Entity Beans in Java

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Entity Beans
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Entity beans correspond to application objects that are meant to be persistent By persistent, I mean that they contain potentially valuable information across sessions Thus, unlike stateful session bean instance variables, which only make sense for a single session (such as a shopping cart), entity beans are associated with state that exists across sessions and perhaps is shared between them For example, historical purchasing information for a particular customer is relevant not only for one session but for every future session It may be more convenient to think of entity beans as similar to tables or relations in a relational database Like relations, they're named objects that consist of one or more attributes Like foreign key attributes in relations, they can contain attributes that relate them to other entity objects As is the case with the relational database model, they use primary keys to enforce entity[*] integrity That is, instances of the same entity bean can be distinguished from one another, just as primary keys enable rows to be distinguished in a relation
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[*] This use of the word entity is slightly different from its use in the phrase Entity bean Entity integrity ensuring that each tuple is unique is typically provided in relational database systems by primary keys Entity beans simply refer to object-oriented representations of data
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Client Interaction There are a few other important properties of entity beans One is that they differ from session beans in how they interact with their clients Recall that session beans may be stateful or stateless and that this choice directly impacts whether a session bean is shared by multiple clients With entity beans, things are different: They can be shared by multiple clients Think of a specific instance of a bean as a row in a database table Just as multiple queries might access the same row concurrently, multiple clients may access the same entity bean concurrently Entity Bean Relationships Another important entity bean property is the notion that entity beans can relate to each other Again, the relational data model analogy is relevant here Just as tables in a relational data model have relationships to other tables, so do entity beans What's more, the relationship that is, the cardinality can be just as flexible as that found in relational data models, for example, one-tomany Consider our benefits management example We can develop a Member entity bean that can be associated with one or more MemberBenefit entity beans In fact, entity bean relationships can be any of the following:
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One-to-one: Each instance of bean type X is related to, at most, one instance of bean type Y Consider members and employees: Member records are always associated with (at most) one employee One-to-many: Each instance of bean type X may be related to one or more instances of bean type Y Consider the member and member benefits example described above Many-to-many: Many instances of bean type X may be related to a single instance of bean type Y, and many instances of bean type Y may be related to a single instance of bean type X Consider the real relationship between members and benefits: Many members can choose the same benefit and, conversely, many benefits can be chosen by a single member In the data modeling world, many-to-many relationships are often normalized using cross-reference tables By designing a MemberBenefit entity bean, we have effectively achieved this
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At the database level, integrity between the entities is enforced through foreign keys For example, in the one-to-many relationship between members and their benefit elections, the MEMBER_BENEFITS table may have a column for MEMBER_ID that exists as a foreign key to the ID column of the MEMBER table At the bean level, integrity enforcement depends on how bean persistence is managed Methods of Bean Persistence When you define an entity bean, you specify that its persistence will be container managed or bean managed Simply put, container-managed persistence (CMP) means that your J2EE vendor figures out how to persist your entity bean whereas bean-managed persistence (BMP) means that you must explicitly code this mapping The obvious reason that BMP exists is that a J2EE implementation can't know everything it can't know where an existing database is or what the corresponding table names might be As it turns out, many people use the following rule of thumb: If you already have a database with tables representing deliberate analogues of the entity EJBs you create, use BMP for those beans If you just need to persist something that isn't part of a preexisting data model, use CMP I should warn you that this is a very simplistic way of thinking: The debate is actually more complicated for those serious about optimizing bean persistence Nevertheless, for light persistence needs or for the casual application designer, it can be a reasonable approach Container-Managed Persistence The main advantages to CMP are simplicity and portability You don't have to write any SQL that describes how your objects are mapped onto relational tables All you have to do is provide a few methods that meet the requirements of your bean contract and specify some key information in the deployment descriptor J2EE does the rest Thus, your beans consist of much less Java code Since you're defaulting persistence to the container, you don't have to worry about dragging your database around to every deployment locale For example, if you develop a J2EE application composed of only session beans and CMP entity beans, you don't need to buy a database wherever you wish to deploy it The responsibility for ensuring persistence lies with the J2EE vendor CMP's method and mechanism for persistence vary between vendors The EJB spec says nothing about how persistence should be achieved, it just conceptually recognizes the need for it That means that CMP could be implemented by writing serializable Java objects to the filesystem or by tight integration with a high-performance database If you choose CMP and you have a complex object/data model or if you store and update a lot of data, it's well worth your while to understand the details of your vendor's CMP implementation
