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copied!<p>Yes, this is possible. This is a special case of the standard bidirectional <code>@ManyToOne</code>/<code>@OneToMany</code> relationship. It is special because the entity on each end of the relationship is the same. The general case is detailed in Section 2.10.2 of the <a href="http://jcp.org/aboutJava/communityprocess/final/jsr317/index.html" rel="noreferrer">JPA 2.0 spec</a>.</p> <p>Here's a worked example. First, the entity class <code>A</code>:</p> <pre><code>@Entity public class A implements Serializable { @Id @GeneratedValue(strategy=GenerationType.AUTO) private Long id; @ManyToOne private A parent; @OneToMany(mappedBy="parent") private Collection<A> children; // Getters, Setters, serialVersionUID, etc... } </code></pre> <p>Here's a rough <code>main()</code> method that persists three such entities:</p> <pre><code>public static void main(String[] args) { EntityManager em = ... // from EntityManagerFactory, injection, etc. em.getTransaction().begin(); A parent = new A(); A son = new A(); A daughter = new A(); son.setParent(parent); daughter.setParent(parent); parent.setChildren(Arrays.asList(son, daughter)); em.persist(parent); em.persist(son); em.persist(daughter); em.getTransaction().commit(); } </code></pre> <p>In this case, all three entity instances must be persisted before transaction commit. If I fail to persist one of the entities in the graph of parent-child relationships, then an exception is thrown on <code>commit()</code>. On Eclipselink, this is a <code>RollbackException</code> detailing the inconsistency.</p> <p>This behavior is configurable through the <code>cascade</code> attribute on <code>A</code>'s <code>@OneToMany</code> and <code>@ManyToOne</code> annotations. For instance, if I set <code>cascade=CascadeType.ALL</code> on both of those annotations, I could safely persist one of the entities and ignore the others. Say I persisted <code>parent</code> in my transaction. The JPA implementation traverses <code>parent</code>'s <code>children</code> property because it is marked with <code>CascadeType.ALL</code>. The JPA implementation finds <code>son</code> and <code>daughter</code> there. It then persists both children on my behalf, even though I didn't explicitly request it.</p> <p>One more note. It is always the programmer's responsibility to update both sides of a bidirectional relationship. In other words, whenever I add a child to some parent, I must update the child's parent property accordingly. Updating only one side of a bidirectional relationship is an error under JPA. Always update both sides of the relationship. This is written unambiguously on page 42 of the JPA 2.0 spec:</p> <blockquote> <p>Note that it is the application that bears responsibility for maintaining the consistency of runtime relationships—for example, for insuring that the “one” and the “many” sides of a bidirectional relationship are consistent with one another when the application updates the relationship at runtime.</p> </blockquote>

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