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Introduction to Entity Beans 135 Relationship between remove() and ejbRemove(). Note that remove() can be called on either the home object or the EJB object. (Diagram happens to assume bean-managed persistence.) EJB Container/Server Entity Bean Instance Home Object EJB Object Client Code 1: remove() 2: ejbRemove() Database Entity Bean Data 3: Remove Database Data 1: remove() 2: ejbRemove() Figure 6.7 Destroying an entity bean s data representation. Note that ejbRemove() does not mean the in-memory entity bean instance is actually going to be destroyed; ejbRemove() destroys only database data and makes the entity bean inaccessible to the client. The bean instance can be recycled by the container to handle a different database data instance, such as a bank account bean representing different bank accounts. ejbRemove() is a required method of all entity beans, and it takes no parameters. There is only one form of ejbRemove(). With entity beans, ejbRemove() is not called if the client times out because the lifetime of an entity bean is longer than the client s session.
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134 Chapter 6 Relationship between create() and ejbCreate(). (Diagram leaves out a few minor steps and happens to assume bean-managed persistence.) EJB Container/Server 3: Create Database Data Entity Bean Instance Home Object EJB Object 2: ejbCreate() Database Entity Bean Data 5: Create EJB Object 4: Return Primary Key Figure 6.6 Creating an entity bean and EJB object. To destroy an entity bean s data in a database, the client must call remove() on the EJB object or home object. This method causes the container to issue an ejbRemove() call on the bean. Figure 6.7 shows the relationship between remove() and ejbRemove(). Note that remove() can be called on either the home object or the EJB object. Figure 6.7 happens to assume bean-managed persistence. 1: create() Client Code 6: Return EJB Object
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Introduction to Entity Beans 133 method is called, the ejbCreate() method is responsible for creating database data. Similarly, when a bean-managed persistent entity bean s ejbRemove() method is called, the ejbRemove() method is responsible for removing database data. If container-managed persistence is used, the container will modify the database for you, and you can leave these methods empty of data access logic. Let s look at this in more detail. Understanding How Entity Beans Are Created and Destroyed In EJB, remember that clients do not directly invoke beans they invoke an EJB object proxy. The EJB object is generated through the home object. Therefore, for each ejbCreate() method signature you define in your bean, you must define a corresponding create() method in the home interface. The client calls the home object s create() method, which delegates to your bean s ejbCreate() method. For example, let s say you have a bank account entity bean class called AccountBean, with a remote interface Account, home interface AccountHome, and primary key class AccountPK. Given the following ejbCreate() method in AccountBean: public AccountPK ejbCreate(String accountID, String owner) throws… you must have this create() method in your home interface (notice there is no ejb prefix): public Account create(String accountID, String owner) throws … Notice that there are two different return values here. The bean instance returns a primary key (AccountPK), while the home object returns an EJB object (Account). This makes sense the bean returns a primary key to the container (that is, to the home object) so that the container can identify the bean. Once the home object has this primary key, it can generate an EJB object and return that to the client. We show this process more rigorously with the sequence diagram in Figure 6.6.
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132 Chapter 6 EJB offers an alternative to bean-managed persistence: You can have your EJB container perform your persistence for you. This is called container-managed persistence. In this case, you would usually strip your bean of any persistence logic. Then, you inform the container about how you d like to be persisted by using the container s tools. The container then generates the data access code for you. For example, if you re using a relational database, the container may automatically perform SQL INSERT statements to create database data. Similarly, it will automatically perform SQL DELETE statements to remove database data, and it will handle any other necessary persistent operations. Even if you are not working with a relational database, you can have your container persist for you. If your container supports a nonrelational persistent store, such as an object database or a VSAM file, the container will generate the appropriate logic as necessary. In fact, you can wait until deployment time before you set up the O/R mapping, which is great because you can write storage-independent data objects and reuse them in a variety of enterprise environments. Container-managed persistence reduces the size of your beans tremendously because you don t need to write JDBC code the container handles all the persistence for you. This is a huge value-add feature of EJB. Of course, it is still evolving technology. Once we ve written a few entity beans, we ll review the trade-offs of bean-managed versus container-managed persistence (see Chapter 16). Creation and Removal of Entity Beans As we mentioned earlier, entity beans are a view into a database, and you should think of an entity bean instance and the underlying database as one and the same (they are routinely synchronized). Because they are one and the same, the initialization of an entity bean instance should entail initialization of database data. Thus, when an entity bean is initialized in memory during ejbCreate(), it makes sense to create some data in an underlying database that correlates with the in-memory instance. That is exactly what happens with entity beans. When a bean-managed persistent entity bean s ejbCreate()
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Introduction to Entity Beans 131 Container Bean Instance Passivation entails a state save. 1: ejbStore() 2: ejbPassivate() Activation entails a state load. 1: ejbActivate() 2: ejbLoad() Figure 6.5 Passivation of entity beans entails a state save, and activation entails a state load. There Are Two Ways to Persist Entity Beans Since entity beans map to storage, someone needs to write the database access code. A bean-managed persistent entity bean is an entity bean that must be persisted by hand. In other words, you as the component developer must write code to translate your in-memory fields into an underlying data store, such as a relational database or an object database. You handle the persistent operations yourself including saving, loading, and finding data within the entity bean. Therefore, you must write to a persistence API, such as JDBC. For example, with a relational database, your entity bean could perform a SQL INSERT statement via JDBC to stick some data into a relational database. You could also perform an SQL DELETE statement via JDBC to remove data from the underlying store.
