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本文主要研究一下Elasticsearch的EvictingQueue

EvictingQueue

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/common/collect/EvictingQueue.java

public class EvictingQueue<T> implements Queue<T> {
    private final int maximumSize;
    private final ArrayDeque<T> queue;

    /**
     * Construct a new {@code EvictingQueue} that holds {@code maximumSize} elements.
     *
     * @param maximumSize The maximum number of elements that the queue can hold
     * @throws IllegalArgumentException if {@code maximumSize} is less than zero
     */
    public EvictingQueue(int maximumSize) {
        if (maximumSize < 0) {
            throw new IllegalArgumentException("maximumSize < 0");
        }
        this.maximumSize = maximumSize;
        this.queue = new ArrayDeque<>(maximumSize);
    }

    /**
     * @return the number of additional elements that the queue can accommodate before evictions occur
     */
    public int remainingCapacity() {
        return this.maximumSize - this.size();
    }

    /**
     * Add the given element to the queue, possibly forcing an eviction from the head if {@link #remainingCapacity()} is
     * zero.
     *
     * @param t the element to add
     * @return true if the element was added (always the case for {@code EvictingQueue}
     */
    @Override
    public boolean add(T t) {
        if (maximumSize == 0) {
            return true;
        }
        if (queue.size() == maximumSize) {
            queue.remove();
        }
        queue.add(t);
        return true;
    }

    /**
     * @see #add(Object)
     */
    @Override
    public boolean offer(T t) {
        return add(t);
    }

    @Override
    public T remove() {
        return queue.remove();
    }


    @Override
    public T poll() {
        return queue.poll();
    }

    @Override
    public T element() {
        return queue.element();
    }

    @Override
    public T peek() {
        return queue.peek();
    }

    @Override
    public int size() {
        return queue.size();
    }

    @Override
    public boolean isEmpty() {
        return queue.isEmpty();
    }

    @Override
    public boolean contains(Object o) {
        return queue.contains(o);
    }

    @Override
    public Iterator<T> iterator() {
        return queue.iterator();
    }

    @Override
    public Object[] toArray() {
        return queue.toArray();
    }

    @Override
    public <T1> T1[] toArray(T1[] a) {
        return queue.toArray(a);
    }

    @Override
    public boolean remove(Object o) {
        return queue.remove(o);
    }

    @Override
    public boolean containsAll(Collection<?> c) {
        return queue.containsAll(c);
    }

    /**
     * Add the given elements to the queue, possibly forcing evictions from the head if {@link #remainingCapacity()} is
     * zero or becomes zero during the execution of this method.
     *
     * @param c the collection of elements to add
     * @return true if any elements were added to the queue
     */
    @Override
    public boolean addAll(Collection<? extends T> c) {
        boolean modified = false;
        for (T e : c)
            if (add(e))
                modified = true;
        return modified;
    }

    @Override
    public boolean removeAll(Collection<?> c) {
        return queue.removeAll(c);
    }

    @Override
    public boolean retainAll(Collection<?> c) {
        return queue.retainAll(c);
    }

    @Override
    public void clear() {
        queue.clear();
    }
}
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• EvictingQueue实现了Queue接口，它的构造器要求输入maximumSize，然后根据maximumSize创建ArrayDeque；其add方法会判断当前队列大小是否等于maximumSize，等于则移除队首的元素然后再添加新元素

实例

elasticsearch-7.0.1/server/src/test/java/org/elasticsearch/search/aggregations/pipeline/MovFnWhitelistedFunctionTests.java

    public void testWindowMax() {
        int numValues = randomIntBetween(1, 100);
        int windowSize = randomIntBetween(1, 50);

        EvictingQueue<Double> window = new EvictingQueue<>(windowSize);
        for (int i = 0; i < numValues; i++) {

            double randValue = randomDouble();
            double expected = -Double.MAX_VALUE;

            if (i == 0) {
                window.offer(randValue);
                continue;
            }

            for (double value : window) {
                expected = Math.max(expected, value);
            }

            double actual = MovingFunctions.max(window.stream().mapToDouble(Double::doubleValue).toArray());
            assertEquals(expected, actual, 0.01 * Math.abs(expected));
            window.offer(randValue);
        }
    }
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• 这里使用EvictingQueue作为一个window的数据，不断根据numValues来offer数据，同时计算window中的最大值

小结

EvictingQueue实现了Queue接口，它的构造器要求输入maximumSize，然后根据maximumSize创建ArrayDeque；其add方法会判断当前队列大小是否等于maximumSize，等于则移除队首的元素然后再添加新元素

doc

• EvictingQueue