@Table is set at the class level; it allows you to define the table, catalog, and schema names for your entity bean 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 also contains a schema and a 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, refer to @Column).

@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.

You can add optimistic locking capability to an entity bean 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 as per the EJB3 spec. Hibernate support any kind of type provided that you define and implement the appropriate UserVersionType.

Every non static non transient property (field or method) of an entity bean 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(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 EJB-QL or Criteria queries.

EJB3 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 overriden through the @Enumerated annotation as shown in the note property example.

In core Java APIs, the temporal precision 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.

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";                                (1)
    boolean unique() default false;                   (2)
    boolean nullable() default true;                  (3)
    boolean insertable() default true;                (4)
    boolean updatable() default true;                 (5)
    String columnDefinition() default "";             (6)
    String table() default "";                        (7)
    int length() default 255;                         (8)
    int precision() default 0; // decimal precision   (9)
    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; }
    ...
}
            

A embeddable object inherit the access type of its owning entity (note that you can override that using the Hibernate specific @AccessType annotations (see Hibernate Annotation Extensions).

The Person entity bean has two component properties, homeAddress and bornIn. Note that the homeAddress property has not been annotated. 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 EJB3 defaults. Overriding columns of embedded objects of embedded objects is currently not supported in the EJB3 spec, however, Hibernate Annotations supports it 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 one more feature that is not explicitly supported by the EJB3 specification. You can annotate a embedded object with the @MappedSuperclass annotation to make the superclass properties persistent (see @MappedSuperclass for more informations).

You cannot use association annotations in an embeddable object (ie no @*ToOne nor @*ToMany). This is disallowed by the spec, and not yet supported in Hibernate Annotations.

If you want to have the same embeddable object type twice in the same entity, the column name defaulting will not work: at least one of the columns will have to be explicit. Hibernate goes beyond the EJB3 spec and allows you to enhance the defaulting mechanism through the NamingStrategy. DefaultComponentSafeNamingStrategy is a small improvement over the default EJB3NamingStrategy that allows embedded objects to be defaulted even if used twice in the same entity.

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

This strategy has many drawbacks (esp. with polymorphic queries and associations) explained in the EJB3 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 support 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 TYPE, and (for Hibernate) the default discriminator value is the fully qualified class name. 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 is 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;
    }
    ...
}

@Entity
@AttributeOverride( name="altitude", column = @Column(name="fld_altitude") )
public class Plane extends FlyingObject {
    ...
}

The altitude property will be persisted in an fld_altitude column of table Plane.

You can associate entity beans through a one-to-one relationship using @OneToOne. There are two cases for one-to-one associations: either the associated entities share the same primary keys values or 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).

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 one to one is marked as true by using the @PrimaryKeyJoinColumn annotation.

In the following example, the associated entities are linked through a 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.

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

@Entity()
public class Flight implements Serializable {
    @ManyToOne( cascade = {CascadeType.CREATE, 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.CREATE, CascadeType.MERGE}, targetEntity=CompanyImpl.class )
    @JoinColumn(name="COMP_ID")
    public Company getCompany() {
        return company;
    }
    ...
}

public interface Company {
    ...
            

You probably have noticed the cascade attribute taking an array of CascadeType as a value. The cascade concept in EJB3 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 EJB3 specification for more information on cascading and create/merge semantics.

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 is the default and will only trigger a select when the associated object is accessed for the first time. EJBQL also 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.

The access type is guessed from the position of @Id or @EmbeddedId in the entity hierarchy. Sub-entities, embedded objects and mapped superclass inherit the access type from the root entity.

In Hibernate, you can override the access type to:

An @AccessType annotation has been introduced to support this behavior. You can define the access type on

The access type is overriden for the annotated element, if overriden on a class, all the properties of the given class inherit the access type. For root entities, the access type is considered to be the default one for the whole hierarchy (overridable at class or property level).

If the access type is marked as "property", the getters are scanned for annotations, if the access type is marked as "field", the fields are scanned for annotations. Otherwise the elements marked with @Id or @embeddedId are scanned.

You can override an access type for a property, but the element to annotate will not be influenced: for example an entity having access type field, can annotate a field with @AccessType("property"), the access type will then be property for this attribute, the the annotations still have to be carried on the field.

If a superclass or an embeddable object is not annotated, the root entity access type is used (even if an access type has been define on an intermediate superclass or embeddable object). The russian doll principle does not apply.

@Entity
public class Person implements Serializable {
    @Id  @GeneratedValue //access type field
    Integer id;

    @Embedded
    @AttributeOverrides({
    @AttributeOverride(name = "iso2", column = @Column(name = "bornIso2")),
    @AttributeOverride(name = "name", column = @Column(name = "bornCountryName"))
            })
    Country bornIn;
}

@Embeddable
@AccessType("property") //override access type for all properties in Country
public class Country implements Serializable {
    private String iso2;
    private String name;

    public String getIso2() {
        return iso2;
    }

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

    @Column(name = "countryName")
    public String getName() {
        return name;
    }

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

Sometimes, you want the Database to do some computation for you rather than in the JVM, you might also create some kind of virtual column. You can use a SQL fragment (aka formula) instead of mapping a property into a column. This kind of property is read only (its value is calculated by your formula fragment).

