ArrayBlockingQueue 和LinkedBlockingQueue 代码解析(JDK8)

在使用线程池的时候，需要指定BlockingQueue 常用的一般有ArrayBlockingQueue和 edBlockingQueue
有一天被问到有什么区别没回答上来，因此从代码的层面解析一下
1 ArrayBlockingQueue
顾名思义，就是用Array来实现的queue Blockqing 则说明是线程安全的

public class ArrayBlockingQueue<E> extends AbstractQueue<E> implements BlockingQueue<E>, Serializable {    private static final long serialVersionUID = -817911632652898426L;    final  [] items;    int takeIndex;    int putIndex;    int count;    final ReentrantLock lock;    private final Condition notEmpty;    private final Condition notFull;}

items 存储数据的数组
takeIndex 取数据时数组的下标
putIndex 放数据时的下标
count 数据的数量
lock 使用ReentrantLock 来保证线程安全
notEmpty 非空信号量，用来进行取数据时的信号量
notFull 非满信号量，在写数据时数据满时的等待信号量
1 构造函数

    public ArrayBlockingQueue(int capacity) {        this(capacity, false);    }    public ArrayBlockingQueue(int capacity, boolean fair) {        if (capacity <= 0)            throw new IllegalArgumentException();        this.items = new  [capacity];//指定数组大小        lock = new ReentrantLock(fair); //根据参数确定lock是否为公平锁，默认为false        notEmpty = lock.newCondition(); //新建两个lock的信号量        notFull =  lock.newCondition();    }

2 写数据
研究代码发现 put add offer三个方法都调用了enqueue方法，ArrayBlockingQueue 将对数组的实际操作在jdk8抽象了出来,相对于jdk7进行了一定优化

    /**     * Inserts element at current put position, advances, and signals.     * Call only when holding lock.     */    //该方法只有在对象获取到锁之后才能调用    private void enqueue(E x) {        // assert lock.getHoldCount() == 1;        // assert items[putIndex] == null;        final  [] items = this.items; //获取数组对象        items[putIndex] = x; //putIndex 默认值为0        if (++putIndex == items.length) //在putIndex到达数组尾部时，重新指向数组第一个位置            putIndex = 0;        count++; //数组元素+1        notEmpty.signal(); //非空信号发送    }

(1) offer
offer方法 尝试插入数据，在数组满时返回false，正常插入 返回true

    public boolean offer(E e) {        checkNotNull(e); //校验数据是否为null        final ReentrantLock lock = this.lock; //获取对象锁        lock.lock(); //对当前对象加锁        try {            if (count == items.length) //如果数组满，返回false                return false;            else {                enqueue(e); //数组没满，插入数据，返回true                return true;            }        } finally {            lock.unlock(); //释放锁        }    }

(2) add
ArrayBlockingQueue 调用了父类AbstractQueue的add方法，
在插入成功时返回true，在插入失败(数组满)时，抛出异常
AbstractQueue 的add方法调用了offer()方法，所以add是offer的功能升级版

    public boolean add(E e) {        if (offer(e))            return true;        else            throw new IllegalStateException("Queue full");    }

(3) put
put方法 在进行数据插入时，会尝试获取锁并相应异常，同时，在数组满时，会一致等待，直到数组有了空闲空间

    public void put(E e) throws InterruptedException {        checkNotNull(e);        final ReentrantLock lock = this.lock;        lock.lockInterruptibly();//尝试获取锁并相应异常        try {            while (count == items.length) //数组满，                notFull.await(); //等待非满信号            enqueue(e);        } finally {            lock.unlock(); //在数据正常插入或者其他线程抛出异常后，解锁        }    }

(4) offer(E e, long timeout, TimeUnit unit)
ArrayBlockingQueue 还提供了一种超时配置的方法，在数组数据满超过timeout后返回fasle

  public boolean offer(E e, long timeout, TimeUnit unit)        throws InterruptedException {        checkNotNull(e);        long nanos = unit.toNanos(timeout);        final ReentrantLock lock = this.lock;        lock.lockInterruptibly();        try {            while (count == items.length) {                if (nanos <= 0)                    return false;                nanos = notFull.awaitNanos(nanos); //condition超时后 返回-1            }            enqueue(e);            return true;        } finally {            lock.unlock();        }    }

