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flush() and FlushModeType
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When you call persist(), merge(), or remove(), these changes are not synchronized with the database until the entity manager decides to flush. You can force synchronization at any time by calling flush(). By default, flushing automatically happens before a correlated query is executed (inefficient implementations may even flush before any query) and at transaction commit time. The exception to this default rule is find(). A flush does not need to happen when find() or getReference() is called, because finding by a primary key is not something that would be affected by any updates.
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You can control and change this default behavior by using the javax.persistence.Flush ModeType enumeration:
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public enum FlushModeType { AUTO, COMMIT }
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AUTO is the default behavior described in the preceding code snippet. COMMIT means that changes are flushed only when the transaction commits, not before any query. You can set the FlushModeType by calling the setFlushMode() method on the EntityManager.
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Why would you ever want to change the FlushModeType The default flush behavior makes a lot of sense. If you are doing a query on your database, you want to make sure that any updates you ve made within your transaction are flushed so that your query will pick up these changes. If the entity manager didn t flush, then these changes might not be reflected in the query. Obviously, you want to flush changes when a transaction commits.
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FlushModeType.COMMIT makes sense for performance reasons. The best way to tune a
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database application is to remove unnecessary calls to the database. Some vendor implementations will do all required updates with a batch JDBC call. Using COMMIT allows the entity manager to execute all updates in one huge batch. Also, an UPDATE usually ends up in the row being write-locked. Using COMMIT limits the amount of time the transaction holds on to this database lock by holding it only for the duration of the JTA commit.
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Locking
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The EntityManager API supports both read and write locks. Because locking behavior is closely related to the concept of transactions, using the lock() method is discussed in detail in 17.
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unwrap() and getDelegate()
The unwrap() method allows you to obtain a reference to the underlying persistence provider object that implements the EntityManager interface. Most vendors will have API extensions to the EntityManager interface that can be executed by obtaining and typecasting this delegate object to a provider s proprietary interface. In theory, you should be able to write vendor-independent code, but in practice, most vendors provide a lot of extensions to Java Persistence that you may want to take advantage of in your applications. The getDelegate() method was provided in JPA1; for now, it s recommended that users call unwrap().
Mapping Persistent Objects
In this chapter, we take a thorough look at the process of developing entity beans specifically, mapping them to a relational database. A good rule of thumb is that entity beans model business concepts that can be expressed as nouns. Although this is a guideline rather than a requirement, it helps determine when a business concept is a candidate for implementation as an entity bean. In grammar school, you learned that nouns are words that describe a person, place, or thing. The concepts of person and place are fairly obvious: a person entity might represent a customer or passenger, and a place entity might represent a city or port of call. Similarly, entity beans often represent things : real-world objects, such as ships and credit cards, and abstractions, such as reservations. Entity beans describe both the state and behavior of real-world objects and allow developers to encapsulate the data and business rules associated with specific concepts; an Employee entity encapsulates the data and business rules associated with an employee, for example. This makes it possible for data associated with a concept to be manipulated consistently and safely. Entities represent data in the database, so changes to an entity bean result in changes to the database. That s ultimately the purpose of an entity bean: to provide programmers with a simpler mechanism for accessing and changing data. It is much easier to change a customer s name by calling Employee.setName() than by executing an SQL command against the database. In addition, using entity beans provides opportunities for software reuse. Once an entity bean has been defined, its definition can be used throughout your application in a consistent manner. The concept of an employee, for example, is used in many areas of business, including booking, task assignment, and accounts payable. An Employee entity is also a unified model to information and thus ensures that access to its data is consistent and simple. Representing data as entity beans can make development easier and more cost-effective. When a new entity is created and persisted into the entity manager service, a new record must be inserted into the database and a bean instance must be associated with that data. As the entity is used and its state changes, these changes must be synchronized with the data in the database: entries must be inserted, updated, and removed. The
process of coordinating the data represented by a bean instance with the database is called persistence. The Java Persistence specification gave a complete overhaul to entity beans. CMP 2.1 had a huge weakness in that applications written to that specification were completely nonportable between vendors because there was no object-to-relational (O/R) mapping. O/R mapping was completely left to the vendor s discretion. These next chapters focus solely on Java Persistence s object mappings to a relational database. This chapter focuses on basic entity bean mappings to a relational database. 11 will discuss how entities can have complex relationships to one another and how Java Persistence can map these to your database. 13 will cover how we interact with entity beans through the Java Persistence Query Language (JPA QL) and Criteria APIs.
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