博客聚焦于Android多线程中的阻塞队列，对ArrayBlockingQueue、SynchronousQueue、DelayQueue进行源码解析。还介绍了LinkedBlockingQueue，它基于链表、先进先出，默认容量大可能占内存多，入队出队操作可并发，采用可重入独占非公平锁及条件变量同步，通过原子变量获取元素个数。
阻塞队列系列
public class LinkedBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
/**
* LinkedBlockingQueue内部的节点类
*/
static class Node<E> {
E item;
Node<E> next;
Node(E x) { item = x; }
}
//队列的容量，默认为Integer.MAX_VALUE
private final int capacity;
//队列的元素数量，初始值为0
private final AtomicInteger count = new AtomicInteger();
//头节点
transient Node<E> head;
//尾部节点
private transient Node<E> last;
//这个重入锁用于take, poll等获取数据的方法，即用于出队的锁
private final ReentrantLock takeLock = new ReentrantLock();
//用于take, poll等获取数据的方法的阻塞条件，即用于入队的锁
private final Condition notEmpty = takeLock.newCondition();
//这个重入锁用于put, offer等添加数据的方法
private final ReentrantLock putLock = new ReentrantLock();
/用于put, offer等添加数据的方法的阻塞条件
private final Condition notFull = putLock.newCondition();
//激活调用notEmpty.await()阻塞后放入notEmpty条件队列中的线程
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}
//激活调用notFull.await()阻塞后放入notEmpty条件队列中的线程
private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
notFull.signal();
} finally {
putLock.unlock();
}
}
//向队列的链表结构的尾部插入数据
private void enqueue(Node<E> node) {
// assert putLock.isHeldByCurrentThread();
// assert last.next == null;
last = last.next = node;
}
//取出链表头部的节点数据，并释放资源
private E dequeue() {
// assert takeLock.isHeldByCurrentThread();
// assert head.item == null;
Node<E> h = head;
Node<E> first = h.next;
h.next = h; // help GC
head = first;
E x = first.item;
first.item = null;
return x;
}
//可以看到默认的构造函数容量大小为Integer.MAX_VALUE
public LinkedBlockingQueue() {
this(Integer.MAX_VALUE);
}
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
last = head = new Node<E>(null);
}
//返回队列中的元素数量，这里直接通过原子变量获取
public int size() {
return count.get();
}
public void put(E e) throws InterruptedException {
//不允许添加null
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;
//生成新的节点
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
//获取可中断的锁
//因为后面调用了条件变量的await()方法，而await()方法会在中断标志设置后抛出InterruptedException异常后退出，所以还不如在加锁时候先看中断标志是不是被设置了，如果设置了直接抛出InterruptedException异常，就不用再去获取锁了
putLock.lockInterruptibly();
try {
/*
* 下面的原文注释很好的解释了为什么这里count是线程安全的
* 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.
*/
//如果队列已经满了，则阻塞当前线程
while (count.get() == capacity) {
notFull.await();
}
//调用notFull的signal方法退出阻塞状态后，执行添加数据的操作
enqueue(node);
c = count.getAndIncrement();
//如果添加数据后还队列还没有满，则继续调用notFull的signal方法唤醒其他等待在入队的线程
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
//c=0说明队列里有一个元素，这时候唤醒出队线程
if (c == 0)
signalNotEmpty();
}
//带超时时间的offer方法
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) {
//如果超时时间过了队列仍然是满的话就直接返回false
if (nanos <= 0L)
return false;
//否则调用awaitNanos等待，超时会返回<= 0L
nanos = notFull.awaitNanos(nanos);
}
//在超时时间内返回则添加元素
enqueue(new Node<E>(e));
c = count.getAndIncrement();
//如果添加数据后还队列还没有满，则继续调用notFull的signal方法唤醒其他等待在入队的线程
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
//c=0说明队列里有一个元素，这时候唤醒出队线程
if (c == 0)
signalNotEmpty();
return true;
}
//不带超时时间的offer方法
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
//如果队列已经满了的话就直接返回false，后面的逻辑基本和上面相同
if (count.get() == capacity)
return false;
int c = -1;
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
if (count.get() < capacity) {
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
}
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return c >= 0;
}
//take方法用于从队列中取数据，队列为空是会阻塞线程
public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
//获取可中断锁
takeLock.lockInterruptibly();
try {
//如果队列为空，则会阻塞线程
while (count.get() == 0) {
notEmpty.await();
}
x = dequeue();
c = count.getAndDecrement();
//如果c > 1说明队列中还有元素，则调用notEmpty的signal方法唤醒出队线程
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
//如果c == capacity就是说队列中有一个空位，唤醒入队线程
if (c == capacity)
signalNotFull();
return x;
}
//带超时时间的poll方法用于从队列中取数据，不会阻塞当前线程
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;
//获取可中断锁
takeLock.lockInterruptibly();
try {
//如果队列位空，那就循环等待
while (count.get() == 0) {
if (nanos <= 0L)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
//在超时时间内返回，则调用dequeue获取队列中的数据
x = dequeue();
c = count.getAndDecrement();
//如果c > 1说明队列中还有元素，则调用notEmpty的signal方法唤醒出队线程
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
//如果c == capacity就是说队列中有一个空位，唤醒入队线程
if (c == capacity)
signalNotFull();
return x;
}
//不带超时时间的poll方法
public E poll() {
final AtomicInteger count = this.count;
//如果队列为空则直接返回null，后面逻辑同上面的超时poll方法类似
if (count.get() == 0)
return null;
E x = null;
int c = -1;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
if (count.get() > 0) {
x = dequeue();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
}
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
//peek方法不会阻塞当前线程
public E peek() {
//如果队列为空，则直接返回null
if (count.get() == 0)
return null;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
//如果队列不为空，则直接返回链表头部的节点元素数据
return (count.get() > 0) ? head.next.item : null;
} finally {
takeLock.unlock();
}
}
//删除队列中的元素
public boolean remove(Object o) {
//如果删除的元素为null，则直接返回false
if (o == null) return false;
//双重加锁，即入队和出队同时锁定，这样能保证在删除操作时队列元素保持不变
fullyLock();
try {
//循环遍历链表，删除元素
for (Node<E> trail = head, p = trail.next;
p != null;
trail = p, p = p.next) {
if (o.equals(p.item)) {
unlink(p, trail);
return true;
}
}
return false;
} finally {
fullyUnlock();
}
}
//双重加锁
void fullyLock() {
putLock.lock();
takeLock.lock();
}
//双重解锁
void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}
...
}
复制代码- LinkedBlockingQueue是一个基于链表的队列，并且是一个先进先出的队列。
- LinkedBlockingQueue如果不指定队列的容量，默认容量大小为Integer.MAX_VALUE，有可能造成占用内存过大的情况
- LinkedBlockingQueue内部对入队和出队操作采用了不同的锁，这样入队和出队操作可以并发进行。但同时只能有一个线程可以进行入队或出队操作。
- LinkedBlockingQueue内部采用的是可重入独占的非公平锁，并且通过重入锁的条件变量来进行出队和入队的同步
- LinkedBlockingQueue通过操作原子变量count来获取当前队列的元素个数
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