data matrix barcode generator java Introducing the EntityManager in Java

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9.1 Introducing the EntityManager
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The EntityManager API is probably the most important and interesting part of the Java Persistence API. It manages the lifecycle of entities. In this section you ll learn about the EntityManager interface and its methods. We ll explore the entity lifecycle, and you ll also learn about persistence contexts and their types.
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9.1.1 The EntityManager interface
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In a sense, the EntityManager is the bridge between the OO and relational worlds, as depicted in figure 9.1. When you request that a domain entity be created, the EntityManager translates the entity into a new database record. When you request
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Introducing the EntityManager
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Figure 9.1 The EntityManager acts as a bridge between the OO and relational worlds. It interprets the O/R mapping specified for an entity and saves the entity in the database.
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that an entity be updated, it tracks down the relational data that corresponds to the entity and updates it. Likewise, the EntityManager removes the relational data when you request that an entity be deleted. From the other side of the translation bridge, when you request that an entity be saved in the database, the EntityManager creates the Entity object, populates it with relational data, and returns the Entity back to the OO world. Besides providing these explicit SQL -like CRUD operations, the EntityManager also quietly tries to keep entities synched with the database automatically as long as they are within the EntityManager s reach (this behind-the-scenes synchronization is what we mean when we talk about managed entities in the next section). The EntityManager is easily the most important interface in JPA and is responsible for most of the ORM black magic in the API. Despite all this under-the-hood power, the EntityManager is a small, simple, and intuitive interface, especially compared to the mapping steps we discussed in the previous chapter and the query API (which we explore in chapter 10). In fact, once we go over some basic concepts in the next few sections, the interface might seem almost trivial. You might already agree if you take a quick look at table 9.1. The table lists some of the most commonly used methods defined in the EntityManager interface.
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Table 9.1 The EntityManager is used to perform CRUD operations. Here are the most commonly used methods of the EntityManager interface. Method Signature public void persist(Object entity); Description Saves (persists) an entity into the database, and also makes the entity managed. Merges an entity to the EntityManager s persistence context and returns the merged entity. continued on next page
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public <T> T merge(T entity);
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Manipulating entities with EntityManager
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Table 9.1 The EntityManager is used to perform CRUD operations. Here are the most commonly used methods of the EntityManager interface. (continued) Method Signature public void remove(Object entity); public <T> T find(Class<T> entityClass, Object primaryKey); public void flush(); Description Removes an entity from the database. Finds an entity instance by its primary key.
Synchronizes the state of entities in the EntityManager s persistence context with the database. Changes the flush mode of the EntityManager s persistent context. The flush mode may either be AUTO or COMMIT. The default flush mode is AUTO, meaning that the EntityManager tries to automatically synch the entities with the database. Retrieves the current flush mode. Refreshes (resets) the entity from the database. Creates a dynamic query using a JPQL statement. Creates a query instance based on a named query on the entity instance. Creates a dynamic query using a native SQL statement.
public void setFlushMode(FlushModeType flushMode);
public FlushModeType getFlushMode(); public void refresh(Object entity);
public Query createQuery(String jpqlString); public Query createNamedQuery(String name); public Query createNativeQuery(String sqlString); public Query createNativeQuery(String sqlString, Class result Class); public Query createNativeQuery(String sqlString, String resultSetMapping); public void close();
Closes an application-managed EntityManager. Checks whether an EntityManager is open. Retrieves a transaction object that can be used to manually start or end a transaction. Asks an EntityManager to join an existing JTA transaction.
public boolean isOpen();
public EntityTransaction getTransaction(); public void joinTransaction();
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