java.util.concurrent.LinkedBlockingQueue

一、简介

LinkedBlockingQueue是BlockingQueue的一种使用Link List的实现，它对头和尾（取和添加操作）采用两把不同的锁，相对于ArrayBlockingQueue提高了吞吐量。它也是一种阻塞型的容器，适合于实现“消费者生产者”模式。

二、具体实现

LinkedBlockingQueue底层的定义如下：

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {

static class Node<E> {
/** The item, volatile to ensure barrier separating write and read */
volatile E item;
Node<E> next;
Node(E x) { item = x; }
}

// 支持原子操作
private final AtomicInteger count = new AtomicInteger(0);

// 链表的头和尾
private transient Node<E> head;
private transient Node<E> last;

// 针对取和添加操作的两把锁及其上的条件
private final ReentrantLock takeLock = new ReentrantLock();
private final Condition notEmpty = takeLock.newCondition();
private final ReentrantLock putLock = new ReentrantLock();
private final Condition notFull = putLock.newCondition();

...
}

LinkedBlockingQueue的添加操作：

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {

private void insert(E x) {
last = last.next = new Node<E>(x);
}

/**
* signal方法在被调用时，当前线程必须拥有该condition相关的锁!
* Signal a waiting take. Called only from put/offer (which do not
* otherwise ordinarily lock takeLock.)
*/
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}

public void put(E o) throws InterruptedException {
if (o == null) throw new NullPointerException();
int c = -1;
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
// 使用putLock
putLock.lockInterruptibly();
try {
try {
// 当容量已满时，等待notFull条件
while (count.get() == capacity)
notFull.await();
} catch (InterruptedException ie) {
notFull.signal(); // propagate to a non-interrupted thread
throw ie;
}
insert(o);
// 取出当前值，并将原数据增加1
c = count.getAndIncrement();
// 容量不满，再次激活notFull上等待的put线程
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
// 必须先释放putLock再在notEmpty上signal，否则会造成死锁
if (c == 0)
signalNotEmpty();
}

...
}

LinkedBlockingQueue的取操作：

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {

private E extract() {
Node<E> first = head.next;
head = first;
E x = first.item;
first.item = null;
return x;
}

private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
notFull.signal();
} finally {
putLock.unlock();
}
}

public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
// 使用takeLock
takeLock.lockInterruptibly();
try {
try {
// 若容量为空，等待notEmpty
while (count.get() == 0)
notEmpty.await();
} catch (InterruptedException ie) {
notEmpty.signal(); // propagate to a non-interrupted thread
throw ie;
}

x = extract();
c = count.getAndDecrement();
// 再次激活notEmpty
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
// take执行之前容量已满，则激活notFull
if (c == capacity)
signalNotFull();
return x;
}

...
}

ConcurrentLinkedQueue是一个无锁的queue实现，它采用了一种无锁算法（在API中有说明），相比于传统的同步的queue来说吞吐量可以大大提高，同时它也不同于BlockingQueue，并不单单提供阻塞操作。它主要的目的是通过采用无锁的算法，使得read/write操作均不需要对容器加锁，提高容器吞吐量