Hibernate Annotations

@Table is set at the class level; it allows you to define the table, catalog, and schema names for your entity mapping. If no @Table is defined the default values are used: the unqualified class name of the entity.

@Entity

@Table(name="tbl_sky")

public class Sky implements Serializable {

   ...

}            

The @Table element contains a schema and catalog attributes, if they need to be defined. You can also define unique constraints to the table using the @UniqueConstraint annotation in conjunction with @Table (for a unique constraint bound to a single column, it is recommended to use the @Column.unique approach (refer to @Column for more information).

@Table(name="tbl_sky",

    uniqueConstraints = {@UniqueConstraint(columnNames={"month", "day"})}

)

A unique constraint is applied to the tuple month, day. Note that the columnNames array refers to the logical column names.

The logical column name is defined by the Hibernate NamingStrategy implementation. The default JPA naming strategy uses the physical column name as the logical column name but it could be different if for example you append fld_ to all your columns using a custom NamingStrategy implementation. Note that the logical column name is not necessarily equals to the property name esp when the column name is explicitly set. Unless you override the NamingStrategy, you shouldn't worry about that.

You can add optimistic locking capability to an entity using the @Version annotation:

@Entity

public class Flight implements Serializable {

...

    @Version

    @Column(name="OPTLOCK")

    public Integer getVersion() { ... }

}           

The version property will be mapped to the OPTLOCK column, and the entity manager will use it to detect conflicting updates (preventing lost updates you might otherwise see with the last-commit-wins strategy).

The version column may be a numeric (the recommended solution) or a timestamp. Hibernate supports any kind of type provided that you define and implement the appropriate UserVersionType.

The application must not alter the version number set up by Hibernate in any way. To artificially increase the version number, check in Hibernate Entity Manager's reference documentation LockModeType.OPTIMISTIC_FORCE_INCREMENT or LockModeType.PESSIMISTIC_FORCE_INCREMENT.

Every non static non transient property (field or method depending on the access type) of an entity is considered persistent, unless you annotate it as @Transient. Not having an annotation for your property is equivalent to the appropriate @Basic annotation. The @Basic annotation allows you to declare the fetching strategy for a property:

public transient int counter; //transient property


private String firstname; //persistent property


@Transient

String getLengthInMeter() { ... } //transient property


String getName() {... } // persistent property


@Basic

int getLength() { ... } // persistent property


@Basic(fetch = FetchType.LAZY)

String getDetailedComment() { ... } // persistent property


@Temporal(TemporalType.TIME)

java.util.Date getDepartureTime() { ... } // persistent property           


@Enumerated(EnumType.STRING)

Starred getNote() { ... } //enum persisted as String in database

counter, a transient field, and lengthInMeter, a method annotated as @Transient, and will be ignored by the entity manager. name, length, and firstname properties are mapped persistent and eagerly fetched (the default for simple properties). The detailedComment property value will be lazily fetched from the database once a lazy property of the entity is accessed for the first time. Usually you don't need to lazy simple properties (not to be confused with lazy association fetching).

The recommended alternative is to use the projection capability of JP-QL (Java Persistence Query Language) or Criteria queries.

JPA support property mapping of all basic types supported by Hibernate (all basic Java types , their respective wrappers and serializable classes). Hibernate Annotations support out of the box enum type mapping either into a ordinal column (saving the enum ordinal) or a string based column (saving the enum string representation): the persistence representation, defaulted to ordinal, can be overridden through the @Enumerated annotation as shown in the note property example.

In plain Java APIs, the temporal precision of time is not defined. When dealing with temporal data you might want to describe the expected precision in database. Temporal data can have DATE, TIME, or TIMESTAMP precision (ie the actual date, only the time, or both). Use the @Temporal annotation to fine tune that.

@Lob indicates that the property should be persisted in a Blob or a Clob depending on the property type: java.sql.Clob, Character[], char[] and java.lang.String will be persisted in a Clob. java.sql.Blob, Byte[], byte[] and serializable type will be persisted in a Blob.

@Lob

public String getFullText() {

    return fullText;

}


@Lob 

public byte[] getFullCode() {

    return fullCode;

}

 

If the property type implements java.io.Serializable and is not a basic type, and if the property is not annotated with @Lob, then the Hibernate serializable type is used.