3 取数据
和写数据一样，取数据jdk8也进行了一定优化 统一调用dequeue方法

   private E dequeue() {        // assert lock.getHoldCount() == 1;        // assert items[takeIndex] != null;        final  [] items = this.items;         @SuppressWarnings("unchecked")        E x = (E) items[takeIndex]; //获取最老数据        items[takeIndex] = null; //最老数据位置置空        if (++takeIndex == items.length) //下标到达最后 置零            takeIndex = 0;        count--;         if (itrs != null) //itrl目前没看到初始化的位置 ，暂时不清楚有什么用            itrs.elementDequeued();        notFull.signal();        return x;    }

(1) poll(E e, long timeout, TimeUnit unit)
很简单 列表为空返回null否则放回对应数据

    public E poll() {        final ReentrantLock lock = this.lock;        lock.lock(); //加锁        try {            return (count == 0) ? null : dequeue(); //列表为空返回null否则放回对应数据        } finally {            lock.unlock(); //解锁        }    }

(2) take(E e, long timeout, TimeUnit unit)
尝试加锁，在数组为空时一直等待，直到有新数据或者被外部中断

    public E take() throws InterruptedException {        final ReentrantLock lock = this.lock;        lock.lockInterruptibly();        try {            while (count == 0)                notEmpty.await();            return dequeue();        } finally {            lock.unlock();        }    }

(3) peek(E e, long timeout, TimeUnit unit)
返回最老数据，但是不弹出数据，仅获取数据。在数组为空时返回null
因此一条数据可以重复peek多次

    public E peek() {        final ReentrantLock lock = this.lock;        lock.lock();        try {            return itemAt(takeIndex); // null when queue is empty        } finally {            lock.unlock();        }    }    final E itemAt(int i) {        return (E) items[i];    }

(4) poll(long timeout, TimeUnit unit)(E e, long timeout, TimeUnit unit)
也提供了等待超过timeout 返回null的poll方法

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {        long nanos = unit.toNanos(timeout);        final ReentrantLock lock = this.lock;        lock.lockInterruptibly();        try {            while (count == 0) {                if (nanos <= 0)                    return null;                nanos = notEmpty.awaitNanos(nanos); //如果超时awaitNanos 返回-1 ，最后返回null            }            return dequeue();        } finally {            lock.unlock();        }    }

2 edBlockingQueue
顾名思义，就是使用链表来存储的线程安全的队列

public class  edBlockingQueue<E> extends AbstractQueue<E> implements BlockingQueue<E>, Serializable {    private static final long serialVersionUID = -6903933977591709194L;    private final int capacity;    private final AtomicInteger count;    transient  edBlockingQueue.Node<E> head;    private transient  edBlockingQueue.Node<E> last;    private final ReentrantLock takeLock;    private final Condition notEmpty;    private final ReentrantLock putLock;    private final Condition notFull;}

capacity 链表的最大长度，默认为Integer.MAX_VALUE
count 元素数量
head 头节点
last 尾节点
takeLock 取数据锁
notEmpty 非空信号量
putLock 写数据锁
notFull 非满信号量
edBlockingQueue 采用了读写锁分离，因此在短时间内产生大量读写操作时，
比arrayBlockingQueue性能更加优秀
1 构造函数

    public  edBlockingQueue() {        this(Integer.MAX_VALUE);    }    public  edBlockingQueue(int capacity) {        if (capacity <= 0) throw new IllegalArgumentException();        this.capacity = capacity; //设置最大长度        last = head = new Node<E>(null); //    }

2写数据
edBlockingQueue同样提供了三个函数 put offer add
同样提供了enqueue方法，该方法仅在获取到putLock 后执行

    private void enqueue(Node<E> node) {        // assert putLock.isHeldByCurrentThread();        // assert last.next == null;        last = last.next = node; //在尾节点添加数据    }

(1) offer(E e)

    public boolean offer(E e) {        if (e == null) throw new NullPointerException();        final AtomicInteger count = this.count; //获取元素数量        if (count.get() == capacity) //如果链表长度到达上限，返回null            return false;        int c = -1;        Node<E> node = new Node<E>(e); //创建新节点        final ReentrantLock putLock = this.putLock;         putLock.lock();  //写锁加锁        try {            if (count.get() < capacity) { //如果没有到达链表上限                enqueue(node);   //新增节点                c = count.getAndIncrement(); //获取元素数量并将count+1(c=count，count++)，                                             //读写锁分离，链表数量可能有减少                if (c + 1 < capacity)  //如果链表数量没有达到上限，非满信号量通知                    notFull.signal();            }        } finally {            putLock.unlock(); //解锁        }        if (c == 0)  //如果链表原来的数量为0            signalNotEmpty(); //非空信号量通知        return c >= 0;  //返回插入结果 成功返回true，失败返回fasle    }    private void signalNotEmpty() {        final ReentrantLock takeLock = this.takeLock; //获取读锁        takeLock.lock(); //读锁加锁，防止数据被读取        try {            notEmpty.signal(); //非空信号量通知        } finally {            takeLock.unlock(); //读锁解锁        }    }

