[concurrency-interest] Thread-safe iterator

chakravarthy.varaga at wipro.com chakravarthy.varaga at wipro.com
Fri Feb 17 01:42:34 EST 2012


Hi,

	Is there a guarantee that your protected final long resetToLastKey() is not accessed outside your iteration and your concrete next() implementation isn't accessed outside the transaction?

Varaga

-----Original Message-----
From: concurrency-interest-bounces at cs.oswego.edu [mailto:concurrency-interest-bounces at cs.oswego.edu] On Behalf Of Johannes.Lichtenberger
Sent: Friday, February 17, 2012 11:26 AM
To: concurrency-interest at cs.oswego.edu
Subject: [concurrency-interest] Thread-safe iterator

Hello,

I want to write a thread safe iterator for a DescendantAxis which returns all descendants of a node in a tree-structure.

All movement is done within a transaction (the mRtx instance):

    @Override
    public synchronized final long nextNode() {
        synchronized (mRtx) {
            long retVal = -1;
            if (hasNext()) {
                retVal = next();
            }
            return retVal;
        }
    }

The "iterator" returns all unique nodeIDs. hasNext() and next() are implementations of the Iterable<T> interface. As I'm synchronizing on the transaction reference I think it's safe (mRtx is also declared "final"). Should be safe even if hasNext() and next() are not synchronized as the first thing is a movement in hasNext():

    /**
     * Make sure the transaction points to the node after the last {@code hasNext()}.
     * This must be called first in {@code hasNext()}.
     *
     * @return key of node where transaction was after the last call of
     *         {@code hasNext()}
     */
    protected final long resetToLastKey() {
        // No check because of IAxis Convention 4.
        mRtx.moveTo(mKey);
        mNext = true;
        return mKey;
    }

Did I miss something?

kind regards,
Johannes

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