By default the access type of a class hierarchy is defined by the position of the @Id or @EmbeddedId annotations. If these annotations are on a field, then only fields are considered for persistence and the state is accessed via the field. If there annotations are on a getter, then only the getters are considered for persistence and the state is accessed via the getter/setter. That works well in practice and is the recommended approach.

However in some situations, you need to:

The best use case is an embeddable class used by several entities that might not use the same access type. In this case it is better to force the access type at the embeddable class level.

To force the access type on a given class, use the @Access annotation as showed below:

@Entity

public class Order {

   @Id private Long id;

   public Long getId() { return id; }

   public void setId(Long id) { this.id = id; }


   @Embedded private Address address;

   public Address getAddress() { return address; }

   public void setAddress() { this.address = address; }

}


@Entity

public class User {

   private Long id;

   @Id public Long getId() { return id; }

   public void setId(Long id) { this.id = id; }


   private Address address;

   @Embedded public Address getAddress() { return address; }

   public void setAddress() { this.address = address; }

}


@Embeddable

@Access(AcessType.PROPERTY)

public class Address {

   private String street1;

   public String getStreet1() { return street1; }

   public void setStreet1() { this.street1 = street1; }


   private hashCode; //not persistent

}

You can also override the access type of a single property while keeping the other properties standard.

@Entity

public class Order {

   @Id private Long id;

   public Long getId() { return id; }

   public void setId(Long id) { this.id = id; }

   @Transient private String userId;

   @Transient private String orderId;


   @Access(AccessType.PROPERTY)

   public String getOrderNumber() { return userId + ":" + orderId; }

   public void setOrderNumber() { this.userId = ...; this.orderId = ...; }

}

In this example, the default access type is FIELD except for the orderNumber property. Note that the corresponding field, if any must be marked as @Transient or transient.

The column(s) used for a property mapping can be defined using the @Column annotation. Use it to override default values (see the EJB3 specification for more information on the defaults). You can use this annotation at the property level for properties that are:

@Entity

public class Flight implements Serializable {

...

@Column(updatable = false, name = "flight_name", nullable = false, length=50)

public String getName() { ... }

            

The name property is mapped to the flight_name column, which is not nullable, has a length of 50 and is not updatable (making the property immutable).

This annotation can be applied to regular properties as well as @Id or @Version properties.

@Column(
    name="columnName";
    boolean unique() default false;
    boolean nullable() default true;
    boolean insertable() default true;
    boolean updatable() default true;
    String columnDefinition() default "";
    String table() default "";
    int length() default 255;
    int precision() default 0; // decimal precision
    int scale() default 0; // decimal scale

It is possible to declare an embedded component inside an entity and even override its column mapping. Component classes have to be annotated at the class level with the @Embeddable annotation. It is possible to override the column mapping of an embedded object for a particular entity using the @Embedded and @AttributeOverride annotation in the associated property:

@Entity

public class Person implements Serializable {


    // Persistent component using defaults

    Address homeAddress;


    @Embedded

    @AttributeOverrides( {

            @AttributeOverride(name="iso2", column = @Column(name="bornIso2") ),

            @AttributeOverride(name="name", column = @Column(name="bornCountryName") )

    } )

    Country bornIn;

    ...

}          

@Embeddable

public class Address implements Serializable {

    String city;

    Country nationality; //no overriding here

}            

@Embeddable

public class Country implements Serializable {

    private String iso2;

    @Column(name="countryName") private String name;


    public String getIso2() { return iso2; }

    public void setIso2(String iso2) { this.iso2 = iso2; }


    

    public String getName() { return name; }

    public void setName(String name) { this.name = name; }

    ...

}            

An embeddable object inherits the access type of its owning entity (note that you can override that using the @Access annotation).

The Person entity has two component properties, homeAddress and bornIn. homeAddress property has not been annotated, but Hibernate will guess that it is a persistent component by looking for the @Embeddable annotation in the Address class. We also override the mapping of a column name (to bornCountryName) with the @Embedded and @AttributeOverride annotations for each mapped attribute of Country. As you can see, Country is also a nested component of Address, again using auto-detection by Hibernate and JPA defaults. Overriding columns of embedded objects of embedded objects is through dotted expressions.

    @Embedded

    @AttributeOverrides( {

            @AttributeOverride(name="city", column = @Column(name="fld_city") ),

            @AttributeOverride(name="nationality.iso2", column = @Column(name="nat_Iso2") ),

            @AttributeOverride(name="nationality.name", column = @Column(name="nat_CountryName") )

            //nationality columns in homeAddress are overridden

    } )

    Address homeAddress;

Hibernate Annotations supports something that is not explicitly supported by the JPA specification. You can annotate a embedded object with the @MappedSuperclass annotation to make the superclass properties persistent (see @MappedSuperclass for more informations).

