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  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  annotation or the <many-to-many> element.

All ManyToMany relationships require a JoinTable. The JoinTable is defined using the  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[]

EMPLOYEE (table)

EMP_PROJ (table)

PROJECT (table)

Example of a ManyToMany relationship annotation[]

@Entitypublic 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
     
       projects; ..... }
     

Example of a ManyToMany relationship XML[]

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