javax.persistence

The AssociationOverride annotation may be applied to an entity that extends a mapped superclass to override a many-to-one or one-to-one mapping defined by the mapped superclass.

 Example:
 @MappedSuperclass
 public class Employee {
 @Id protected Integer id;
 @Version protected Integer version;
 @ManyToOne protected Address address;
 @OneToOne protected Locker locker;
 public Integer getId() { ...

The AttributeOverride annotation may be applied to an entity that extends a mapped superclass or to an embedded field or property to override a basic mapping defined by the mapped superclass or embeddable class.

 Example:
 @Embedded
 @AttributeOverrides({
 @AttributeOverride(name="startDate", column=@Column("
 EMP_START")),
 @AttributeOverride(name="endDate", column=@Column("EMP_END"))
 })
 public EmploymentPeriod getEmploymentPeriod() { ...

The strategy and the discriminator column are only specified in the root of an entity class hierarchy or subhierarchy in which a different inheritance strategy is applied

If the DiscriminatorColumn annotation is missing, and a discriminator column is required, the name of the discriminator column defaults to "DTYPE" and the discriminator type to DiscriminatorType.STRING DiscriminatorType.STRING .

 Example:
 @Entity
 @Table(name="CUST")
 @Inheritance(strategy=SINGLE_TABLE)
 @DiscriminatorColumn(name="DISC", discriminatorType=STRING,length=20)
 public class Customer { ...

An EntityManager instance is associated with a persistence context.

When queries are executed within a transaction, if FlushModeType.AUTO is set on the Query object, or if the flush mode setting for the persistence context is AUTO (the default) and a flush mode setting has not been specified for the Query object, the persistence provider is responsible for ensuring that all updates to the state of all entities in the persistence context which could potentially affect the result of the query are visible to the processing of the query.

The semantics of requesting locks of type LockModeType.READ LockModeType.READ and LockModeType.WRITE LockModeType.WRITE are the following.

If transaction T1 calls lock(entity, LockModeType.READ LockModeType.READ ) on a versioned object, the entity manager must ensure that neither of the following phenomena can occur:

• P1 (Dirty read): Transaction T1 modifies a row.

If a persistent field or property other than the primary key is used as a map key then it is expected to have a uniqueness constraint associated with it.

 Example 1:
 @Entity
 public class Department {
 ...
 @OneToMany(mappedBy="department")
 @MapKey(name="empId")
 public Map getEmployees() {...

The syntax of the value ordering element is an orderby_list, as follows:

 orderby_list::= orderby_item [,orderby_item]*
 orderby_item::= property_or_field_name [ASC | DESC]
 

If ASC or DESC is not specified, ASC (ascending order) is assumed.

If the ordering element is not specified, ordering by the primary key of the associated entity is assumed.

The property or field name must correspond to that of a persistent property or field of the associated class.

Vendor specific properties may be included in the set of properties, and are passed to the persistence provider by the container when the entity manager is created.

 Example 1: ValuedCustomer subclass
 @Entity
 @Table(name="VCUST")
 @DiscriminatorValue("VCUST")
 @PrimaryKeyJoinColumns({
 @PrimaryKeyJoinColumn(name="CUST_ID", 
 referencedColumnName="ID"),
 @PrimaryKeyJoinColumn(name="CUST_TYPE",
 referencedColumnName="TYPE")
 })
 public class ValuedCustomer extends Customer { ...

 Example 1: Multiple secondary tables assuming primary key columns are named the same in all tables.
 @Entity
 @Table(name="EMPLOYEE")
 @SecondaryTables({
 @SecondaryTable(name="EMP_DETAIL"),
 @SecondaryTable(name="EMP_HIST")
 })
 public class Employee { ...

 Example:
 @Entity
 @Table(
 name="EMPLOYEE", 
 uniqueConstraints=
 @UniqueConstraint(columnNames={"EMP_ID", "EMP_NAME"})
 )
 public class Employee { ...