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1.2 Understanding EJB types
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If you re like most developers, you always have a tight deadline to meet. Most of us try to beg, borrow, or steal reusable code to make our lives easier. Gone are those days when developers had the luxury to create their own infrastructure
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Understanding EJB types
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when building a commercial application. While several commercial and open source frameworks are available that can simplify application development, EJB is a compelling framework that has a lot to offer. We expect that by now you re getting excited about EJB and you re eager to learn more. So let s jump right in and see how you can use EJB as a framework to build your business logic and persistence tier of your applications, starting with the beans. In EJB -speak, a component is a bean. If your manager doesn t find the Java coffee bean play on words cute either, blame Sun s marketing department. Hey, at least we get to hear people in suits use the words enterprise and bean in close sequence as if it were perfectly normal As we mentioned, EJB classifies beans into three types, based on what they are used for:
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Session beans Message-driven beans Entities
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Each bean type serves a purpose and can use a specific subset of EJB services. The real purpose of bean types is to safeguard against overloading them with services that cross wires. This is akin to making sure the accountant in the horn-rimmed glasses doesn t get too curious about what happens when you touch both ends of a car battery terminal at the same time. Bean classification also helps you understand and organize an application in a sensible way; for example, bean types help you develop applications based on a layered architecture. As we ve briefly mentioned, session beans and message-driven beans (MDBs) are used to build business logic, and they live in the container, which manages these beans and provides services to them. Entities are used to model the persistence part of an application. Like the container, it is the persistence provider that manages entities. A persistence provider is pluggable within the container and is abstracted behind the Java Persistence API (JPA). This organization of the EJB 3 API is shown in figure 1.6. We ll discuss the container and the persistence provider in section 1.3. For the time being, all you need to know is that these are separate parts of an EJB implementation, each of which provide support for different EJB component types. Let s start digging a little deeper into the various EJB component types, starting with session beans.
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What s what in EJB 3
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Figure 1.6 Overall organization of the EJB 3 API. The Java persistence API is completely separable from the EJB 3 container. The business logic processing is carried out by through two component types: session beans and message-driven beans. Both components are managed by the container. Persistence objects are called entities, which are managed by the persistent provider through the EntityManager interface.
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1.2.1 Session beans
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A session bean is invoked by a client for the purpose of performing a specific business operation, such as checking the credit history for a customer. The name session implies that a bean instance is available for the duration of a unit of work and does not survive a server crash or shutdown. A session bean can model any application logic functionality. There are two types of session beans: stateful and stateless. A stateful session bean automatically saves bean state between client invocations without your having to write any additional code. A typical example of a state-aware process is the shopping cart for a web merchant like Amazon. In contrast, stateless session beans do not maintain any state and model application services that can be completed in a single client invocation. You could build stateless session beans for implementing business processes such as charging a credit card or checking customer credit history. A session bean can be invoked either locally or remotely using Java RMI. A stateless session bean can be exposed as a web service.
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1.2.2 Message-driven beans Like session beans, MDBs process business logic. However, MDBs are different in one important way: clients never invoke MDB methods directly. Instead, MDBs are triggered by messages sent to a messaging server, which enables sending asynchronous messages between system components. Some typical examples of messaging servers are IBM WebSphere MQ, SonicMQ, Oracle Advanced Queueing, and TIBCO. MDBs are typically used for robust system integration or asynchronous processing. An example of messaging is sending an inventory-restocking
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