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4: Applicability of JEE Technology
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JEE Best Practices: Data Access Objects
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Unfortunately, most systems in use today rely on specific features of the enterprise s standard system resources, such as a vendor DBMS; they merge business logic and data access mechanisms. The result is lack of portability. As these standard resources become obsolete, the application systems tied to a resource become a real chore to upgrade. A good architect wants to avoid tying an application s business logic components to a resource, so the architect upgrades to a system with the least amount of resistance. The data-access object (DAO) pattern separates the interface to a system resource from the code used to access that resource by encapsulating the access to the data. Each enterprise bean that accesses a back-end resource in the application may have an associated DAO class, which defines an abstract API of operations on the resource. This allows a clean separation of bean and database access code. This also ensures easier migration to and from bean to container-managed persistence for entity beans and allows for cross-database and cross-schema capability. This abstract API makes no reference to how the resource is implemented. The DAO simply has to know how to operate from the persistent store given some identity information such as a filename. For example, an enterprise bean uses data it obtains from the DAO, not directly from the database. In this way, the enterprise bean defers its persistence mechanism to the DAO, allowing it to concentrate entirely on implementing business methods.
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Some enterprise information objects have values that are used together. For example, in a shopping cart application, the fields of the Address object are always used together. Using a complete remote interface for such entity beans is overkill and results in unacceptably high server communication. The data for an Address object can be retrieved once, sent to the client from the server in serialized form, and instantiated on the client. From then on, the local copy of the Address information can serve as a proxy for the Address property of the remote Order object. Subsequent accesses to the Address object s state are local, require no server communication, and use fewer resources. If the Address object is updated and sent to the server to update the server-side object, the entire object is sent. Furthermore, local accesses obviously have lower latency than accesses deferred through a remote interface.
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Such a client-side object is called a Value Object, because it represents a composite value from the server, not a reference to an object on the server. Value Objects tend to be more-or-less ad hoc groupings of data values to support a use case (or group of use cases). Use Value Objects for business objects that represent structure with accessor get and set behavior only. You should use a Value Object when the business entity being modeled has
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n Only methods that get values from the object s internal state (that is,
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n A life cycle that is completely controlled by another object n A relatively small size
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Whenever you update a bean by passing it a Value Object, the code should inspect all attributes of the Value Object and update the corresponding model bean attributes with their values. It should also check whether the version number of the Value Object is different from the model bean s version number. If this is the case, it should throw an exception, which indicates that the bean has been updated by another client.
JEE Best Practices: Session Bean Facade
The session bean facade (SBF), shown in Figure 4-13, provides a simple, single point of entry to shared entity beans. It shields the client from complex entity bean relationships. The most obvious rationale for using session beans to abstract entity beans is that the approach also abstracts the structure of your data stores. The presumption is that you do not want to expose the inner workings of your application s data store (such as the database tables and columns), or even the specifics of how that data is stored. In other words, letting users (potential hackers) know your database schema is a not a good idea. Problems can arise when you allow direct access to the entity bean layer. The methods in entity beans typically map directly to underlying fields in the data schema. This will become more important as service-based computing increases. Instead of providing complete applications, the JEE specification (or Web Services: UDDI, SOAP) indicates that organizations are focusing more on components than on complete applications. Interchanging data components from enterprise A s application with presentation components from enterprise B s application is
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