@Formula("obj_length * obj_height * obj_width")
public long getObjectVolume()

The SQL fragment can be as complex as you want avec even include subselects.

@org.hibernate.annotations.Type overrides the default hibernate type used: this is generally not necessary since the type is correctly inferred by Hibernate. Please refer to the Hibernate reference guide for more informations on the Hibernate types.

@org.hibernate.annotations.TypeDef and @org.hibernate.annotations.TypeDefs allows you to declare type definitions. These annotations are placed at the class or package level. Note that these definitions will be global for the session factory (even at the class level) and that type definition has to be defined before any usage.

@TypeDefs(
    {
    @TypeDef(
        name="caster",
        typeClass = CasterStringType.class,
        parameters = {
            @Parameter(name="cast", value="lower")
        }
    )
    }
)
package org.hibernate.test.annotations.entity;

...
public class Forest {
    @Type(type="caster")
    public String getSmallText() {
    ...
}
         

When using composite user type, you will have to express column definitions. The @Columns has been introduced for that purpose.

@Type(type="org.hibernate.test.annotations.entity.MonetaryAmountUserType")
@Columns(columns = {
    @Column(name="r_amount"),
    @Column(name="r_currency")
})
public MonetaryAmount getAmount() {
    return amount;
}


public class MonetaryAmount implements Serializable {
    private BigDecimal amount;
    private Currency currency;
    ...
}

You can define an index on a particular column using the @Index annotation on a one column property, the columnNames attribute will then be ignored

@Column(secondaryTable="Cat1")
@Index(name="story1index")
public String getStoryPart1() {
    return storyPart1;
}

It is possible to set

You can also declare a sort comparator. Use the @Sort annotation. Expressing the comparator type you want between unsorted, natural or custom comparator. If you want to use your own comparator implementation, you'll also have to express the implementation class using the comparator attribute.

    @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
    @JoinColumn(name="CUST_ID")
    @Sort(type = SortType.COMPARATOR, comparator = TicketComparator.class)
    @Where(clause="1=1")
    @OnDelete(action=OnDeleteAction.CASCADE)
    public SortedSet<Ticket> getTickets() {
        return tickets;
    }

Please refer to the previous descriptions of these annotations for more informations.

Beyond EJB3, Hibernate Annotations supports true List and Array. Map your collection the same way as usual and add the @IndexColumn. This annotation allows you to describe the column that will hold the index. You can also declare the index value in DB that represent the first element (aka as base index). The usual value is 0 or 1.

@OneToMany(cascade = CascadeType.ALL)
@IndexColumn(name = "drawer_position", base=1)
public List<Drawer> getDrawers() {
    return drawers;
}

Hibernate Annotations also supports collections of core types (Integer, String, Enums, ...), collections of embeddable objects and even arrays of primitive types. This is known as collection of elements.

A collection of elements as to be annotated as @CollectionOfElements (as a replacement of @OneToMany) To define the collection table, the @JoinTable annotation is used on the association property, joinColumns defines the join columns between the entity primary table and the collection table (inverseJoincolumn is useless and should be left empty). For collection of core types or array of primitive types, you can override the element column definition using a @Column on the association property. You can also override the columns of a collection of embeddable object using @AttributeOverride.

@Entity
public class Boy {
    private Integer id;
    private Set<String> nickNames = new HashSet<String>();
    private int[] favoriteNumbers;
    private Set<Toy> favoriteToys = new HashSet<Toy>();
    private Set<Character> characters = new HashSet<Character>();

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

    @CollectionOfElements
    public Set<String> getNickNames() {
        return nickNames;
    }

    @CollectionOfElements
    @JoinTable(
            table=@Table(name="BoyFavoriteNumbers"),
            joinColumns = @JoinColumn(name="BoyId")
    )
    @Column(name="favoriteNumber", nullable=false)
    @IndexColumn(name="nbr_index")
    public int[] getFavoriteNumbers() {
        return favoriteNumbers;
    }

    @CollectionOfElements
    @AttributeOverride( name="serial", column=@Column(name="serial_nbr") )
    public Set<Toy> getFavoriteToys() {
        return favoriteToys;
    }

    @CollectionOfElements
    public Set<Character> getCharacters() {
        return characters;
    }
    ...
}

public enum Character {
    GENTLE,
    NORMAL,
    AGGRESSIVE,
    ATTENTIVE,
    VIOLENT,
    CRAFTY
}

@Embeddable
public class Toy {
    public String name;
    public String serial;

    public String getName() {
        return name;
    }

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

    public String getSerial() {
        return serial;
    }

    public void setSerial(String serial) {
        this.serial = serial;
    }

    public boolean equals(Object o) {
        if ( this == o ) return true;
        if ( o == null || getClass() != o.getClass() ) return false;

        final Toy toy = (Toy) o;

        if ( !name.equals( toy.name ) ) return false;
        if ( !serial.equals( toy.serial ) ) return false;

        return true;
    }

    public int hashCode() {
        int result;
        result = name.hashCode();
        result = 29 * result + serial.hashCode();
        return result;
    }
}