static class Node {
E item; // 节点元素

Node next; // 后继节点

Node(E x) { item = x; }
}

PS: 从 Node 定义可以看出该链表是一个单链表。

// 链表的容量（若不指定则为 Integer.MAX_VALUE）
private final int capacity;

// 当前元素的数量（即链表中元素的数量）
private final AtomicInteger count = new AtomicInteger();

// 链表的头节点（节点元素为空）

// 链表的尾结点（节点元素为空）
private transient Node last;

// take、poll 等出队操作持有的锁
private final ReentrantLock takeLock = new ReentrantLock();

/** Wait queue for waiting takes */
// 出队锁的条件队列
private final Condition notEmpty = takeLock.newCondition();

// put、offer 等入队操作的锁
private final ReentrantLock putLock = new ReentrantLock();

/** Wait queue for waiting puts */
// 入队锁的条件队列
private final Condition notFull = putLock.newCondition();

// 构造器 1：无参构造器，初始容量为 Integer.MAX_VALUE，即 2^31-1
this(Integer.MAX_VALUE);
}

// 构造器 2：指定容量的构造器
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
// 初始化链表的头尾节点
last = head = new Node(null);
}

// 构造器 3：用给定集合初始化的构造器
// 调用构造器 2 进行初始化
this(Integer.MAX_VALUE);
final ReentrantLock putLock = this.putLock;
putLock.lock(); // Never contended, but necessary for visibility
try {
int n = 0;
for (E e : c) {
if (e == null)
throw new NullPointerException();
if (n == capacity)
throw new IllegalStateException(“Queue full”);
// 将集合中的元素封装成 Node 对象，并添加到链表末尾
enqueue(new Node(e));
++n;
}
count.set(n);
} finally {
putLock.unlock();
}
}

enqueue 方法如下：

// 将 node 节点添加到链表末尾
private void enqueue(Node node) {
last = last.next = node;
}

1. put(E) 代码如下：

public void put(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
// Note: convention in all put/take/etc is to preset local var
// holding count negative to indicate failure unless set.
int c = –1;
// 把 E 封装成 Node 节点
Node node = new Node(e);
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
putLock.lockInterruptibly();
try {
/*
* Note that count is used in wait guard even though it is
* not protected by lock. This works because count can
* only decrease at this point (all other puts are shut
* out by lock), and we (or some other waiting put) are
* signalled if it ever changes from capacity. Similarly
* for all other uses of count in other wait guards.
*/
// 若队列已满，notFull 等待（类比生产者）
while (count.get() == capacity) {
notFull.await();
}
// node 入队
enqueue(node);
c = count.getAndIncrement();
// 若该元素添加后，队列仍未满，唤醒一个其他生产者线程
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
// c==0 说明之前队列为空，出队线程处于等待状态，
// 添加一个元素后，将出队线程唤醒（消费者）
if (c == 0)
signalNotEmpty();
}

signalNotEmpty 方法：

private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
// 唤醒 notEmpty 条件下的一个线程（消费者）
notEmpty.signal();
} finally {
takeLock.unlock();
}
}

2. offer(E, timeout, TimeUnit):

public boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) throw new NullPointerException();
long nanos = unit.toNanos(timeout);
int c = –1;
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
putLock.lockInterruptibly();
try {
while (count.get() == capacity) {
// 等待超时，返回 false
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
// 入队
enqueue(new Node(e));
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return true;
}

3. offer(E):

public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
// 若队列已满，直接返回 false
if (count.get() == capacity)
return false;
int c = –1;
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
// 队列未满，入队
if (count.get() < capacity) {
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
// 队列未满，唤醒 notFull 下的线程，继续入队
notFull.signal();
}
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return c >= 0;
}

1. put(E): 若队列已满，则等待， 无返回值

2.  offer(E, timeout, TimeUnit): 与 put 方法类似，有超时等待和返回值；

3.  offer(E): 立即返回，没有循环等待。

1. take():

public E take() throws InterruptedException {
E x;
int c = –1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
// 队列为空，notEmpty 条件下的线程等待（消费者）
while (count.get() == 0) {
notEmpty.await();
}
// 从队列头部删除节点
x = dequeue();
c = count.getAndDecrement();
// 若队列不为空，唤醒一个 notEmpty 条件下的线程（消费者）
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
// 队列已经不满了，唤醒 notFull 条件下的线程（生产者）
if (c == capacity)
signalNotFull();
return x;
}

dequeue 方法：

private E dequeue() {
Node h = head; // 头节点
Node first = h.next; // 头节点的后继节点
h.next = h; // help GC // 后继节点指向自己（从链表中删除）
E x = first.item; // 获取要删除节点的数据
first.item = null; // 清空数据（新的头节点）
return x;
}

signalNotFull:

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

2. poll(timeout, TimeUnit) :

public E poll(long timeout, TimeUnit unit) throws InterruptedException {
E x = null;
int c = –1;
long nanos = unit.toNanos(timeout);
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
// 队列已空
while (count.get() == 0) {
// 超时返回 null
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
x = dequeue();
// 若队列不空，唤醒一个 notEmpty 条件下的线程（消费者）
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
// 队列不满，唤醒 notFull 条件下的线程（生产者）
if (c == capacity)
signalNotFull();
return x;
}

3. poll():

public E poll() {
final AtomicInteger count = this.count;
// 队列为空，返回 null
if (count.get() == 0)
return null;
E x = null;
int c = –1;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
// 队列不为空，出队
if (count.get() > 0) {
x = dequeue();
c = count.getAndDecrement();
// 该元素出队后，队列仍不为空，唤醒其他消费者
if (c > 1)
notEmpty.signal();
}
} finally {
takeLock.unlock();
}
// 队列已经不满，唤醒生产者
if (c == capacity)
signalNotFull();
return x;
}

4. peek()

public E peek() {
if (count.get() == 0)
return null;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
// 头节点的后继节点
if (first == null)
return null;
else
return first.item;
} finally {
takeLock.unlock();
}

}

peek() 方法只返回头节点，并不删除。严格来说该方法并不属于出队操作，只是查询。

1. take():  获取 队列头部 元素，并将 其移除 ，队列为空时 阻塞等待；

2.  poll(long, unit):  获取队列头部元素，并将其移除 ，队列为空时等待一段时间，若超时返回 null；

3 . poll() : 获取队列头 部元素，并将其移除，队列为空时返回 null；

2. 内部使用了 ReentrantLock 保证线程安全；

3. 常用方法：