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)
| ID | FIRSTNAME | LASTNAME |
| 1 | Bob | Way |
| 2 | Sarah | Smith |
EMP_PROJ (table)
| EMP_ID | PROJ_ID |
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
PROJECT (table)
| ID | NAME |
| 1 | GIS |
| 2 | SIG |
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