You can also use association annotations in an embeddable object (ie @OneToOne, @ManyToOne, @OneToMany or @ManyToMany). To override the association columns you can use @AssociationOverride.

If you want to have the same embeddable object type twice in the same entity, the column name defaulting will not work as several embedded objects would share the same set of columns. In plain JPA, you need to override at least one set of columns. Hibernate, however, allows you to enhance the default naming mechanism through the NamingStrategy interface. You can write a strategy that prevent name clashing in such a situation. DefaultComponentSafeNamingStrategy is an example of this.

If a property is not annotated, the following rules apply:

JPA defines five types of identifier generation strategies:

Hibernate provides more id generators than the basic JPA ones. Check Section 2.4, “Hibernate Annotation Extensions” for more informations.

The following example shows a sequence generator using the SEQ_STORE configuration (see below)

@Id @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="SEQ_STORE")

public Integer getId() { ... }         

The next example uses the identity generator:

@Id @GeneratedValue(strategy=GenerationType.IDENTITY)

public Long getId() { ... }         

The AUTO generator is the preferred type for portable applications (across several DB vendors). The identifier generation configuration can be shared for several @Id mappings with the generator attribute. There are several configurations available through @SequenceGenerator and @TableGenerator. The scope of a generator can be the application or the class. Class-defined generators are not visible outside the class and can override application level generators. Application level generators are defined at XML level (see Chapter 3, Overriding metadata through XML):

<table-generator name="EMP_GEN"

            table="GENERATOR_TABLE"

            pk-column-name="key"

            value-column-name="hi"

            pk-column-value="EMP"

            allocation-size="20"/>


//and the annotation equivalent


@javax.persistence.TableGenerator(

    name="EMP_GEN",

    table="GENERATOR_TABLE",

    pkColumnName = "key",

    valueColumnName = "hi"

    pkColumnValue="EMP",

    allocationSize=20

)


<sequence-generator name="SEQ_GEN" 

    sequence-name="my_sequence"

    allocation-size="20"/>


//and the annotation equivalent


@javax.persistence.SequenceGenerator(

    name="SEQ_GEN",

    sequenceName="my_sequence",

    allocationSize=20

)

         

If JPA XML (like META-INF/orm.xml) is used to define the generators, EMP_GEN and SEQ_GEN are application level generators. EMP_GEN defines a table based id generator using the hilo algorithm with a max_lo of 20. The hi value is kept in a table "GENERATOR_TABLE". The information is kept in a row where pkColumnName "key" is equals to pkColumnValue "EMP" and column valueColumnName "hi" contains the the next high value used.

SEQ_GEN defines a sequence generator using a sequence named my_sequence. The allocation size used for this sequence based hilo algorithm is 20. Note that this version of Hibernate Annotations does not handle initialValue in the sequence generator. The default allocation size is 50, so if you want to use a sequence and pickup the value each time, you must set the allocation size to 1.

The next example shows the definition of a sequence generator in a class scope:

@Entity

@javax.persistence.SequenceGenerator(

    name="SEQ_STORE",

    sequenceName="my_sequence"

)

public class Store implements Serializable {

    private Long id;


    @Id @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="SEQ_STORE")

    public Long getId() { return id; }

}         

This class will use a sequence named my_sequence and the SEQ_STORE generator is not visible in other classes. Note that you can check the Hibernate Annotations tests in the org.hibernate.test.annotations.id package for more examples.

Finally, you can ask Hibernate to copy the identifier from another associated entity. In the Hibernate jargon, it is known as a foreign generator but the JPA mapping reads better and is encouraged.

@Entity

class MedicalHistory implements Serializable {

  @Id @OneToOne

  @JoinColumn(name = "person_id")

  Person patient;

}


@Entity

public class Person implements Serializable {

  @Id @GeneratedValue Integer id;

}

Or alternatively

@Entity

class MedicalHistory implements Serializable {

  @Id Integer id;


  @MapsId @OneToOne

  @JoinColumn(name = "patient_id")

  Person patient;

}


@Entity

class Person {

  @Id @GeneratedValue Integer id;

}

If you are interested in more examples of "derived identities", the JPA 2 specification has a great set of them in chapter 2.4.1.3.

