Consider a simple entity Node with two properties:

• name (required with a @NotNull and the db column is not allowing nulls)
• label (optional and the db column is allowing nulls)

Then consider this test:

@Test
public void test1()
{
	accept(em ->
	{
		Node n = new Node();
		em.persist(n);
		
		n.setName("node-1");
		n.setLabel("label-1");
	});
}

test1 will fail with:

Caused by: java.sql.SQLIntegrityConstraintViolationException: Column 'NAME'  cannot accept a NULL value.

The incoherence is in the fact that no consistent behavior is met.
A consistent behavior is either one:

• a javax.validation.ConstraintViolationException (for @NotNull) should be thrown (on persist or flush/commit)
• or test1 should pass

Supposing that the expected behavior is a validation exception being thrown, the validator is executed on the entity on flush/commit time, but at that time the entity has the "name" set.
Then, this leads to an out-of-sync between the entity being validated and the generated statement to be executed, making validation return a false positive.

To show it, consider a second simple test:

@Test
public void test2()
{
	accept(em ->
	{
		Node n = new Node();
		em.persist(n);
	});
}

Correctly, this is failing with:

Caused by: javax.validation.ConstraintViolationException: Validation failed for classes [hibernate.model.Node] during persist time for groups [javax.validation.groups.Default, ]
List of constraint violations:[
	ConstraintViolationImpl{interpolatedMessage='may not be null', propertyPath=name, rootBeanClass=class hibernate.model.Node, messageTemplate='{javax.validation.constraints.NotNull.message}'}
]

On the other hand, supposing that the expected behavior is that test1 should pass, the incoherence is due to statement generation time.

To show it, consider a second simple test:

@Test
public void test3()
{
	accept(em ->
	{
		Node n = new Node();
		n.setName("node-3");

		em.persist(n);
		
		n.setLabel("label-3");
	});
	
	Node n = apply(em -> em.createQuery("select x from Node x", Node.class).getSingleResult());
	
	Assert.assertEquals("label-3", n.getLabel());
}

Even if the test passes, two statements are generated (and executed).

Hibernate: insert into Node (label, name, id) values (?, ?, ?)
Hibernate: update Node set label=?, name=? where id=?

I suppose the first statement is generated on persist and the second on flush/commit;
but, in this case, I'm expecting a single insert statement generated on flush/commit time and immediately after the entity has been validated.

In conclusion, I see to possible solutions:

• run the validator inside persist()
• postpone statements generation to flush/commit time

I will upload a test case as soon as I can.
Thank you.