- LinkedBlockingQueue是一种基于单向链表实现的有界的(可选的，不指定默认int最大值)阻塞队列。队列中的元素遵循先入先出 (FIFO)的规则。新元素插入到队列的尾部，从队列头部取出元素。(在并发程序中，基于链表实现的队列和基于数组实现的队列相比，往往具有更高的吞吐 量，但性能稍差一些)
- 首先看下LinkedBlockingQueue内部的数据结构：
public class LinkedBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = -6903933977591709194L;
/**
* Linked list node class
*/
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; }
}
/** The capacity bound, or Integer.MAX_VALUE if none */
private final int capacity;
/** 这里的count为原子量，避免了一些使用count的地方需要加两把锁。 */
private final AtomicInteger count = new AtomicInteger(0);
/** Head of linked list */
private transient Node<E> head;
/** Tail of linked list */
private transient Node<E> last;
/** Lock held by take, poll, etc */
private final ReentrantLock takeLock = new ReentrantLock();
/** Wait queue for waiting takes */
private final Condition notEmpty = takeLock.newCondition();
/** Lock held by put, offer, etc */
private final ReentrantLock putLock = new ReentrantLock();
/** Wait queue for waiting puts */
private final Condition notFull = putLock.newCondition();
/**
* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
* {@link Integer#MAX_VALUE}.
*/
public LinkedBlockingQueue() {
this(Integer.MAX_VALUE);
}
/**
* Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
*
* @param capacity the capacity of this queue
* @throws IllegalArgumentException if <tt>capacity</tt> is not greater
* than zero
*/
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
last = head = new Node<E>(null);
}
public LinkedBlockingQueue(Collection<? extends E> c) {
this(Integer.MAX_VALUE);
for (E e : c)
add(e);
}
首先可见，内部为单向链表；其次，内部为两把锁：存锁和取锁，并分别关联一个条件(是一种双锁队列)。
- 还是从put和take入手，先看下put方法：
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;
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.
*/
try {
while (count.get() == capacity)
notFull.await();
} catch (InterruptedException ie) {
notFull.signal(); // propagate to a non-interrupted thread
throw ie;
}
insert(e);
c = count.getAndIncrement();
if (c + 1 < capacity)
/*
* 注意这里的处理：和单锁队列不同，count为原子量，不需要锁保护。
* put过程中可能有其他线程执行多次get,所以这里需要判断一下当前
* 如果还有剩余容量，那么继续唤醒notFull条件上等待的线程。
*/
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0) //如果count又0变为1，说明在队列是空的情况下插入了1个元素，唤醒notNull条件上等待的线程。
signalNotEmpty();
}
/**
* Creates a node and links it at end of queue.
* @param x the item
*/
private void insert(E x) {
last = last.next = new Node<E>(x);
}
/**
* Signals 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();
}
}
代码很容易看懂，再看下take方法实现：
public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
try {
while (count.get() == 0)
notEmpty.await();
} catch (InterruptedException ie) {
notEmpty.signal(); // propagate to a non-interrupted thread
throw ie;
}
x = extract();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
/**
* Removes a node from head of queue,
* @return the node
*/
private E extract() {
Node<E> first = head.next;
head = first;
E x = first.item;
first.item = null;
return x;
}
/**
* Signals a waiting put. Called only from take/poll.
*/
private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
notFull.signal();
} finally {
putLock.unlock();
}
}
和put对等的逻辑，也很容易看懂。
- 上面看到，主要方法里并没有同时用两把锁，但有些方法里会同时使用两把锁，比如remove方法等：
public boolean remove(Object o) {
if (o == null) return false;
boolean removed = false;
fullyLock();
try {
Node<E> trail = head;
Node<E> p = head.next;
while (p != null) {
if (o.equals(p.item)) {
removed = true;
break;
}
trail = p;
p = p.next;
}
if (removed) {
p.item = null;
trail.next = p.next;
if (last == p)
last = trail;
if (count.getAndDecrement() == capacity)
notFull.signalAll();
}
} finally {
fullyUnlock();
}
return removed;
}
/**
* Lock to prevent both puts and takes.
*/
private void fullyLock() {
putLock.lock();
takeLock.lock();
}
/**
* Unlock to allow both puts and takes.
*/
private void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}