But an identifier does not have to be a single property, it can be composed of several properties.

You can define a composite primary key through several syntaxes:

As you can see the last case is far from obvious. It has been inherited from the dark ages of EJB 2 for backward compatibilities and we recommend you not to use it (for simplicity sake).

Let's explore all three cases using examples.

@Entity

class User {

  @EmbeddedId

  @AttributeOverride(name="firstName", column=@Column(name="fld_firstname")

  UserId id;


  Integer age;

}


@Embeddable

class UserId implements Serializable {

  String firstName;

  String lastName;

}

@Entity

class Customer {

  @EmbeddedId CustomerId id;

  boolean preferredCustomer;


  @MapsId("userId")

  @JoinColumns({

    @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

    @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

  })

  @OneToOne User user;

}


@Embeddable

class CustomerId implements Serializable {

  UserId userId;

  String customerNumber;

}


@Entity 

class User {

  @EmbeddedId UserId id;

  Integer age;

}


@Embeddable

class UserId implements Serializable {

  String firstName;

  String lastName;

}

@Entity

class Customer {

  @EmbeddedId CustomerId id;

  boolean preferredCustomer;

}


@Embeddable

class CustomerId implements Serializable {

  @OneToOne

  @JoinColumns({

    @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

    @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

  }) 

  User user;

  String customerNumber;

}


@Entity 

class User {

  @EmbeddedId UserId id;

  Integer age;

}


@Embeddable

class UserId implements Serializable {

  String firstName;

  String lastName;

}

@Entity

class Customer implements Serializable {

  @Id @OneToOne

  @JoinColumns({

    @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

    @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

  })

  User user;

  

  @Id String customerNumber;


  boolean preferredCustomer;

}


@Entity 

class User {

  @EmbeddedId UserId id;

  Integer age;

}


@Embeddable

class UserId implements Serializable {

  String firstName;

  String lastName;

}

@Entity

class Customer {

  @Id @OneToOne

  @JoinColumns({

    @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

    @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

  }) 

  User user;

  

  @Id String customerNumber;


  boolean preferredCustomer;

}


class CustomerId implements Serializable {

  UserId user;

  String customerNumber;

}


@Entity 

class User {

  @EmbeddedId UserId id;

  Integer age;

}


@Embeddable

class UserId implements Serializable {

  String firstName;

  String lastName;

}

@Entity

class Customer {

  @Id @OneToOne

  @JoinColumns({

    @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),

    @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")

  }) 

  User user;

  

  @Id String customerNumber;


  boolean preferredCustomer;

}


class CustomerId implements Serializable {

  @OneToOne User user;

  String customerNumber;

}


@Entity 

class User {

  @EmbeddedId UserId id;

  Integer age;

}


@Embeddable

class UserId implements Serializable {

  String firstName;

  String lastName;

}

@Entity

public class CustomerInventory implements Serializable {

  @Id

  @TableGenerator(name = "inventory",

    table = "U_SEQUENCES",

    pkColumnName = "S_ID",

    valueColumnName = "S_NEXTNUM",

    pkColumnValue = "inventory",

    allocationSize = 1000)

  @GeneratedValue(strategy = GenerationType.TABLE, generator = "inventory")

  Integer id;



  @Id @ManyToOne(cascade = CascadeType.MERGE)

  Customer customer;

}


@Entity

public class Customer implements Serializable {

   @Id

   private int id;

}

This strategy has many drawbacks (esp. with polymorphic queries and associations) explained in the JPA spec, the Hibernate reference documentation, Hibernate in Action, and many other places. Hibernate work around most of them implementing this strategy using SQL UNION queries. It is commonly used for the top level of an inheritance hierarchy:

@Entity

@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)

public class Flight implements Serializable { ... }            

This strategy supports one-to-many associations provided that they are bidirectional. This strategy does not support the IDENTITY generator strategy: the id has to be shared across several tables. Consequently, when using this strategy, you should not use AUTO nor IDENTITY.

