579 words

javax.persistence.ManyToMany - JPA annotation

javax.persistence
Annotation ManyToMany

Target:
Fields (including property get methods)
Defines a many-valued association with many-to-many multiplicity.

Every many-to-many association has two sides, the owning side and the non-owning, or inverse, side. The join table is specified on the owning side. If the association is bidirectional, either side may be designated as the owning side. If the relationship is bidirectional, the non-owning side must use the mappedBy element of the ManyToMany annotation to specify the relationship field or property of the owning side.

The join table for the relationship, if not defaulted, is specified on the owning side.

The ManyToMany annotation may be used within an embeddable class contained within an entity class to specify a relationship to a collection of entities. If the relationship is bidirectional and the entity containing the embeddable class is the owner of the relationship, the non-owning side must use the mappedBy element of the ManyToMany annotation to specify the relationship field or property of the embeddable class. The dot (".") notation syntax must be used in the mappedBy element to indicate the relationship attribute within the embedded attribute. The value of each identifier used with the dot notation is the name of the respective embedded field or property.

Example 1:

    // In Customer class:
 
    @ManyToMany
    @JoinTable(name="CUST_PHONES")
    public Set<PhoneNumber> getPhones() { return phones; }
 
    // In PhoneNumber class:
 
    @ManyToMany(mappedBy="phones")
    public Set<Customer> getCustomers() { return customers; }

Example 2:

    // In Customer class:
 
    @ManyToMany(targetEntity=com.acme.PhoneNumber.class)
    public Set getPhones() { return phones; }
 
    // In PhoneNumber class:
 
    @ManyToMany(targetEntity=com.acme.Customer.class, mappedBy="phones")
    public Set getCustomers() { return customers; }

Example 3:

    // In Customer class:
 
    @ManyToMany
    @JoinTable(name="CUST_PHONE",
        joinColumns=
            @JoinColumn(name="CUST_ID", referencedColumnName="ID"),
        inverseJoinColumns=
            @JoinColumn(name="PHONE_ID", referencedColumnName="ID")
        )
    public Set<PhoneNumber> getPhones() { return phones; }
 
    // In PhoneNumberClass:
 
    @ManyToMany(mappedBy="phones")
    public Set<Customer> getCustomers() { return customers; }

Since:
JPA 1.0
See Also:
JoinTable
(Optional) The operations that must be cascaded to the target of the association.
(Optional) The operations that must be cascaded to the target of the association.

When the target collection is a java.util.Map, the cascade element applies to the map value.

Defaults to no operations being cascaded.

Default value:
{}
Since:
JPA 1.0
(Optional) Whether the association should be lazily loaded or must be eagerly fetched.
(Optional) Whether the association should be lazily loaded or must be eagerly fetched. The EAGER strategy is a requirement on the persistence provider runtime that the associated entities must be eagerly fetched. The LAZY strategy is a hint to the persistence provider runtime.
Default value:
javax.persistence.FetchType.LAZY
Since:
JPA 1.0
String mappedBy
The field that owns the relationship.
The field that owns the relationship. Required unless the relationship is unidirectional.
Default value:
""
Since:
JPA 1.0
(Optional) The entity class that is the target of the association.
(Optional) The entity class that is the target of the association. Optional only if the collection-valued relationship property is defined using Java generics. Must be specified otherwise.

Defaults to the parameterized type of the collection when defined using generics.

Default value:
void.class
Since:
JPA 1.0

This documentation page is derived (with some adjustments) from the open source JPA 2 RI (EclipseLink)
and is available under the terms of the Eclipse Public License, v. 1.0 and Eclipse Distribution License, v. 1.0.
Object Relational Mapping (ORM) JPA 2 providers include Hibernate, EclipseLink, TopLink, OpenJPA and DataNucleus.
Object DB is not an ORM JPA implementation but an Object Database for Java with built in JPA 2 support.