Java: How to do double-checked-locking with an array element?

Question

This is what my code currently looks like:

private boolean[] isInitialized = new boolean[COUNT];

private void ensureInitialized(int i) {
    if (! isInitialized[i]) {
        initialize(i);
        isInitialized[i] = true;
    }
}

Now I want to have it thread-safe. I know that double-checked-locking in Java is "teh 3vilness!!1", but since ensureInitialized may be called very often, I don't want it to be synchronized. So I am thinking of doing this:

private boolean[] isInitialized = new boolean[COUNT];

private void ensureInitialized(int i) {
    if (! isInitialized[i]) {
        synchronized (this) {
            if (! isInitialized[i]) {
                initialize(i);
                isInitialized[i] = true;
            }
        }
    }
}

Now what do I have to do to make this actually thread safe?
Some subquestions:

• Making isInitialized volatile is not necessary, since the variable is not changed, right?
• The array elements are changed, how can I make those volatile?
• Are there generally better ways to do this?

(Also notice that this is an instance method, so static initializer will no work)

Answer 1

You could also consider using Suppliers.memoize(Supplier<T> delegate) from guava library.

Answer 2

Please note that java implementation of double-checking is called "broken pattern" because it was proven as fail (for example see http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html). To work around just use atomic operation. Follow is sample how to build thread safe singleton:

static AtomicReferenceArray<Boolean> instance = 
   new AtomicReferenceArray<Boolean>(COUNT);

private void ensure(int i)
{
    if(!instance.get(i) )
    {
        synchronized(this){
        if( !instance.get(i) ){
           initialize(i);
           instance.set(i, true);
        }
    }
}