How to avoid excessive hibernate flushes to DB

Question

I'm using Play Framework, with Hibernate as the JPA provider, and I noticed that if an entity has a Blob member, it will always get flushed to the DB, even if I haven't changed anything in it, and even if I don't read the Blob's value.

Blob is a UserType defined in Play. Here's the source code. The basic idea of the class is to save the actual data in the file system, and only save a pointer (a UUID) in the DB table.

From this SO answer, I gather that something in the Blob code must be making a change between the time it gets loaded from the DB and the time Hibernate inspects it. There's also this answer that suggests it might be something else.

How can I change the Blob class to avoid the DB flush if no changes are made?

---

GreyBeardedGeek's answer proved correct. The current equals implementation returns true only on objects with the same identity, and always returns false on nulls.

Changing this:

public boolean equals(Object o, Object o1) throws HibernateException {
    return o == null ? false : o.equals(o1);
}

to this:

private static boolean equal(Object a, Object b) {
  return a == b || (a != null && a.equals(b));
}

public boolean equals(Object a, Object b) throws HibernateException {
    if(a instanceof Blob && b instanceof Blob) {
        return equal(((Blob)a).UUID, ((Blob)b).UUID) &&
                equal(((Blob)a).type, ((Blob)b).type);
    }
    return equal(a, b);
}

made all the excessive DB updates go away.

Answer 1

You'd have to actually debug this to find out, but I suspect a problem with hashcode() and equals(). The first thing that I'd check is the implementation of hashcode() and equals() in the class that you are persisting with the Blob UserType - make sure that they work consistently, and follow the rules - .e.g. if equals() returns false when it is used to compare the 'old' and the 'new' state of your object, it's going to get persisted.