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Singleton beans
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Sometimes we don t need any more than one backing instance for our business objects. EJB 3.1 therefore introduces a new session component type, the singleton bean. Because all requests upon a singleton are destined for the same bean instance, the Container doesn t have much work to do in choosing the target (Figure 2-4). The singleton session bean may be marked to eagerly load when an application is deployed; therefore, it may be leveraged to fire application lifecycle events. This draws a relationship where deploying a singleton bean implicitly leads to the invocation of its lifecycle callbacks. We ll put this to good use when we discuss singleton beans in 7. Dealers in a casino aren t granted complete autonomy; they ve got to clear a set of privileged tasks such as changing money, cashout, and large betting with the pit boss. The pit boss, in turn, is the sole authority over what s permitted on his floor. If we model the boss as a singleton, all requests are passed along to the same source one
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able to make informed, consistent decisions. In addition, at the start of the pit boss s shift, he can make the rounds and do whatever startup tasks he deems necessary.
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Message-Driven Beans (MDBs)
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Asynchronous messaging is a paradigm in which two or more applications communicate via a message describing a business event. EJB 3.1 interacts with messaging systems via the Java Connector Architecture (JCA) 1.6 (http://jcp.org/en/jsr/detail id=322), which acts as an abstraction layer that enables any system to be adapted as a valid sender. The message-driven bean, in turn, is a listener that consumes messages and may either handle them directly or delegate further processing to other EJB components. The asynchronous characteristic of this exchange means that a message sender is not waiting for a response, so no return to the caller is provided (Figure 2-5).
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One common provider of asynchronous messaging is the Java Message Service (JMS), and the EJB specification dictates that JMS is supported implicitly. If a message is sent
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EJB 3.1 Specification 5.4.17.1
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to a JMS Topic or Queue, an MDB may be created to take action upon that event. By extension, any service with a valid JCA Resource Adapter may use MDB as an endpoint. We ll untangle the various technologies that piece together asynchronous messaging in 8. Like SLSBs, MDBs have no conversational state. Any instance may be used in servicing a message. In fact, the client has no view or knowledge of the MDB at all! Once the client sends a message, it s out of scope to worry about what may be listening to the event fired. After a full night of gambling, visitors to our casino are likely to need their cars back from the valet. We may provide our valet service via MDBs; once a ticket (message) is received, the valet should fetch the car while its owner continues on to grab a cup of coffee or visit the gift shop. The casino visitor will have to frequently check back to see whether his car has arrived. Message-driven beans are explored in 8.
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While session beans are our verbs, entity beans are the nouns. Their aim is to express an object view of resources stored within a Relational Database Management System (RDBMS) a process commonly known as object-relational mapping. Like session beans, the entity type is modeled as a POJO, and becomes a managed object only when associated with a construct called the javax.persistence.EntityManager, a container-supplied service that tracks state changes and synchronizes with the database as necessary. A client who alters the state of an entity bean may expect any altered fields to be propagated to persistent storage. Frequently the EntityManager will cache both reads and writes to transparently streamline performance, and may enlist with the current transaction to flush state to persistent storage automatically upon invocation completion (Figure 2-6).
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Unlike session beans and MDBs, entity beans are not themselves a server-side component type. Instead, they are a view that may be detached from management and used
just like any stateful object. When detached (disassociated from the EntityManager), there is no database association, but the object may later be re-enlisted with the EntityManager such that its state may again be synchronized. Just as session beans are EJBs only within the context of the Container, entity beans are managed only when registered with the EntityManager. In all other cases entity beans act as POJOs, making them extremely versatile. Users familiar with EJB 2.x forms may be used to the former notion of Container Managed Persistence (CMP), which required dependence upon a verbose API. Although entity bean classes were once required to implement specific interfaces and their callback methods explicitly, these restrictions have been removed as of the 3.0 revision of the EJB Specification. Our casino exists primarily to move money. Taking bets, paying winners, making change all are transactions that need to be recorded for bookkeeping purposes. By modeling currency as an entity bean, all functions that interact with cash and chips may share the same object view that ultimately will be preserved in the database for future auditing. Entity beans are introduced in Part III.
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