All properties of all super- and subclasses are mapped into the same table, instances are distinguished by a special discriminator column:

@Entity

@Inheritance(strategy=InheritanceType.SINGLE_TABLE)

@DiscriminatorColumn(

    name="planetype",

    discriminatorType=DiscriminatorType.STRING

)

@DiscriminatorValue("Plane")

public class Plane { ... }


@Entity

@DiscriminatorValue("A320")

public class A320 extends Plane { ... }          

Plane is the superclass, it defines the inheritance strategy InheritanceType.SINGLE_TABLE. It also defines the discriminator column through the @DiscriminatorColumn annotation, a discriminator column can also define the discriminator type. Finally, the @DiscriminatorValue annotation defines the value used to differentiate a class in the hierarchy. All of these attributes have sensible default values. The default name of the discriminator column is DTYPE. The default discriminator value is the entity name (as defined in @Entity.name) for DiscriminatorType.STRING. A320 is a subclass; you only have to define discriminator value if you don't want to use the default value. The strategy and the discriminator type are implicit.

@Inheritance and @DiscriminatorColumn should only be defined at the top of the entity hierarchy.

The @PrimaryKeyJoinColumn and @PrimaryKeyJoinColumns annotations define the primary key(s) of the joined subclass table:

@Entity

@Inheritance(strategy=InheritanceType.JOINED)

public class Boat implements Serializable { ... }


@Entity

public class Ferry extends Boat { ... }


@Entity

@PrimaryKeyJoinColumn(name="BOAT_ID")

public class AmericaCupClass  extends Boat { ... }            

All of the above entities use the JOINED strategy, the Ferry table is joined with the Boat table using the same primary key names. The AmericaCupClass table is joined with Boat using the join condition Boat.id = AmericaCupClass.BOAT_ID.

This is sometimes useful to share common properties through a technical or a business superclass without including it as a regular mapped entity (ie no specific table for this entity). For that purpose you can map them as @MappedSuperclass.

@MappedSuperclass

public class BaseEntity {

    @Basic

    @Temporal(TemporalType.TIMESTAMP)

    public Date getLastUpdate() { ... }

    public String getLastUpdater() { ... }

    ...

}


@Entity class Order extends BaseEntity {

    @Id public Integer getId() { ... }

    ...

}

In database, this hierarchy will be represented as an Order table having the id, lastUpdate and lastUpdater columns. The embedded superclass property mappings are copied into their entity subclasses. Remember that the embeddable superclass is not the root of the hierarchy though.

You can override columns defined in entity superclasses at the root entity level using the @AttributeOverride annotation.

@MappedSuperclass

public class FlyingObject implements Serializable {


    public int getAltitude() {

        return altitude;

    }


    @Transient

    public int getMetricAltitude() {

        return metricAltitude;

    }


    @ManyToOne

    public PropulsionType getPropulsion() {

        return metricAltitude;

    }

    ...

}


@Entity

@AttributeOverride( name="altitude", column = @Column(name="fld_altitude") )

@AssociationOverride( 

   name="propulsion", 

   joinColumns = @JoinColumn(name="fld_propulsion_fk") 

)

public class Plane extends FlyingObject {

    ...

}

The altitude property will be persisted in an fld_altitude column of table Plane and the propulsion association will be materialized in a fld_propulsion_fk foreign key column.

You can define @AttributeOverride(s) and @AssociationOverride(s) on @Entity classes, @MappedSuperclass classes and properties pointing to an @Embeddable object.

You can associate entities through a one-to-one relationship using @OneToOne. There are three cases for one-to-one associations: either the associated entities share the same primary keys values, a foreign key is held by one of the entities (note that this FK column in the database should be constrained unique to simulate one-to-one multiplicity), or a association table is used to store the link between the 2 entities (a unique constraint has to be defined on each fk to ensure the one to one multiplicity).

First, we map a real one-to-one association using shared primary keys:

@Entity

public class Body {

    @Id

    public Long getId() { return id; }


    @OneToOne(cascade = CascadeType.ALL)

    @PrimaryKeyJoinColumn

    public Heart getHeart() {

        return heart;

    }

    ...

}            

@Entity

public class Heart {

    @Id

    public Long getId() { ...}

}            

The @PrimaryKeyJoinColumn annotation does say that the primary key of the entity is used as the foreign key value to the associated entity.

In the following example, the associated entities are linked through an explicit foreign key column:

@Entity

public class Customer implements Serializable {

    @OneToOne(cascade = CascadeType.ALL)

    @JoinColumn(name="passport_fk")

    public Passport getPassport() {

        ...

    }


@Entity

public class Passport implements Serializable {

    @OneToOne(mappedBy = "passport")

    public Customer getOwner() {

    ...

