Problem:
With bidirectional relationships, if you do not update both sides in your object model, you might have an object tree that is not consistent with the Database. (See [url]http://www.hibernate.org/Documentation/InsideExplanationOfInverseTrue[/url]

Solution:
Update child and parent at the same time as follows:

In the child:
[code]
public void setParent(Parent parent)
{
// Do something only if parent changes.
if (this.parent != parent) {
// Remove child from current parent (if not null)
if (this.parent != null) this.parent.removeChild(this);
// Set new parent
this.parent = parent;
// Add child to new parent (if not null)
if (parent != null) {
parent.addChild(this);
}
}
}
[/code]

In the parent:
[code]
public void addChild(Child child) {
this.children.add(child);
child.setParent(this);
}

public void removeChild(Child child) {
this.children.remove(child);
child.setParent(null);
}
[/code]

Comments:
This works fine if the children collection in the parent has the "lazy" flag set to "false". As long as this flag is set to "true", you get a [code]Failed to lazily initialize a collection[/code] exception. The reason is that you try to add a child to the parent collection during the lazy initialization.
If you update the setParent method as follows, it works fine:
[code]
public void setParent(Parent parent)
{
// Do something only if parent changes.
if (this.parent != parent) {
// Remove child from current parent (if not null)
if (this.parent != null) this.parent.removeChild(this);
// Set new parent
this.parent = parent;
// Add child to new parent (if not null)
if (parent != null) {

try {
if (((PersistentCollection)parent.getChildren()).wasInitialized()) {
parent.addChild(this);
}
} catch (Exception e) {

}
}
}
}
[/code]

Question:
Could it be possible to synchronize the collection "add" methods during the lazy initialization?

It does not work because your session is closed: not the usual case. Capability of adding elements to a collection wo initializing it depends wether it's an indexed collection or not (see Chapter 13. Understanding Collection Performance).
If you manipulate collections (that need init to add) outside session, then initialize them before.

PersistentCollection is internal implementation, prefer using Hibernate.isInitialized()

Thanks for your quick answer.
The code is cleaner and more generic with the Hibernate.isInitialized() method.

I tried with a list instead of a set. As expected the test on the collection initialization is useless, but I have each child twice in the collection (probably my mistake in the mapping files).