Resetting a field lazy-loaded with the double-check idiom

Question

Consider the "double-check idiom for lazy initialization of instance fields":

// Item 71 in Effective Java copied from this interview with Bloch.
private volatile FieldType field;
FieldType getField() {
    FieldType result = field;
    if (result == null) { // First check (no locking)
        synchronized(this) {
            result = field;
            if (result == null) // Second check (with locking)
                field = result = computeFieldValue();
        }
    }
     return result;
}

I want to be able to reset the field in a safe way (force it to load again from the database, in my case). I assume that we could do this by having a reset method:

void reset() {
   field = null;
}

Is this the standard way of doing resetting the field? Is it safe? Any pitfalls? I'm asking because Bloch gave the following warning about double-checked lazy-loading: "The idiom is very fast but also complicated and delicate, so don't be tempted to modify it in any way. Just copy and paste -- normally not a good idea, but appropriate here."

Thanks in advance, Playa from the Himalayas.

Answer 1

Yes, this is thread safe.

The synchronized block is to prevent multiple threads from unnecessarily calling computeFieldValue(). Since field is volatile, the accesses in reset and getField are all well-ordered.

If the first check is non-null, getField is done; result is returned.

Otherwise, a lock is acquired, excluding any other thread that might set the field to non-null, but permitting any thread to set field to null. If any thread does set field to null, nothing should have changed; that's the condition that got the thread into the synchronized block. If another thread had already acquired the lock after the current thread's check, and set the field to a non-null value, the second check will detect that.

Answer 2

I think this should be safe, but only because you're storing the field in a local variable. After this is done, there's no way for the local variable reference to magically change to null, even if another thread is resetting field's value half-way through.