IteratorCursor.java

/*
 * Copyright 2006-2012 Amazon Technologies, Inc. or its affiliates.
 * Amazon, Amazon.com and Carbonado are trademarks or registered trademarks
 * of Amazon Technologies, Inc. or its affiliates.  All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.amazon.carbonado.cursor;

import java.util.Iterator;
import java.util.NoSuchElementException;

import java.util.concurrent.locks.Lock;

import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

/**
 * Adapts an Iterator into a Cursor.
 *
 * @author Brian S O'Neill
 */
public class IteratorCursor<S> extends AbstractCursor<S> {
    private static final AtomicReferenceFieldUpdater<IteratorCursor, Lock> lockUpdater =
        AtomicReferenceFieldUpdater.newUpdater(IteratorCursor.class, Lock.class, "mLock");

    private volatile Iterator<S> mIterator;
    private volatile Lock mLock;

    /**
     * @param iterable collection to iterate over, or null for empty cursor
     */
    public IteratorCursor(Iterable<S> iterable) {
        this(iterable, null);
    }

    /**
     * @param iterable collection to iterate over, or null for empty cursor
     * @param lock optional lock to hold while cursor is open
     */
    public IteratorCursor(Iterable<S> iterable, Lock lock) {
        if (iterable == null) {
            mIterator = null;
            mLock = null;
        } else {
            if (lock != null) {
                lock.lock();
            }
            mIterator = iterable.iterator();
            mLock = lock;
        }
    }

    /**
     * @param iterator iterator to wrap, or null for empty cursor
     */
    public IteratorCursor(Iterator<S> iterator) {
        mIterator = iterator;
        mLock = null;
    }

    public void close() {
        mIterator = null;
        // Use AtomicReferenceFieldUpdater to allow close method to be safely
        // called multiple times without unlocking multiple times.
        Lock lock = lockUpdater.getAndSet(this, null);
        if (lock != null) {
            lock.unlock();
        }
    }

    public boolean hasNext() {
        Iterator it = mIterator;
        return it != null && it.hasNext();
    }

    public S next() {
        Iterator<S> it = mIterator;
        if (it == null) {
            throw new NoSuchElementException();
        }
        return it.next();
    }
}