(2) add(E e)
和ArrayBlockingQueue一样，直接调用offer方法，在新增成功后返回true，在新增失败后直接抛出异常
(3) put(E e)
put操作 和offer操作基本一致，只不过在链表满时进行等待，知道链表节点减少

 public void put(E e) throws InterruptedException {        if (e == null) throw new NullPointerException();        int c = -1;        Node<E> node = new Node<E>(e);        final ReentrantLock putLock = this.putLock; //获取写锁        final AtomicInteger count = this.count;        putLock.lockInterruptibly(); //对写锁加锁并相应异常        try {                   while (count.get() == capacity) { //如果节点数量到达上限                notFull.await(); //等待非满信号量的通知            }            enqueue(node); //在数据被弹出后，插入新节点            c = count.getAndIncrement();              if (c + 1 < capacity)                notFull.signal();        } finally {            putLock.unlock(); //释放写锁        }        if (c == 0)             signalNotEmpty();    }

(4) offer(E e, long timeout, TimeUnit unit)
和ArrayBlockingQueue一样，如果链表ch长度到达上限，就等待timeout ，超时后直接返回fasle
3 读取数据
上dequeue

    private E dequeue() {        // assert takeLock.isHeldByCurrentThread();        // assert head.item == null;        Node<E> h = head; // 获取头节点        Node<E> first = h.next; //first设置为新的头节点        h.next = h; // help GC //需要移除的节点next指向自己帮助gc        head = first;  //head 置为新的头节点        E x = first.item; //获取返回值得item        first.item = null; //first item设置为null         return x; //返回item    }

(1) poll(E e, long timeout, TimeUnit unit)

    public E poll() {        final AtomicInteger count = this.count; //获取数量        if (count.get() == 0) //如果链表节点数量为空 返回null            return null;        E x = null;        int c = -1;        final ReentrantLock takeLock = this.takeLock; //获取读锁并加锁        takeLock.lock();        try {            if (count.get() > 0) {                x = dequeue(); //获取数据                c = count.getAndDecrement(); //数量-1                if (c > 1)  //剩余节点>1                    notEmpty.signal(); //非空信号通知            }        } finally {            takeLock.unlock(); //读锁解锁        }        if (c == capacity) //可能有线程在等在非满信号，-1前数量=限定长度            signalNotFull();        return x;    }    private void signalNotFull() {        final ReentrantLock putLock = this.putLock; //获取写锁并加锁        putLock.lock();        try {            notFull.signal(); //非满信号通知        } finally {            putLock.unlock(); //写锁解锁        }     }

(2) peek(E e)
存在返回数据，不存在返回null，链表节点不便，仅获取数据

    public E peek() {        if (count.get() == 0)            return null;        final ReentrantLock takeLock = this.takeLock;        takeLock.lock();        try {            Node<E> first = head.next;            if (first == null)                return null;            else                return first.item;        } finally {            takeLock.unlock();        }    }

(3) take(E e)
链表到达最大长度。等待，可以被异常中断

    public E take() throws InterruptedException {        E x;        int c = -1;        final AtomicInteger count = this.count;        final ReentrantLock takeLock = this.takeLock;        takeLock.lockInterruptibly();        try {            while (count.get() == 0) {                notEmpty.await();            }            x = dequeue();            c = count.getAndDecrement();            if (c > 1)                notEmpty.signal();        } finally {            takeLock.unlock();        }        if (c == capacity)            signalNotFull();        return x;    }

(4) poll(long timeout, TimeUnit unit)
等待超过timeout 返回null

3 两者的区别
1 ed读写锁分离，在短时间内发生大量读写交替操作时性能高
2 Array在读写操作时不需要维护额外节点，空间较少
3 Array使用int count ed使用AtomicInteger ，
因此：Array使用唯一Lock来保证count强一致性， ed使用Atomic来保证count的准确性