}            

A Customer is linked to a Passport, with a foreign key column named passport_fk in the Customer table. The join column is declared with the @JoinColumn annotation which looks like the @Column annotation. It has one more parameters named referencedColumnName. This parameter declares the column in the targeted entity that will be used to the join. Note that when using referencedColumnName to a non primary key column, the associated class has to be Serializable. Also note that the referencedColumnName to a non primary key column has to be mapped to a property having a single column (other cases might not work).

The association may be bidirectional. In a bidirectional relationship, one of the sides (and only one) has to be the owner: the owner is responsible for the association column(s) update. To declare a side as not responsible for the relationship, the attribute mappedBy is used. mappedBy refers to the property name of the association on the owner side. In our case, this is passport. As you can see, you don't have to (must not) declare the join column since it has already been declared on the owners side.

If no @JoinColumn is declared on the owner side, the defaults apply. A join column(s) will be created in the owner table and its name will be the concatenation of the name of the relationship in the owner side, _ (underscore), and the name of the primary key column(s) in the owned side. In this example passport_id because the property name is passport and the column id of Passport is id.

The third possibility (using an association table) is quite exotic.

@Entity

public class Customer implements Serializable {

    @OneToOne(cascade = CascadeType.ALL)

    @JoinTable(name = "CustomerPassports",

        joinColumns = @JoinColumn(name="customer_fk"),

        inverseJoinColumns = @JoinColumn(name="passport_fk")

    )

    public Passport getPassport() {

        ...

    }


@Entity

public class Passport implements Serializable {

    @OneToOne(mappedBy = "passport")

    public Customer getOwner() {

    ...

}          

A Customer is linked to a Passport through a association table named CustomerPassports ; this association table has a foreign key column named passport_fk pointing to the Passport table (materialized by the inverseJoinColumn, and a foreign key column named customer_fk pointing to the Customer table materialized by the joinColumns attribute.

You must declare the join table name and the join columns explicitly in such a mapping.

Many-to-one associations are declared at the property level with the annotation @ManyToOne:

@Entity()

public class Flight implements Serializable {

    @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )

    @JoinColumn(name="COMP_ID")

    public Company getCompany() {

        return company;

    }

    ...

}            

The @JoinColumn attribute is optional, the default value(s) is like in one to one, the concatenation of the name of the relationship in the owner side, _ (underscore), and the name of the primary key column in the owned side. In this example company_id because the property name is company and the column id of Company is id.

@ManyToOne has a parameter named targetEntity which describes the target entity name. You usually don't need this parameter since the default value (the type of the property that stores the association) is good in almost all cases. However this is useful when you want to use interfaces as the return type instead of the regular entity.

@Entity

public class Flight implements Serializable {

    @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE}, targetEntity=CompanyImpl.class )

    @JoinColumn(name="COMP_ID")

    public Company getCompany() {

        return company;

    }

    ...

}


public interface Company {

    ...

}

You can also map a many-to-one association through an association table. This association table described by the @JoinTable annotation will contains a foreign key referencing back the entity table (through @JoinTable.joinColumns) and a a foreign key referencing the target entity table (through @JoinTable.inverseJoinColumns).

@Entity

public class Flight implements Serializable {

    @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )

    @JoinTable(name="Flight_Company",

        joinColumns = @JoinColumn(name="FLIGHT_ID"),

        inverseJoinColumns = @JoinColumn(name="COMP_ID")

    )

    public Company getCompany() {

        return company;

    }

    ...

}       

You can map Collection, List, Map and Set pointing to associated entities as one-to-many or many-to-many associations using the @OneToMany or @ManyToMany annotation respectively. If the collection is of a basic type or of an embeddable type, use @ElementCollection. We will describe that in more detail in the following subsections.

@Entity

public class Troop {

    @OneToMany(mappedBy="troop")

    public Set<Soldier> getSoldiers() {

    ...

}


@Entity

public class Soldier {

    @ManyToOne

    @JoinColumn(name="troop_fk")

    public Troop getTroop() {

    ...

}              

@Entity

public class Troop {

    @OneToMany

    @JoinColumn(name="troop_fk") //we need to duplicate the physical information

    public Set<Soldier> getSoldiers() {

    ...

}


@Entity

public class Soldier {

    @ManyToOne

    @JoinColumn(name="troop_fk", insertable=false, updatable=false)

    public Troop getTroop() {

    ...

}

@Entity

public class Customer implements Serializable {

    @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)

    @JoinColumn(name="CUST_ID")

    public Set<Ticket> getTickets() {

    ...

