barcode 128 generator vb.net Uses of Entity Beans in Java

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Uses of Entity Beans
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The following are common uses of entity beans:
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n Entity beans can be used to enforce the integrity of data that will be persisted
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as well as data that might potentially be persisted.
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n Entity beans can be reused to cache data, therefore saving trips to the
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FIGURE 7-6
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Entity beans representing legacy application based data
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7: Enterprise JavaBeans and the EJB Container Model
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n Entity beans can be used to model domain objects with unique identities that
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might be shared by multiple clients.
n Unlike session beans, entity beans are intended to model records in a data
set, not to maintain conversations with clients.
n Entity beans can be used for wrapping JDBC code, hence giving the
application an object-oriented interface for the data set.
n Entity beans can be wrapped by session beans, giving the developer more
control in determining how clients can control data.
n Entity beans can be used in either bean-managed persistence (BMP) or
container-managed persistence (CMP) mode. CMP mode should be used if at all possible, allowing the enterprise bean provider to concentrate on writing business logic instead of JDBC logic.
Entity Bean Life Cycle States
As they execute the bean methods described later in this objective, entity bean instances can be in one of three states (see Figure 7-7):
n Null n Pooled n Ready
The bean instance doesn t exist. The bean exists but isn t associated with any specific entity object. The bean instance has been assigned an entity object identity.
The EJB container invokes the following entity bean interface methods when life cycle events occur. The enterprise bean provider is responsible for placing business
FIGURE 7-7
Various states of an entity bean
Distinguish Between Stateful and Stateless Session Beans
logic in this container to handle the events of the application. The container invokes the following methods:
n ejbActivate() n ejbLoad()
An entity bean instance is chosen, removed from the available pool, and assigned to a specific EJBObject. The container synchronizes its state by loading data from the underlying data source. An entity bean instance is about to be disassociated with a specific EJBObject and returned to the available pool. An EJBObject that is associated with an entity bean instance is removed by a client-invoked remove operation on the entity bean s home or remote interface. The container needs to synchronize the underlying data source, or persistent state, with the entity bean instance by storing data to the underlying data source.
n ejbPassivate() n ejbRemove()
n ejbStore()
Developing Entity Beans
The preceding sections covered most of the components of an entity bean. The following exercise completes a checklist of tasks, annotated with helpful hints, required to develop an entity bean. Often, a development tool will be provided with the particular application or EJB server in use, such as SilverStream, WebLogic, or WebSphere. When using development tools, many of these tasks will be wrapped in a graphical user interface (GUI) integrated development environment (IDE) for creating interfaces and deployment descriptor files.
EXERCISE 7-3 Review Code for Entity Bean Using Container-Managed Persistence (Prior to EJB 3.0)
Provide the code for an entity bean. Your code should look similar to the code shown in the following sections. To increase your familiarity with this process, we review the code for an entity bean that uses CMP. We use a stepwise approach that covers all the things required to create, package, deploy, and call the bean.
7: Enterprise JavaBeans and the EJB Container Model
1. Create the CMP Entity Bean Class The following CMP entity bean class example assumes that the following table definition exists in the database:
create table tb_product ( productId varchar(64), name varchar(64), productPx numeric(18), description varchar(64) );
The in-line documentation points out the required methods along with the business methods:
package javaee.architect.EntityCMP; import javax.ejb.*; import java.util.*; // CMP Entity Bean abstract public class EntityCMPBean implements EntityBean { EntityContext entityContext; // Methods required by the EJB specification. // They are called by the container only. // This method corresponds to the create() method // found in the home interface. Because this // is CMP, the method will return void and the // EJB Container will make the primary key. public java.lang.String ejbCreate(java.lang.String productId, java.lang.String name, java.lang.Double productPx, java.lang.String description) throws CreateException { log("ejbCreate() [primary key="+productId+"]"); // However, with CMP we must set the Bean's fields // with the parameters passed in, so that the EJB // Container is able to inspect our Bean and // insert the corresponding database entries. setProductId(productId); setName(name); setProductPx(productPx); setDescription(description); return null; } // Called after ejbCreate() method. public void ejbPostCreate(java.lang.String productId, java.lang.String name, java.lang.Double productPx, ava.lang.String description) throws CreateException { log("ejbPostCreate() [primary key="+getProductId()+"]");
Distinguish Between Stateful and Stateless Session Beans
} public void ejbRemove() throws RemoveException { log("ejbRemove() [primary key="+getProductId()+"]"); } // Loads/re-loads the entity bean instance with // the current value in database. We can leave // this basically empty for CMP. The EJB container // will set public fields to the current values. public void ejbLoad() { log("ejbLoad() [primary key="+getProductId()+"]"); } // Updates the database value with the current // value of this entity bean instance. We can leave // this basically empty for CMP. The EJB container // will save public fields to the database. public void ejbStore() { log("ejbStore() [primary key="+getProductId()+"]"); } public void ejbActivate() { log("ejbActivate() [primary key="+getProductId()+"]"); } public void ejbPassivate() { log("ejbPassivate() [primary key="+getProductId()+"]"); } public void unsetEntityContext() { log("unsetEntityContext() [primary key="+getProductId()+"]"); this.entityContext = null; } public void setEntityContext(EntityContext entityContext) { log("setEntityContext() called."); this.entityContext = entityContext; } // No finder methods are required because // they are implemented by Container // Abstract getters and setters public abstract void setProductId(java.lang.String productId); public abstract void setName(java.lang.String name); public abstract void setProductPx(java.lang.Double productPx); public abstract void setDescription(java.lang.String description); public abstract java.lang.String getProductId(); public abstract java.lang.String getName(); public abstract java.lang.Double getProductPx(); public abstract java.lang.String getDescription();
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