java 固定大小的queue thread safe 线程安全

用于统计特别好用( 数据时刻更新 但只取最近的几次数据) 采用数组实现

package com.yiwugou.homer.core;

import java.util.AbstractQueue;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.concurrent.locks.ReentrantLock;

/**
 *
 * LimitQueue
 *
 * @author [email protected]
 *
 * @since 2017年7月14日 上午10:45:42
 */
public class LimitQueue extends AbstractQueue implements Queue, java.io.Serializable {

    private static final long serialVersionUID = 4361189502222039104L;

    /** The queued items */
    final Object[] items;

    /** items index for next take, poll, peek or remove */
    int nextTakeIndex;

    /** items index for next put, offer, or add */
    int nextPutIndex;

    /** Number of elements in the queue */
    int count;

    /** Main lock guarding all access */
    final ReentrantLock lock;

    public LimitQueue(int capacity) {
        this.items = new Object[capacity];
        this.lock = new ReentrantLock();
    }

    @Override
    public boolean offer(E e) {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (this.items[this.nextPutIndex] != null) {
                this.nextTakeIndex = this.inc(this.nextTakeIndex);
            }
            this.items[this.nextPutIndex] = e;
            this.nextPutIndex = this.inc(this.nextPutIndex);
            this.count = (this.items.length <= this.count) ? this.items.length : (this.count + 1);
            return true;
        } finally {
            lock.unlock();
        }
    }

    @Override
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (this.count == 0) {
                return null;
            } else {
                final Object[] items = this.items;
                E x = (E) (items[this.nextTakeIndex]);
                items[this.nextTakeIndex] = null;
                this.nextTakeIndex = this.inc(this.nextTakeIndex);
                --this.count;
                return x;
            }
        } finally {
            lock.unlock();
        }
    }

    @Override
    public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (this.count == 0) ? null : (E) this.items[(this.nextTakeIndex)];
        } finally {
            lock.unlock();
        }
    }

    @Override
    public Iterator iterator() {
        return new Itr();
    }

    @Override
    public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return this.count;
        } finally {
            lock.unlock();
        }
    }

    final int inc(int i) {
        return (++i == this.items.length) ? 0 : i;
    }

    final int dec(int i) {
        return ((i == 0) ? this.items.length : i) - 1;
    }

    private static void checkNotNull(Object v) {
        if (v == null) {
            throw new NullPointerException("LimitQueue item must not be null!");
        }
    }

    void removeAt(int i) {
        final Object[] items = this.items;
        if (i == this.nextTakeIndex) {
            items[this.nextTakeIndex] = null;
            this.nextTakeIndex = this.inc(this.nextTakeIndex);
        } else {
            for (;;) {
                int nexti = this.inc(i);
                if (nexti != this.nextPutIndex) {
                    items[i] = items[nexti];
                    i = nexti;
                } else {
                    items[i] = null;
                    this.nextPutIndex = i;
                    break;
                }
            }
        }
        --this.count;
    }

    private class Itr implements Iterator {
        private int remaining; // Number of elements yet to be returned
        private int nextIndex; // Index of element to be returned by next
        private E nextItem; // Element to be returned by next call to next
        private E lastItem; // Element returned by last call to next
        private int lastRet; // Index of last element returned, or -1 if none

        Itr() {
            final ReentrantLock lock = LimitQueue.this.lock;
            lock.lock();
            try {
                this.lastRet = -1;
                if ((this.remaining = LimitQueue.this.count) > 0) {
                    this.nextIndex = LimitQueue.this.nextTakeIndex;
                    this.nextItem = (E) LimitQueue.this.items[this.nextIndex];
                }
            } finally {
                lock.unlock();
            }
        }

        @Override
        public boolean hasNext() {
            return this.remaining > 0;
        }

        @Override
        public E next() {
            final ReentrantLock lock = LimitQueue.this.lock;
            lock.lock();
            try {
                if (this.remaining <= 0) {
                    throw new NoSuchElementException();
                }
                this.lastRet = this.nextIndex;
                E x = (E) LimitQueue.this.items[this.nextIndex]; // check for
                                                                 // fresher
                                                                 // value
                if (x == null) {
                    x = this.nextItem; // we are forced to report old value
                    this.lastItem = null; // but ensure remove fails
                } else {
                    this.lastItem = x;
                }
                while (--this.remaining > 0
                        && (this.nextItem = (E) LimitQueue.this.items[(this.nextIndex = LimitQueue.this
                                .inc(this.nextIndex))]) == null) {
                    ;// skip over nulls
                }
                return x;
            } finally {
                lock.unlock();
            }
        }

        @Override
        public void remove() {
            final ReentrantLock lock = LimitQueue.this.lock;
            lock.lock();
            try {
                int i = this.lastRet;
                if (i == -1) {
                    throw new IllegalStateException();
                }
                this.lastRet = -1;
                E x = this.lastItem;
                this.lastItem = null;
                // only remove if item still at index
                if (x != null && x == LimitQueue.this.items[i]) {
                    boolean removingHead = (i == LimitQueue.this.nextTakeIndex);
                    LimitQueue.this.removeAt(i);
                    if (!removingHead) {
                        this.nextIndex = LimitQueue.this.dec(this.nextIndex);
                    }
                }
            } finally {
                lock.unlock();
            }
        }
    }

    
}