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130 Chapter 6 EJB Container/Server EJB Object 1 (John Smith’s Bank Account) Client 1 John Smith Client 2 Mary Jane Client 3 Bob Hall Remote Interface Remote Interface Remote Interface Entity Bean Instances Bean Pool EJB Object 2 (Mary Jane’s Bank Account) EJB Object 3 (Bob Hall’s Bank Account) The EJB container can dynamically assign entity bean instances to represent different data. Figure 6.4 EJB container pooling of entity beans. ejbPassivate() is the callback that your container will invoke when transitioning your bean into a generic instance pool. This process is called passivation, and it indicates that the container is disassociating your bean from a specific EJB object and a specific primary key. Your bean s ejbPassivate() method should release resources, such as sockets, that your bean acquired during ejbActivate(). When an entity bean instance is passivated, it must not only release held resources but also save its state to the underlying storage; that way, the storage is updated to the latest entity bean instance state. To save the instance s fields to the database, the container invokes the entity bean s ejbStore() method prior to passivation. Similarly, when the entity bean instance is activated, it must not only acquire any resources it needs but also load the most recent data from the database. To load data into the bean instance, the container invokes the entity bean s ejbLoad() method after activation. This is shown in Figure 6.5.
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Introduction to Entity Beans 129 Unfortunately this is not a scalable way to build an application server. Creation and destruction of objects is expensive, especially if client requests come frequently. How can we save on this overhead? One thing to remember is that an entity bean class describes the fields and rules for your entity bean, but it does not dictate any specific data. For example, an entity bean class may specify that all bank accounts have the following fields: The name of the bank account owner An account ID An available balance That bean class can then represent any distinct instance of database data, such as a particular bank account record. The class itself, though, is not specific to any particular bank account. To save precious time-instantiating objects, entity bean instances are therefore recyclable objects and may be pooled depending on your container s policy. The container may pool and reuse entity bean instances to represent different instances of the same type of data in an underlying storage. For example, a container could use a bank account entity bean instance to represent different bank account records. When you re done using an entity bean instance, that instance may be assigned to handle a different client s request and may represent different data. The container performs this by dynamically assigning the entity bean instance to different client-specific EJB objects. Not only does this save the container from unnecessarily instantiating bean instances, but this scheme also saves on the total amount of resources held by the system. We show this in Figure 6.4. Instance pooling is an interesting optimization that containers may provide, and it is not at all unique to entity beans. However, complications arise when reassigning entity bean instances to different EJB objects. When your entity bean is assigned to a particular EJB object, it may be holding resources such as socket connections. But when it s in the pool, it may not need that socket. Thus, to allow the bean to release and acquire resources, your entity bean class must implement two callback methods: ejbActivate() is the callback that your container will invoke on your bean instance when transitioning your bean out of a generic instance pool. This process is called activation, and it indicates that the container is associating your bean with a specific EJB object and a specific primary key. Your bean s ejbActivate() method should acquire resources, such as sockets, that your bean needs when assigned to a particular EJB object.
