ArrayBlockingQueue
初始化
//指定初始化队列的容量，锁的公平性
public ArrayBlockingQueue(int capacity, boolean fair) {
if (capacity <= 0)
throw new IllegalArgumentException();
this.items = new Object[capacity];
lock = new ReentrantLock(fair);
notEmpty = lock.newCondition();
notFull = lock.newCondition();
}
//可初始化队列中的元素
public ArrayBlockingQueue(int capacity, boolean fair,
Collection c) {
this(capacity, fair);
final ReentrantLock lock = this.lock;
lock.lock(); // Lock only for visibility, not mutual exclusion
try {
int i = 0;
try {
for (E e : c) {
checkNotNull(e);
items[i++] = e;
}
} catch (ArrayIndexOutOfBoundsException ex) {
throw new IllegalArgumentException();
}
count = i;
putIndex = (i == capacity) ? 0 : i;
} finally {
lock.unlock();
}
}
列出主要的成员变量
/** The queued items */
final Object[] items;
/** items index for next take, poll, peek or remove */
int takeIndex;
/** items index for next put, offer, or add */
int putIndex;
/** Number of elements in the queue */
int count;
/** Main lock guarding all access */
final ReentrantLock lock;
/** Condition for waiting takes */
private final Condition notEmpty;
/** Condition for waiting puts */
private final Condition notFull;
- 插入 add、offer、put
//java.util.concurrent.ArrayBlockingQueue.add(E)
public boolean add(E e) {
return super.add(e);
}
//java.util.AbstractQueue.add(E)
public boolean add(E e) {
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}
//java.util.concurrent.ArrayBlockingQueue.offer(E)
public boolean offer(E e) {
//若e为null抛出空指针异常
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
//队列满了，直接返回false
if (count == items.length)
return false;
else {
//元素入队
enqueue(e);
return true;
}
} finally {
lock.unlock();
}
}
//java.util.concurrent.ArrayBlockingQueue.put(E)
public void put(E e) throws InterruptedException {
//若e为null抛出空指针异常
checkNotNull(e);
final ReentrantLock lock = this.lock;
//可响应中断的加锁操作
lock.lockInterruptibly();
try {
while (count == items.length)
//阻塞，直到队列中有空间可用
notFull.await();
enqueue(e);
} finally {
lock.unlock();
}
}
//java.util.concurrent.ArrayBlockingQueue.enqueue(E)
//添加元素到队列中
private void enqueue(E x) {
// assert lock.getHoldCount() == 1;
// assert items[putIndex] == null;
final Object[] items = this.items;
items[putIndex] = x;
//重置putIndex
if (++putIndex == items.length)
putIndex = 0;
count++;
//唤醒在notEmpty条件队列上等待的某个线程
notEmpty.signal();
}
添加元素API的一些特点：add内部调用了offer，返回成功或抛出异常。offer返回成功或失败。put阻塞直到元素放入队列，可响应中断。（add的元素不能是null，否则会产生空指针异常）
- 删除（出队）remove、poll、take
//java.util.AbstractQueue.remove()
public E remove() {
E x = poll();
if (x != null)
return x;
else
throw new NoSuchElementException();
}
//java.util.concurrent.ArrayBlockingQueue.poll()
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
//出队一个元素，若队列为空返回一个null
return (count == 0) ? null : dequeue();
} finally {
lock.unlock();
}
}
//java.util.concurrent.ArrayBlockingQueue.take()
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
//可响应中断的加锁操作
lock.lockInterruptibly();
try {
//阻塞直到队列中有元素能够出队
while (count == 0)
notEmpty.await();
return dequeue();
} finally {
lock.unlock();
}
}
//java.util.concurrent.ArrayBlockingQueue.dequeue()
//元素出队
private E dequeue() {
// assert lock.getHoldCount() == 1;
// assert items[takeIndex] != null;
final Object[] items = this.items;
@SuppressWarnings("unchecked")
E x = (E) items[takeIndex];
items[takeIndex] = null;
//重置takeIndex
if (++takeIndex == items.length)
takeIndex = 0;
count--;
if (itrs != null)
itrs.elementDequeued();
//唤醒在notFull上等待的线程中的一个
notFull.signal();
return x;
}
删除元素API的一些特点：remove内部调用了poll，返回出队元素或抛出异常。poll返回出队元素或null。take阻塞直到元素出队，可响应中断。
LinkedBlockingQueue
初始化
public LinkedBlockingQueue() {
//默认初始化一个20多亿长度的队列
this(Integer.MAX_VALUE);
}
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
//初始化head和last
last = head = new Node(null);
}
public LinkedBlockingQueue(Collection c) {
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");
enqueue(new Node(e));
++n;
}
count.set(n);
} finally {
putLock.unlock();
}
}
列出主要的成员变量
/** The capacity bound, or Integer.MAX_VALUE if none */
private final int capacity;
/** Current number of elements */
private final AtomicInteger count = new AtomicInteger();
/**
* Head of linked list.
