Alternatively, we could include the target server runtime dependency instead of the JavaEE APIs, but that does reduce portability between different containers. Stateless , Stateful or Singleton. These annotations come from the javax.
A message-driven bean or MDB is an enterprise bean that allows you to process messages asynchronously. This type of bean normally acts as a JMS message listener, which is similar to an event listener but receives JMS messages instead of events.
They are in many ways similar to a Stateless session bean but they are not invoked by a client. 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. Session Bean: Session bean contains business logic that can be invoked by local, remote or webservice client. There are two types of session beans: i Stateful session bean and ii Stateless session bean.
Stateful session bean can be used to access various method calls by storing the information in an instance variable. Some of the applications require information to be stored across separate method calls.
In a shopping site, the items chosen by a customer must be stored as data is an example of stateful session bean. Stateless session bean can be used in situations where information is not required to used across call methods. Message Driven Bean: Like Session Bean, it contains the business logic but it is invoked by passing message.
Entity Bean: It summarizes the state that can be remained in the database. It is deprecated. There are two types of entity bean: i Bean Managed Persistence : In a bean managed persistence type of entity bean, the programmer has to write the code for database calls.
It persists across multiple sessions and multiple clients. Skip to content. Change Language. 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. The enterprise bean can store the reference to the context object in an instance variable, for use in transaction management. Beans that manage their own transactions can use the session context to get the transaction context.
Using setSessionContext You use this method to obtain a reference to the context of the bean. Session beans have session contexts that the container maintains and makes available to the beans. The bean may use the methods in the session context to make callback requests to the container.
The container invokes setSessionContext method, after it first instantiates the bean, to enable the bean to retrieve the session context. The container will never call this method from within a transaction context. If the bean does not save the session context at this point, the bean will never gain access to the session context. When the container calls this method, it passes the reference of the SessionContext object to the bean.
The bean can then store the reference for later use. The following example shows the bean saving the session context in the sessctx variable. A bean needs the session context when it wants to perform the operations listed in Table This is valid only for beans that have been designated transactional. There are two types of session beans:. Stateful Session Beans --Stateful session beans are useful for conversational sessions, in which it is necessary to maintain state, such as instance variable values or transactional state, between method invocations.
These session beans are mapped to a single client for the life of that client. Stateless Session Beans A stateless session bean does not maintain any state for the client. It is strictly a single invocation bean. It is employed for reusable business services that are not connected to any specific client, such as generic currency calculations, mortgage rate calculations, and so on.
Stateless session beans may contain client-independent, read-only state across a call. Subsequent calls are handled by other stateless session beans in the pool. The information is used only for the single invocation. The EJB container maintains a pool of these stateless beans to service multiple clients. An instance is taken out of the pool when a client sends a request.
There is no need to initialize the bean with any information. In addition, there is no need for the intended use for these methods in a stateless session bean. Instead, these methods are used mostly for EJBs with state--for stateful session beans and entity beans. Thus, these methods should be empty or extremely simple.
Implementation Methods Home Interface Extends javax. EJBHome and requires a single create factory method, with no arguments, and a single remove method. Remote Interface Extends javax. EJBObject and defines the business logic methods, which are implemented in the bean implementation.
Bean implementation Implements SessionBean. This class must be declared as public, contain a public, empty, default constructor, no finalize method, and implements the methods defined in the remote interface. Must contain a single ejbCreate method, with no arguments, to match the create method in the home interface.
Contains empty implementations for the container service methods, such as ejbRemove , and so on. Stateful Session Beans A stateful session bean maintains its state between method calls.
Thus, there is one instance of a stateful session bean created for each client. Each stateful session bean contains an identity and a one-to-one mapping with an individual client. The state of this type of bean is maintained across several calls through serialization of its state, called passivation.
This is why the state that you passivate must be serializable. However, this information does not survive system crashes. To maintain state for several stateful beans in a pool, it serializes the conversational state of the least recently used stateful bean to a secondary storage. When the bean instance is requested again by its client, the state is activated to a bean within the pool.
Thus, all resources are used performantly, and the state is not lost. The type of state that is saved does not include resources. The container invokes the ejbPassivate method within the bean to provide the bean with a chance to clean up its resources, such as sockets held, database connections, and hash tables with static information. All these resources can be reallocated and recreated during the ejbActivate method.
If the bean instance fails, the state can be lost--unless you take action within your bean to continually save state. However, if you must make sure that state is persistently saved in the case of failovers, you may want to use an entity bean for your implementation.
Alternatively, you could also use the SessionSynchronization interface to persist the state transactionally.
For example, a stateful session bean could implement the server side of a shopping cart on-line application, which would have methods to return a list of objects that are available for purchase, put items in the customer's cart, place an order, change a customer's profile, and so on. EJBHome and requires one or more create factory methods, and a single remove method. This class must be declared as public, contain a public, empty, default constructor, no finalize method, and implement the methods defined in the remote interface.
Must contain ejbCreate methods equivalent to the create methods defined in the home interface.
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