@Entity
@Table(name = "USERS")
public class User {
	@Id
	@GeneratedValue
	@Column(name = "USER_ID")
	private long id;

	@OneToMany(targetEntity = Group.class)
	@JoinColumn(name = "USER_ID", foreignKey = @ForeignKey(name = "FK_USER_GROUP"))
	private List<Group> groups;
}

@Entity
@Table(name = "GROUP")
public class Group implements Serializable {
	@Id
	@Column(name = "GROUP_ID")
	private Long id;
}