15 jpa

Professional Open Source™

JPA

© JBoss, Inc. 2003, 2004. 07/17/04 1

The @Id annotation

 Using the javax.persistence.Id annotations is the simplest way of
telling the persistence provider where the entity identity is stored.

 The @Id annotation marks afield or property as identity for an entity.

© JBoss, Inc. 2003, 2004. 2

The @IdClass annotation

 The @IdClass annotation enables you to use more than one @Id annotation in a sensible way

 This is the basic problem with using more than one @Id field or property in an entity class: it is not
obvious how to compare two instances in an automated fashion. This is especially true since in
cases where composite keys are necessary

© JBoss, Inc. 2003, 2004. 3

Using @IdClass

© JBoss, Inc. 2003, 2004. 4

The @EmbeddedId annotation

 Using the @EmbeddedId annotation is like moving the IdClass right
into your entity and using the identity fields nested inside it to store
entity data.

 See the example in next page

© JBoss, Inc. 2003, 2004. 5

The @EmbeddedId annotation

© JBoss, Inc. 2003, 2004. 6


GENERATING PRIMARY KEYS

© JBoss, Inc. 2003, 2004. 7

Database sequences as generators

 To use sequence generators, first define a sequence in the database.
The following is a sample sequence for the USER_ID column in an
Oracle database:

 CREATE SEQUENCE USER_SEQUENCE START WITH 1
INCREMENT BY 10;

 @SequenceGenerator(name="USER_SEQUENCE_GENERATOR",
 sequenceName="USER_SEQUENCE",
 initialValue=1, allocationSize=10)

 The @SequenceGenerator annotation creates a sequence generator
named USER_SEQUENCE_GENERATOR referencing the Oracle
sequence we created and matching its setup.

© JBoss, Inc. 2003, 2004. 8

Using Sequence Generator

© JBoss, Inc. 2003, 2004. 9

Sequence tables as generators

 The first step is creating a table to use for generating values as
follows :
 CREATE TABLE SEQUENCE_GENERATOR_TABLE
 (SEQUENCE_NAME VARCHAR2(80) NOT NULL,
 SEQUENCE_VALUE NUMBER(15) NOT NULL,
 PRIMARY KEY (SEQUENCE_NAME));

 The next step is to prepare the table for use by inserting the initial
value manually as follows:

 INSERT INTO
 SEQUENCE_GENERATOR_TABLE (SEQUENCE_NAME,
SEQUENCE_VALUE)
 VALUES ('USER_SEQUENCE', 1);

© JBoss, Inc. 2003, 2004. 10

Sequence tables as generators

 Next step :
 @TableGenerator (name="USER_TABLE_GENERATOR",
 table="SEQUENCE_GENERATOR_TABLE",
 pkColumnName="SEQUENCE_NAME",
 valueColumnName="SEQUENCE_VALUE",
 pkColumnValue="USER_SEQUENCE")

 Next step :

 @Id
 @GeneratedValue(strategy=GenerationType.TABLE,
 generator="USER_TABLE_GENERATOR")
 @Column(name="USER_ID")
 protected Long userId;

© JBoss, Inc. 2003, 2004. 11

Mapping embeddable classes

See next page for sample code

© JBoss, Inc. 2003, 2004. 12

Mapping embeddable classes

© JBoss, Inc. 2003, 2004. 13

Unidirectional one-to-one relationship

© JBoss, Inc. 2003, 2004. 14

Bidirectional one-to-one

 you can access EJBContext through DI. For example, a
SessionContext could be injected into a bean as follows:

See next page for explanation
© JBoss, Inc. 2003, 2004. 15

Bidirectional one-to-one

 The @OneToOne annotation on the user field has two
interesting things going on.
– The first is the mappedBy="billingInfo" specification . This tells the
container that the “owning” side of the relationship exists in the
User class’s billingInfo instance variable.
– The second interesting feature of the @OneToOne annotation on
the user field is that the optional parameter is set to false this time.
This means that a BillingInfo object cannot exist without a related
User object.

© JBoss, Inc. 2003, 2004. 16

One-to-many bidirectional relationship

© JBoss, Inc. 2003, 2004. 17

Many-to-one as owning-side of relationship

 For bidirectional one-to-many relationships,ManyToOne is always the
owning side of the relationship.

 Because of this fact, the mappedBy element does not exist in the
definition of the @ManyToOne annotation:

© JBoss, Inc. 2003, 2004. 18

@ManyToMany

© JBoss, Inc. 2003, 2004. 19

Mapping an entity

© JBoss, Inc. 2003, 2004. 20

Specifying the table

© JBoss, Inc. 2003, 2004. 22

Mapping the columns

If the insertable parameter is set to false, the field or property will not be
included in the INSERT statement generated by the persistence provider
to create a new record corresponding to the entity. Likewise, setting the
updatable parameter to false excludes the field or property from being
updated when the entity is saved. These two parameters are usually
helpful in dealing with read-only data, like primary keys generated by the
database.