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128 Chapter 6 idea, it is not very appropriate for EJB, for two reasons. First, if we d like an entity bean instance to service many concurrent clients, we d need to make that instance thread-safe. Writing thread-safe code is difficult and error-prone. Remember that the EJB value proposition is rapid application development. Mandating that component vendors produce stable thread-safe code does not encourage this. Second, having multiple threads of execution makes transactions almost impossible to control by the underlying transaction system. For these reasons, EJB dictates that only a single thread can ever be running within a bean instance. With session beans and message-driven beans, as well as entity beans, all bean instances are single-threaded. Mandating that each bean can service only one client at a time could result in performance bottlenecks. Because each instance is single-threaded, clients need to effectively run in lockstep, each waiting their turn to use a bean. This could easily grind performance to a halt in any large enterprise deployment. To boost performance, we could allow containers to instantiate multiple instances of the same entity bean class. This would allow many clients to interact concurrently with separate instances, each representing the same underlying entity data. Indeed, this is exactly what EJB allows containers to do. Thus, client requests do not necessarily need to be processed sequentially, but rather concurrently. Having multiple bean instances represent the same data now raises a new problem: data corruption. If many bean instances are representing the same underlying data through caching (see Chapter 19), we re dealing with multiple in-memory cached replicas. Some of these replicas could become stale, representing data that is not current. To achieve entity bean instance cache consistency, each entity bean instance needs to be routinely synchronized with the underlying storage. The container synchronizes the bean with the underlying storage by calling the bean s ejbLoad() and ejbStore() callbacks, as described in the previous section. The frequency with which beans are synchronized with an underlying storage is dictated by transactions, a topic we cover in Chapter 12. Transactions enable each client request to be isolated from every other request. They enable clients to believe they are dealing with a single in-memory bean instance, when in fact many instances are behind the scenes. Transactions give clients the illusion that they have exclusive access to data when in fact many clients are touching the same data. Entity Bean Instances Can Be Pooled Let s say you ve decided to author your own EJB container/server. Your product is responsible for instantiating entity beans as necessary, with each bean representing data in an underlying storage. As clients connect and disconnect, you could create and destroy beans as necessary to service those clients.
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Introduction to Entity Beans 127 This ejbLoad()-business method-ejbStore() cycle may be repeated many times. EJB Container/Server Entity Bean Instance 3: Business Methods 1: ejbLoad() 4: ejbStore() 2: Read from 5: Write to Database Database Database Entity Bean Data Figure 6.3 Loading and storing an entity bean. Several Entity Bean Instances May Represent the Same Underlying Data Let s consider the scenario in which many threads of execution want to access the same database data simultaneously. In banking, interest might be applied to a bank account, while at the same time a company directly deposits a check into that same account. In e-commerce, many different client browsers may be simultaneously interacting with a catalog of products. To facilitate many clients accessing the same data, we need to design a high- performance access system to our entity beans. One possibility is to allow many clients to share the same entity bean instance; that way, an entity bean could service many client requests simultaneously. While this is an interesting
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126 Chapter 6 Entity Bean Instances Are a View into a Database When you load entity bean data into an in-memory entity bean instance, you read in the data stored in a database so that you can manipulate the data within a Java Virtual Machine. However, you should think of the in-memory object and the database itself as one and the same. This means if you update the in-memory entity bean instance, the database should automatically be updated as well. You should not think of the entity bean as a separate version of the data in the database. The in-memory entity bean is simply a view or lens into the database. Of course, in reality there are multiple physical copies of the same data: the in-memory entity bean instance and the entity bean data itself stored in the database. Therefore, there must be a mechanism to transfer information back and forth between the Java object and the database. This data transfer is accomplished with two special methods that your entity bean class must implement, called ejbLoad() and ejbStore(). ejbLoad() reads the data in from the persistent storage into the entity bean s in-memory fields. ejbStore() saves your bean instance s current fields to the underlying data storage. It is the complement of ejbLoad(). So who decides when to transfer data back and forth between the in-memory bean and the database? That is, who calls ejbLoad() and ejbStore()? The answer is your EJB container. ejbLoad() and ejbStore() are callback methods that the container invokes. They are management methods required by EJB. The container worries about the proper time to call ejbLoad() and ejbStore() this is one of the value-adds of the container. This is shown in Figure 6.3. Your beans should be prepared to accept an ejbLoad() or ejbStore() call at almost any time (but not during a business method). The container automatically figures out when each of your instances needs to be refreshed depending on the current transactional state (see Chapter 12). This means that you never explicitly call your own ejbLoad() or ejbStore() methods. This is one of the advantages of EJB: You don t have to worry about synchronizing your objects with the underlying database. The EJB black box handles it for you. That is why you can think of the entity bean and the database as the same; there should never be a time when the two are transactionally out of sync.
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