Get the array from an AtomicLongArray

问题

Using Java 1.6 and the AtomicLongArray, I'd like to "copy" the original AtomicLongArray into a new one. There is a constructor that takes an array (AtomicLongArray(long[])), so I thought I could just get the array from the original one and give it to the constructor.

Sadly, the actual long[] in the AtomicLongArray is private and there seem to be no getters for it. Is there any way to do this, meaning copy the values from one AtomicLongArray to another? I can't create my own class based on this class, as the sun.misc.Unsafe class is not available to me.

This is needed because I'm going to iterate over the values, and I don't want them modified by another thread during iteration. So I thought I could make a copy and use that for the iteration...

Thanks! Phillip

回答1:

I suspect you have to create your own long[] and populate it first, or just iterate over the original:

AtomicLongArray copy = new AtomicLongArray(original.length());
for (int i = 0; i < copy.length(); i++)
{
    copy.set(i, original.get(i));
}

Note that although each individual operation in AtomicLongArray is atomic, there are no bulk operations - so there's no way of getting a "snapshot" of the whole array at time T. If you want that sort of behaviour, I believe you'll need to use synchronization.

回答2:

This data structure allows concurrent updates to individual entries in the collection. There is not overall lock, so you can't prevent another thread changing the contents while you are iterating over it.

If you need this, you need a workaround, e.g. copy the array and loop again to check it hasn't changed. If changed, repeat. Or you need a collection which supports a global lock.

long[] copy = new long[original.length()];
boolean changed = true;
// repeat until we get an unchanged copy.
while(true) {
    for (int i = 0; i < copy.length(); i++) {
        long l = original.get(i);
        changed |= copy[i] != l;
        copy[i] = l;
    }
    if (!changed) break;
    changed = false;
}

This is not completely safe, but may be enough for what you need.

来源：https://stackoverflow.com/questions/9408112/get-the-array-from-an-atomiclongarray

标签

java

java.util.concurrent