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Introduction to Entity Beans 125 table definition. In this case, an entity bean instance of that class would map to a row in that table. Your entity bean class can expose simple methods, such as a method to decrease a bank account balance, to manipulate or access that data. Like a session bean class, EJB also requires that an entity bean class must fill in some standard callback methods. The EJB container will call these methods appropriately to manage the entity bean. The primary key class makes every entity bean different. For example, if you have one million bank account entity beans, each bank account needs to have a unique ID (such as a bank account ID string) that can never be repeated in any other bank account. A primary key is an object that may contain any number of attributes. This could be any data necessary to identify uniquely an entity bean data instance. In some advanced cases, when the entity bean represents a complex relationship, the primary key might be an entire object. EJB gives you the flexibility to define what your unique identifier is by including a primary key class with your entity bean. The one rule is that your primary key class must be serializable and follow the rules for Java object serialization. The rules for object serialization are covered in Appendix A. Features of Entity Beans Let s take a look at the features of entity beans. Entity Beans Survive Failures Entity beans are long lasting. They survive critical failures, such as application servers crashing, or even databases crashing. This is because entity beans are just representations of data in a permanent, fault-tolerant, underlying storage. If a machine crashes, the entity bean can be reconstructed in memory. All we need to do is read the data back in from the permanent database and instantiate an entity bean Java object instance with fields that contain the data read in from the database. This is a huge difference between session and entity beans. Entity beans have a much longer life cycle than a client s session, perhaps years long, depending on how long the data sits in the database. In fact, the database records representing an object could have existed before the company even decided to go with a Java-based solution, because a database structure can be language independent. This makes sense you definitely would want your bank account to last for a few years, regardless of technology changes in your bank.
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