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The EJB 20 spec has made some important changes to CMP One has to do with the introduction of the so-called "local model" of CMP, which allows entity beans to interact more easily and optimally than in the model defined by EJB 11 Since entity beans have relationships with other entity beans, there's a considerable likelihood that they'll be chatting it up quite a bit Just as navigating a relational data model can result in many queries to the database, navigating through entity EJB objects can result in substantial cross-object communication EJBs are actually distributed objects, which means that substantial marshalling and network communication may be necessary EJB 20 addresses this by offering a local model for use by entity beans that tend to act as stepping-stones to other data The best way to understand this is with an example Suppose that, in our benefits processing system, application clients need to query member information Part of this information might be statistics (name of health plan, name of dental plan, etc); another part might be employee information (name, date hired, etc) From the end-user point of view, however, this information is combined into one visual record We can imagine this relationship as that shown in Figure 8-3 Figure 8-3 Relationship between client data and server side entitybeans
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It might seem natural to implement such an application with at least two entity beans one for members and one for employees and maintain a one-to-one relationship between the two A client request for an employee record contacts the Member entity bean, which contacts an Employee entity bean and returns the resulting compounded information to the client This relationship is shown in Figure 8-4 Figure 8-4 Requesting member and employee information from EJBs
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However, since application clients never directly access employee objects (they do so only indirectly through the Member entity bean), having the Employee entity bean maintain a
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remote interface is a waste In other words, we know that this bean will only be contacted by other server-side entity beans, yet the same marshalling cost continues to be applied The EJB 20 solution for this dilemma is to develop the Employee bean with a local interface, making it a local object Incorporating local objects into your application design offers two basic advantages:
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More efficient argument marshalling: Communication with local objects enables parameters to be passed by reference, not by value as they would be over the network in a normal call to an entity bean More flexibility in terms of security enforcement: Under EJB 20, calls to local objects can be classified as "unchecked"; thus, the deployer can avoid unnecessarily secure communication between objects
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A second important CMP feature found in EJB 20 is container-managed relationships In this model, a developer can specify relationships between entity beans and be sure that the relationships are enforced properly This is a referential integrity issue In our member/benefits example, suppose a request is made to delete a member from the system The relational database answer to this request is to prevent the deletion until all child records (ie, benefit elections) are deleted or to support a cascading-delete model, wherein deleting the student record implies deleting all transcript entries as well In the EJB 20 spec, CMP now supports better enforcement of referential integrity and offers powerful automation features like cascading deletes in entity bean relationships Finally, CMP includes dependent objects, which you can think of as extensions to entity beans Dependent objects allow complex CMP fields to be represented in a separate class For example, each benefit entry might include the attributes benefitID, benefitName, and benefitRestriction Each benefitRestriction may actually be complex (ie, composed of several subfields, such as max_age, min_age, min_seniority, etc) and could thus be represented as a single dependent object The key to dependent objects is that they're not the same as local EJB objects that expose only local interfaces They're simply a way of breaking up a more complex object into distinct parts Local objects are a way of simplifying and optimizing communication with truly distinct objects but ones that the client never interacts with directly Bean-Managed Persistence Although it's more complex to implement, BMP generally provides greater flexibility and potentially better performance than CMP Depending on your application architecture, it may even be a necessity As mentioned above, if you need to connect your entity beans to a database not supported by your J2EE vendor's CMP implementation, or if you need to do things like persist an application entity across multiple tables in the underlying database, you really have no choice but to use BMP for those beans In doing so, you code via JDBC and SQL just how the object is mapped onto a table It's true, BMP does result in more time-consuming and complex development responsibilities Engineers must code methods that describe how bean instances are loaded, stored, and located, which means that they must be familiar with the underlying database and how to achieve their goals via JDBC and SQL Not only that, but continued integration with the underlying data model must be maintained When the data model changes, BMP entity beans must be reevaluated to ensure that the code they contain is consistent with the data model modifications The code maintenance demands I'm hinting at here can probably be most appreciated by those who have struggled with keeping code bases in sync with continuously evolving data models Nevertheless, many people are quick to point out the flexibility of BMP Complex data relationships can be more easily resolved because the developer has explicit control Thus, if it's necessary to store an object across multiple tables or, conversely, to query an object by joining
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multiple tables, this can easily be done with BMP Also, consider the flexibility of the data source With BMP, we can use any mechanism for persistence storage, even legacy or nonstandard storage systems In contrast, CMP limits us to the options for persistence provided by the J2EE vendor In terms of performance, BMP may be more desirable I emphasize "may" because it really depends on the developer and the application Since the developer is writing the database integration code, she is obviously in the driver's seat in terms of how it's optimized The effectiveness and efficiency of persistent storage thus depends on how well data source integration in particular, JDBC and SQL query optimization is understood It also depends on whether or not performance-enhancing mechanisms like caches exist in the development environment CMP already handles much of this optimization using its own buffering and caching scheme Theoretically, depending on the application, a developer could write a more optimal caching mechanism and thus exceed CMP's benefits, but this may translate into a lot of extra programmer hours that, despite its optimality, lead to just a small performance improvement We'll revisit this discussion at the end of the chapter For now, however, let's get back on track with our description of the EJB types Our next focus is message-driven beans, a new type provided by the EJB 20 specification
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