© JBoss, Inc. 2003, 2004. 23

Mapping CLOBs and BLOBs

• Whether a field or property designated @Lob is a CLOB or a BLOB is
determined by its type.
• If the data is of type char[] or String, the persistence provider maps
the data to a CLOB column. Otherwise, the column is mapped as a
BLOB.
• An extremely useful annotation to use in conjunction with @Lob is
@Basic. @Basic can be marked on any attribute with direct-to-field
mapping. Just as we have done for the picture field, the
@Basic(fetch=FetchType.LAZY) specification causes the BLOB or
CLOB data to be loaded from the database only when it is first
accessed.
• Postponing of loading of entity data from the database is known as
lazy loading.

© JBoss, Inc. 2003, 2004. 24

Mapping temporal types

 The @Temporal annotation specifies which of these data types we
want to map a java.util.Date or java.util.Calendar persistent data type
to.
 Note this explicit mapping is redundant while using the java.sql.Date,
java.sql.Time or java.sql.Timestamp Java types.
 If we do not specify a parameter for @Temporal annotation or omit it
altogether, the persistence provider will assume the data type
mapping to be TIMESTAMP

© JBoss, Inc. 2003, 2004. 25

Mapping an entity to multiple tables

© JBoss, Inc. 2003, 2004. 26

Generating primary keys

© JBoss, Inc. 2003, 2004. 27

Mapping one-to-one relationships

 Depending on where the foreign key resides, the relationship could
be implemented in two different ways: using the @JoinColumn or the
@PrimaryKeyJoinColumn annotation.

User has a one-to-one
unidirectional relationship
with BillingInfo. The User
and BillingInfo entities are
mapped to the USERS and
BILLING_INFO tables,
respectively, and the USERS
table has a foreign key
reference to the
BILLING_INFO table. Such
associations are mapped
using @JoinColumn.

© JBoss, Inc. 2003, 2004. 28

Using @JoinColumn

© JBoss, Inc. 2003, 2004. 29

Using @JoinColumn

 Like the @Column annotation, the @JoinColumn annotation contains
the updatable,insertable, table, and unique elements. The elements
serve the same purposeas the elements of the @Column annotation.
In our case, updatable is set to false, which means that the
persistence provider would not update the foreign key even if the
billingInfo reference were changed.

 If you have more than one column in the foreign key, you can use the
JoinColumns annotation instead.

© JBoss, Inc. 2003, 2004. 30

Using @PrimaryKeyJoinColumn

User has a one-to-one
unidirectional relationship with
BillingInfo. The User and
BillingInfo entities are mapped to
the USERS and BILLING_INFO
tables, respectively, and the
BILLING_INFO and USERS
tables share the same primary
key; the primary key of the
BILLING_INFO table is also a
foreign key referencing the
primary key of the USERS table.
Such associations are mapped
using
@PrimaryKeyJoinColumn.

© JBoss, Inc. 2003, 2004. 31

Many-to-many

Many-to-many relationships are modeled in the database world
using join tables. A join table essentially pairs foreign keys
pointing to primary keys on either side of the relationship.

© JBoss, Inc. 2003, 2004. 35

Mapping inheritance

 Entities in the hierarchy in below figure can be mapped to
database tables using different types of inheritance
mapping strategies supported by JPA:
– Single table
– Joined tables
– Table per class

© JBoss, Inc. 2003, 2004. 37

Single-table strategy

 In the single-table strategy, all classes in the inheritance hierarchy are
mapped to a single table. This means that the single table will contain
a superset of all data stored in the class hierarchy. Different objects in
the OO hierarchy are identified using a special column called a
discriminator column. In effect, the discriminator column contains a
value unique to the object type in a given row.

© JBoss, Inc. 2003, 2004. 38

Joined-tables strategy

 The joined-tables inheritance strategy uses one-to-one relationships
to model OO inheritance. In effect, the joined-tables strategy involves
creating separate tables for each entity in the OO hierarchy and
relating direct descendants in the hierarchy with one-to-one
relationships.
 The discriminator column in the USERS table is still used, primarily as
a way of easily differentiating data types in the hierarchy.

© JBoss, Inc. 2003, 2004. 40

Table-per-class strategy

 Entity data are stored in their own tables even if they are inherited
from the superclass. This is true even for the USER_ID primary key.

© JBoss, Inc. 2003, 2004. 42

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