The Transient annotation specifies that a field is non-persistent. Use it to exclude fields from management that would otherwise be persistent. Transient is a marker annotation only; it has no properties.

The equivalent XML element is transient. It has a single attribute:

Annotate your simple identity fields with Id. This annotation has no properties. We explore entity identity and identity fields in Section 1.3, “ Identity Fields ”.

The equivalent XML element is id. It has one required attribute:

The previous section showed you how to declare your identity fields with the Id annotation. It is often convenient to allow the persistence implementation to assign a unique value to your identity fields automatically. JPA includes the the GeneratedValue annotation for this purpose. It has the following properties:

The equivalent XML element is generated-value, which includes the following attributes:

If your entity has multiple identity values, you may declare multiple @Id fields, or you may declare a single @EmbeddedId field. The type of a field annotated with EmbeddedId must be an embeddable entity class. The fields of this embeddable class are considered the identity values of the owning entity. We explore entity identity and identity fields in Section 1.3, “ Identity Fields ”.

The EmbeddedId annotation has no properties.

The equivalent XML element is embedded-id. It has one required attribute:

Use the Version annotation to designate a version field. Section 1.4, “ Version Field ” explained the importance of version fields to JPA. This is a marker annotation; it has no properties.

The equivalent XML element is version, which has a single attribute:

Basic signifies a standard value persisted as-is to the datastore. You can use the Basic annotation on persistent fields of the following types: primitives, primitive wrappers, java.lang.String, byte[], Byte[], char[], Character[], java.math.BigDecimal, java.math.BigInteger, java.util.Date, java.util.Calendar, java.sql.Date, java.sql.Timestamp, Enums, and Serializable types.

Basic declares these properties:

The equivalent XML element is basic. It has the following attributes:

Use the Embedded marker annotation on embeddable field types. Embedded fields are mapped as part of the datastore record of the declaring entity. In our sample model, Author and Company each embed their Address, rather than forming a relation to an Address as a separate entity.

The equivalent XML element is embedded, which expects a single attribute:

When an entity A references a single entity B, and other As might also reference the same B, we say there is a many to one relation from A to B. In our sample model, for example, each magazine has a reference to its publisher. Multiple magazines might have the same publisher. We say, then, that the Magazine.publisher field is a many to one relation from magazines to publishers.

JPA indicates many to one relations between entities with the ManyToOne annotation. This annotation has the following properties:

The equivalent XML element is many-to-one. It accepts the following attributes:

@ManyToOne(cascade={CascadeType.PERSIST,CascadeType.REMOVE,
    CascadeType.REFRESH,CascadeType.MERGE})
private Company publisher;

@ManyToOne(cascade=CascadeType.ALL)
private Company publisher;

<many-to-one name="publisher">
    <cascade>
        <cascade-persist/>
        <cascade-merge/>
        <cascade-remove/>
        <cascade-refresh/>
    </cascade>
</many-to-one>

<many-to-one name="publisher">
    <cascade>
        <cascade-all/>
    </cascade>
</many-to-one>

When an entity A references multiple B entities, and no two As reference the same B, we say there is a one to many relation from A to B.

One to many relations are the exact inverse of the many to one relations we detailed in the preceding section. In that section, we said that the Magazine.publisher field is a many to one relation from magazines to publishers. Now, we see that the Company.mags field is the inverse - a one to many relation from publishers to magazines. Each company may publish multiple magazines, but each magazine can have only one publisher.

JPA indicates one to many relations between entities with the OneToMany annotation. This annotation has the following properties:

The equivalent XML element is one-to-many, which includes the following attributes:

You may also nest the cascade element within a one-to-many element.

When an entity A references a single entity B, and no other As can reference the same B, we say there is a one to one relation between A and B. In our sample model, Magazine has a one to one relation to Article through the Magazine.coverArticle field. No two magazines can have the same cover article.

JPA indicates one to one relations between entities with the OneToOne annotation. This annotation has the following properties:

The equivalent XML element is one-to-one which understands the following attributes:

You may also nest the cascade element within a one-to-one element.

When an entity A references multiple B entities, and other As might reference some of the same Bs, we say there is a many to many relation between A and B. In our sample model, for example, each article has a reference to all the authors that contributed to the article. Other articles might have some of the same authors. We say, then, that Article and Author have a many to many relation through the Article.authors field.

JPA indicates many to many relations between entities with the ManyToMany annotation. This annotation has the following properties:

The equivalent XML element is many-to-many. It accepts the following attributes:

You may also nest the cascade element within a many-to-many element.

Datastores such as relational databases do not preserve the order of records. Your persistent List fields might be ordered one way the first time you retrieve an object from the datastore, and a completely different way the next. To ensure consistent ordering of collection fields, you must use the OrderBy annotation. The OrderBy annotation's value is a string defining the order of the collection elements. An empty value means to sort on the identity value(s) of the elements in ascending order. Any other value must be of the form:

<field name>[ ASC|DESC][, ...]

Each <field name> is the name of a persistent field in the collection's element type. You can optionally follow each field by the keyword ASC for ascending order, or DESC for descending order. If the direction is omitted, it defaults to ascending.

The equivalent XML element is order-by which can be listed as a sub-element of the one-to-many or many-to-many elements. The text within this element is parsed as the order by string.

JPA supports persistent Map fields through either a OneToMany or ManyToMany association. The related entities form the map values. JPA derives the map keys by extracting a field from each entity value. The MapKey annotation designates the field that is used as the key. It has the following properties:

The equivalent XML element is map-key which can be listed as a sub-element of the one-to-many or many-to-many elements. The map-key element has the following attributes:

In the absence of any of the annotations above, JPA defines the following default behavior for declared fields:

Note that according to these defaults, all relations between entities must be annotated explicitly. Without an annotation, a relation field will default to serialized storage if the related entity type is serializable, or will default to being non-persistent if not.