}


@Entity

public class Ticket implements Serializable {

    ... //no bidir

}              

@Entity

public class Trainer {

    @OneToMany

    @JoinTable(

            name="TrainedMonkeys",

            joinColumns = @JoinColumn( name="trainer_id"),

            inverseJoinColumns = @JoinColumn( name="monkey_id")

    )

    public Set<Monkey> getTrainedMonkeys() {

    ...

}


@Entity

public class Monkey {

    ... //no bidir

}         

@Entity

public class Trainer {

    @OneToMany

    public Set<Tiger> getTrainedTigers() {

    ...

}


@Entity

public class Tiger {

    ... //no bidir

}               

@Entity

public class Employer implements Serializable {

    @ManyToMany(

        targetEntity=org.hibernate.test.metadata.manytomany.Employee.class,

        cascade={CascadeType.PERSIST, CascadeType.MERGE}

    )

    @JoinTable(

        name="EMPLOYER_EMPLOYEE",

        joinColumns=@JoinColumn(name="EMPER_ID"),

        inverseJoinColumns=@JoinColumn(name="EMPEE_ID")

    )

    public Collection getEmployees() {

        return employees;

    }

    ...

}               

@Entity

public class Employee implements Serializable {

    @ManyToMany(

        cascade = {CascadeType.PERSIST, CascadeType.MERGE},

        mappedBy = "employees",

        targetEntity = Employer.class

    )

    public Collection getEmployers() {

        return employers;

    }

}               

@Entity

public class Store {

    @ManyToMany(cascade = CascadeType.PERSIST)

    public Set<City> getImplantedIn() {

        ...

    }

}


@Entity

public class City {

    ... //no bidirectional relationship

}

               

@Entity

public class Store {

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})

    public Set<Customer> getCustomers() {

        ...

    }

}


@Entity

public class Customer {

    @ManyToMany(mappedBy="customers")

    public Set<Store> getStores() {

        ...

    }

}               

@Entity

public class User {

   [...]

   public String getLastname() { ...}


   @ElementCollection

   @CollectionTable(name="Nicknames", joinColumns=@JoinColumn(name="user_id"))

   @Column(name="nickname")

   public Set<String> getNicknames() { ... } 

}

@Entity

public class User {

   [...]

   public String getLastname() { ...}


   @ElementCollection

   @CollectionTable(name="Addresses", joinColumns=@JoinColumn(name="user_id"))

   @AttributeOverrides({

      @AttributeOverride(name="street1", column=@Column(name="fld_street"))

   })

   public Set<Address> getAddresses() { ... } 

}


@Embeddable

public class Address {

   public String getStreet1() {...}

   [...]

}

@Entity

public class User {

   @ElementCollection

   @AttributeOverrides({

      @AttributeOverride(name="key.street1", column=@Column(name="fld_street")),

      @AttributeOverride(name="value.stars", column=@Column(name="fld_note"))

   })

   public Map<Address,Rating> getFavHomes() { ... } 

@Entity

public class Customer {

   @Id @GeneratedValue public Integer getId() { return id; }

   public void setId(Integer id) { this.id = id; }

   private Integer id;


   @OneToMany(mappedBy="customer")

   @OrderBy("number")

   public List<Order> getOrders() { return orders; }

   public void setOrders(List<Order> orders) { this.orders = orders; }

   private List<Order> orders;

}


@Entity

public class Order {

   @Id @GeneratedValue public Integer getId() { return id; }

   public void setId(Integer id) { this.id = id; }

   private Integer id;


   public String getNumber() { return number; }

   public void setNumber(String number) { this.number = number; }

   private String number;


   @ManyToOne

   public Customer getCustomer() { return customer; }

   public void setCustomer(Customer customer) { this.customer = customer; }

   private Customer number;

}


-- Table schema

|-------------| |----------|

| Order       | | Customer |

|-------------| |----------|

| id          | | id       |

| number      | |----------| 

| customer_id |

|-------------|

@Entity

public class Customer {

   @Id @GeneratedValue public Integer getId() { return id; }

   public void setId(Integer id) { this.id = id; }

   private Integer id;


   @OneToMany(mappedBy="customer")

   @OrderColumn(name"orders_index")

   public List<Order> getOrders() { return orders; }

   public void setOrders(List<Order> orders) { this.orders = orders; }

   private List<Order> orders;

