7: Enterprise JavaBeans and the EJB Container Model in Java

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7: Enterprise JavaBeans and the EJB Container Model
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Object Equality The EJB framework doesn t specify object equality that is, the use of the == operator. Instead, the isIdentical() method should be used to support this functionality. The equals() Method The java.lang.Object equals() method relies heavily on memory addresses. For this reason, the enterprise bean provider should either override this method or simply not use it. It is recommended that the isIdentical() method be used when possible. The hashCode() Method The EJB framework doesn t specify the behavior of the Object.hashCode() method on object references pointing to entity objects. Instead, the isIdentical() method should be used to determine whether two entity object references refer to the same entity object. If the enterprise bean provider wants to use the hashCode() method, the following condition must be enforced:
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if custHome1.equals(custHome2) { if (custHome1.hashCode() == custHome2.hashCode()) System.out.println(this + " is implemented correctly"); else System.out.println(this + " is NOT implemented correctly"); }
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The EntityBean Class and Life Cycle Event Methods
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Much like the primary key class, the bean class can be extended from any Java class. If the enterprise bean provider extends an EntityBean class from another Java class, the methods of the base class will be available to the client by defining the base class methods in the stub for the EntityBean.
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The Public Constructor To control the initial state of all objects, the enterprise bean provider creates a public constructor. This constructor enables the container to create stable instances of the EntityBean class. It is specified that this constructor should take no arguments. The container calls the public constructor to create a bean instance. The ejbCreate() and ejbPostCreate() methods are invoked to initialize that bean instance. Accessor Methods When using CMP, the enterprise bean provider doesn t make direct read and write calls to persistent storage devices. Instead, relational data is accessed via get and set accessor methods. These accessor methods, as well as the persistent fields and relationships, are declared in the abstract persistence schema of the XML deployment descriptor file.
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Distinguish Between Stateful and Stateless Session Beans
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The cmr-field-name element in the abstract persistence schema corresponds to the name used for the get and set accessor methods used for the relationship. The cmr-field-type element is used only for collection-valued cmr-fields.
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The ejbCreate( ) Method When you re using the create() and remove() methods, it is important to note the difference between session beans and entity beans. When these methods are used with session beans, bean objects are being created and destroyed. When these methods are used with entity beans, records in a database are being created and destroyed. An entity bean can have zero or more ejbCreate() methods. However, the signature of each method must map to the entity bean home interface create() methods. When a client invokes a create() method to create an entity object, the container invokes the appropriate ejbCreate() method. After the ejbCreate() method is completed for a CMP entity bean, the EJB container performs a database insert. The ejbPostCreate( ) Method For each ejbCreate() method declared, a matching ejbPostCreate() method with a void return type should be declared. Immediately after the EJB container invokes the ejbCreate() method on an EntityBean instance, it will call the corresponding ejbPostCreate() method on that same instance. The ejbPostCreate() method can be used to refine the instance created by the ejbCreate() method before this instance becomes available to the client. The ejbCreate() method can be used to initialize persistent data, whereas the ejbPostCreate() method might do initialization involving the entity s context. Context information isn t available while the ejbCreate() method is being invoked, but it is available when the ejbPostCreate() is being invoked. After the ejbPostCreate() method is invoked, the instance can discover the primary key by calling the getPrimaryKey() method on its entity context object. The ejbRemove( ) Method When a container invokes the ejbRemove() method on a bean instance or a client calls the corresponding remove() method in its remote home or remote interface, it not only removes the entity bean instance, but it also destroys physical data that is related to the bean instance. Another way to destroy an entity object and its corresponding physical data is by use of the deployment descriptor s cascade-delete deployment descriptor element, contained in the ejb-relationship-role element.
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