A ManyToMany relationship in Java is where the source object has an attribute that stores a collection of target objects and (if) those target objects had the inverse relationship back to the source object it would also be a ManyToManyrelationship. All relationships in Java and JPA are unidirectional, in that if a source object references a target object there is no guarantee that the target object also has a relationship to the source object. This is different than a relational database, in which relationships are defined through foreign keys and querying such that the inverse query always exists.

JPA also defines a OneToMany relationship, which is similar to a ManyToMany relationship except that the inverse relationship (if it were defined) is a ManyToOne relationship. The main difference between a OneToMany and a ManyToManyrelationship in JPA is that a ManyToMany always makes use of a intermediate relational join table to store the relationship, where as a OneToMany can either use a join table, or a foreign key in target object's table referencing the source object table's primary key.

In JPA a ManyToMany relationship is defined through the @ManyToMany annotation or the <many-to-many> element.

All ManyToMany relationships require a JoinTable. The JoinTable is defined using the @JoinTable annotation and <join-table> XML element. The JoinTable defines a foreign key to the source object's primary key (joinColumns), and a foreign key to the target object's primary key (inverseJoinColumns). Normally the primary key of the JoinTable is the combination of both foreign keys.

Example of a ManyToMany relationship database[edit]

EMPLOYEE (table)

EMP_PROJ (table)

PROJECT (table)

Example of a ManyToMany relationship annotation[edit]

@Entity

public class Employee {

  @Id

  @Column(name="ID")

  private long id;

  ...

  @ManyToMany

  @JoinTable(

      name="EMP_PROJ",

      joinColumns=@JoinColumn(name="EMP_ID", referencedColumnName="ID"),

      inverseJoinColumns=@JoinColumn(name="PROJ_ID", referencedColumnName="ID"))

  private List<Project> projects;

  .....

}

Example of a ManyToMany relationship XML[edit]

<entity name="Employee" class="org.acme.Employee" access="FIELD">

	<attributes>

		<id name="id">

			<column name="EMP_ID"/>

		</id>

		<set name="projects" table="EMP_PROJ" lazy="true" cascade="none" sort="natural" optimistic-lock="false">

			<key column="EMP_ID" not-null="true" />

			<many-to-many class="com.flipswap.domain.Project" column="PROJ_ID" />

		</set>

	</attributes>

</entity>

https://en.wikibooks.org/wiki/Java_Persistence/ManyToMany