* Invariant: head.item == null
*/
transient Node head;
/**
* Tail of linked list.
* Invariant: last.next == null
*/
private transient Node 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();
- 插入 add、offer、put
//java.util.AbstractQueue.add(E)
public boolean add(E e) {
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}
//java.util.concurrent.LinkedBlockingQueue.offer(E)
public boolean offer(E e) {
//入队的元素e为null，会抛出空指针异常
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
//使用原子类计数，判断队列是否已满。（应该比加锁快）
if (count.get() == capacity)
return false;
int c = -1;
Node node = new Node(e);
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
//队列中还有空间
if (count.get() < capacity) {
//入队
enqueue(node);
//count加1
c = count.getAndIncrement();
//元素入队后，队列还有空间。唤醒在notFull上等待的线程中的一个
if (c + 1 < capacity)
notFull.signal();
}
} finally {
putLock.unlock();
}
//若c==0，表明元素入队前队列是空的。(此时删除元素那边的线程肯定在notEmpty上等待，这里需要执行唤醒)
if (c == 0)
signalNotEmpty();
//c大于等于0时表明元素入队成功
return c >= 0;
}
//java.util.concurrent.LinkedBlockingQueue.put(E)
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;
Node node = new Node(e);
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
putLock.lockInterruptibly();
try {
//这里上的时写锁，删除（出队）元素的操作可能也再同时执行。这里count可能会不断变化
while (count.get() == capacity) {
notFull.await();
}
enqueue(node);
c = count.getAndIncrement();
//队列中还有空间，唤醒在notFull上等待的线程中的一个
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
//
if (c == 0)
signalNotEmpty();
}
//java.util.concurrent.LinkedBlockingQueue.enqueue(Node)
//这里enqueue和下面的dequeue可能同时被调用，会有问题嘛？
private void enqueue(Node node) {
// assert putLock.isHeldByCurrentThread();
// assert last.next == null;
last = last.next = node;
}
//java.util.concurrent.LinkedBlockingQueue.signalNotEmpty()
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}
添加元素API的一些特点：同ArrayBlockingQueue
- 删除（出队）remove、poll、take
//java.util.AbstractQueue.remove()
public E remove() {
E x = poll();
if (x != null)
return x;
else
throw new NoSuchElementException();
}
//java.util.concurrent.LinkedBlockingQueue.poll()
public E poll() {
final AtomicInteger count = this.count;
if (count.get() == 0)
return null;
E x = null;
int c = -1;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
//队列非空，元素出队
if (count.get() > 0) {
x = dequeue();
c = count.getAndDecrement();
//若c为2表明元素出队前，队列中有2个元素。若c为1，元素出队后队列就空了。所以这里需要判断c > 1
if (c > 1)
notEmpty.signal();
}
} finally {
takeLock.unlock();
}
//这里c==capacity，说明在出队之前队列是满的。添加元素的线程一定在等待，这里进行唤醒。
if (c == capacity)
signalNotFull();
return x;
}
//java.util.concurrent.LinkedBlockingQueue.take()
public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
//队列为空时，线程在这里阻塞
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;
}
//java.util.concurrent.LinkedBlockingQueue.dequeue()
////这里dequeue和上面的enqueue可能同时被调用，会有问题嘛？
private E dequeue() {
// assert takeLock.isHeldByCurrentThread();
// assert head.item == null;
Node h = head;
Node first = h.next;
//h的next指向自己
h.next = h; // help GC
head = first;
E x = first.item;
first.item = null;
return x;
}
//java.util.concurrent.LinkedBlockingQueue.signalNotFull()
private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
notFull.signal();
} finally {
putLock.unlock();
}
}
删除元素API的一些特点：同ArrayBlockingQueue
LinkedBlockingQueue相比于ArrayBlockingQueue，它在元素入队和出队这两种操作上使用了不同的锁。这意味着LinkedBlockingQueue可以同时执行入队和出队操作，而ArrayBlockingQueue中元素入队和出队时使用的是同一把锁，所以ArrayBlockingQueue入队和出队操作是不可能同时执行的。
总结
| 常用的队列 | 特点 |
|---|---|
| ArrayListBlockingQueue | 数组实现的有界阻塞队列，支持公平和非公平配置（ReentrantLock） |
| LinkedBlockingQueue | 链表实现的有界阻塞队列，两把独立锁提高并发 |
| PriorityBlockingQueue | 数组实现的无界队列，数据结构为二叉堆（compareTo排序） |
| DelayQueue | 队列使用PriorityQueue实现，队列中的元素需要是java.util.concurrent.Delayed |
| SynchronousQueue | 不存储元素的阻塞队列，每个put操作必须等待一个take操作，反之亦然 |
| LinkedTransferQueue | 相比其他阻塞队列，多了tryTransfer和transfer方法（生产者将元素传递给消费者） |
| LinkedBlockingDeque | 链表实现的双向阻塞队列，可以从队列两端插入和移除元素 |