}


@Entity

public class Order {

   @Id @GeneratedValue public Integer getId() { return id; }

   public void setId(Integer id) { this.id = id; }

   private Integer id;


   public String getNumber() { return number; }

   public void setNumber(String number) { this.number = number; }

   private String number;


   @ManyToOne

   public Customer getCustomer() { return customer; }

   public void setCustomer(Customer customer) { this.customer = customer; }

   private Customer number;

}


-- Table schema

|--------------| |----------|

| Order        | | Customer |

|--------------| |----------|

| id           | | id       |

| number       | |----------| 

| customer_id  |

| orders_index |

|--------------|

@Entity

public class Customer {

   @Id @GeneratedValue public Integer getId() { return id; }

   public void setId(Integer id) { this.id = id; }

   private Integer id;


   @OneToMany(mappedBy="customer")

   @MapKey(name"number")

   public Map<String,Order> getOrders() { return orders; }

   public void setOrders(Map<String,Order> order) { this.orders = orders; }

   private Map<String,Order> orders;

}


@Entity

public class Order {

   @Id @GeneratedValue public Integer getId() { return id; }

   public void setId(Integer id) { this.id = id; }

   private Integer id;


   public String getNumber() { return number; }

   public void setNumber(String number) { this.number = number; }

   private String number;


   @ManyToOne

   public Customer getCustomer() { return customer; }

   public void setCustomer(Customer customer) { this.customer = customer; }

   private Customer number;

}


-- Table schema

|-------------| |----------|

| Order       | | Customer |

|-------------| |----------|

| id          | | id       |

| number      | |----------| 

| customer_id |

|-------------|

@Entity

public class Customer {

   @Id @GeneratedValue public Integer getId() { return id; }

   public void setId(Integer id) { this.id = id; }

   private Integer id;


   @OneToMany @JoinTable(name="Cust_Order")

   @MapKeyColumn(name"orders_number")

   public Map<String,Order> getOrders() { return orders; }

   public void setOrders(Map<String,Order> orders) { this.orders = orders; }

   private Map<String,Order> orders;

}


@Entity

public class Order {

   @Id @GeneratedValue public Integer getId() { return id; }

   public void setId(Integer id) { this.id = id; }

   private Integer id;


   public String getNumber() { return number; }

   public void setNumber(String number) { this.number = number; }

   private String number;


   @ManyToOne

   public Customer getCustomer() { return customer; }

   public void setCustomer(Customer customer) { this.customer = customer; }

   private Customer number;

}


-- Table schema

|-------------| |----------| |---------------|

| Order       | | Customer | | Cust_Order    |

|-------------| |----------| |---------------|

| id          | | id       | | customer_id   |

| number      | |----------| | order_id      |

| customer_id |              | orders_number |

|-------------|              |---------------|

You probably have noticed the cascade attribute taking an array of CascadeType as a value. The cascade concept in JPA is very is similar to the transitive persistence and cascading of operations in Hibernate, but with slightly different semantics and cascading types:

Please refer to the chapter 6.3 of the JPA specification for more information on cascading and create/merge semantics.

You can also enable the orphan removal semantic. If an entity is removed from a @OneToMany collection or an associated entity is dereferenced from a @OneToOne association, this associated entity can be marked for deletion if orphanRemoval is set to true. In a way, it means that the associated entity's lifecycle is bound to the owning entity just like an embeddable object is.

@Entity class Customer {

   @OneToMany(orphanRemoval=true) public Set<Order> getOrders() { return orders; }

   public void setOrders(Set<Order> orders) { this.orders = orders; }

   private Set<Order> orders;


   [...]

}


@Entity class Order { ... }


Customer customer = em.find(Customer.class, 1l);

Order order = em.find(Order.class, 1l);

customer.getOrders().remove(order); //order will be deleted by cascade

You have the ability to either eagerly or lazily fetch associated entities. The fetch parameter can be set to FetchType.LAZY or FetchType.EAGER. EAGER will try to use an outer join select to retrieve the associated object, while LAZY will only trigger a select when the associated object is accessed for the first time. @OneToMany and @ManyToMany associations are defaulted to LAZY and @OneToOne and @ManyToOne are defaulted to EAGER. For more information about static fetching, check Section 2.4.5.1, “Lazy options and fetching modes”.

The recommanded approach is to use LAZY on all static fetching definitions and override this choice dynamically through JP-QL. JP-QL has a fetch keyword that allows you to override laziness when doing a particular query. This is very useful to improve performance and is decided on a use case to use case basis.