BlockingQueue学习笔记
1. BlockingQueue接口的常用实现类如下：
BlockingQueue定义的常用方法如下:
抛出异常 特殊值 阻塞 超时
插入 add(e) offer(e) put(e) offer(e, time, unit)
移除 remove() poll() take() poll(time, unit)
检查 element() peek() 不可用 不可用
解释：
add： 如果当前队列已满，执行add操作将抛出异常
offer： 如果当前队列已满，执行offer操作将返回false（JDK6）
put： 如果当前队列已满，则无限期阻塞下去直到队列可插入
take： 如果当前队列为空，则无限期阻塞下去直到队列非空
2. LinkedBlockingQueue
实例化代码：
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
last = head = new Node<E>(null);//创建一个Node，令head和last都指向这位对象
}
从实例化源码可以看出LinkedBlockingQueue是用链表结构实现的，实例化时并不分配所有空间
add():
public boolean add(E e) {
if (offer(e))//add操作实际是调用了offer
return true;
else
throw new IllegalStateException("Queue full");//如果队列已满则抛出异常
}
offer():
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
if (count.get() == capacity)//队列已满，链表长度等于初始化时的capacity
return false;
int c = -1;
final ReentrantLock putLock = this.putLock;
putLock.lock();//对enqueue操作加锁
try {
if (count.get() < capacity) {
enqueue(e);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();//唤醒读线程
}
} finally {
putLock.unlock();//ReentrantLock一定要在finally中释放锁
}
if (c == 0)
signalNotEmpty();
return c >= 0;
}
可见队列满的情况下仅仅返回false
put()
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.
*/
while (count.get() == capacity) {
notFull.await();//如果队列已满，则阻塞写线程
}
enqueue(e);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();//唤醒写线程
} finally {
putLock.unlock();//释放锁
}
if (c == 0)
signalNotEmpty();
}
//插到队尾
private void enqueue(E x) {
// assert putLock.isHeldByCurrentThread();
last = last.next = new Node<E>(x);
}
take():
public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
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;
}
//将队列头的元素返回
private E dequeue() {
// assert takeLock.isHeldByCurrentThread();
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;
}
poll():
public E poll() {
final AtomicInteger count = this.count;
if (count.get() == 0)
return null;//如果队列空则返回null
E x = null;
int c = -1;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
if (count.get() > 0) {
x = dequeue();//队列非空，获取头元素
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();//唤醒写线程
}
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
3. ArrayBlockingQueue
实例化代码：
public ArrayBlockingQueue(int capacity, boolean fair) {
if (capacity <= 0)
throw new IllegalArgumentException();
//实例化的时候就分配一个数组空间 注意与LinkedBlockingQueue的区别
this.items = (E[]) new Object[capacity];
lock = new ReentrantLock(fair);
notEmpty = lock.newCondition();
notFull = lock.newCondition();
}
add():
//同LinkedBlockingQueue的add
public boolean add(E e) {
return super.add(e);
}
offer():
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final ReentrantLock lock = this.lock;
lock.lock();
try {
if (count == items.length)
return false;//数组已满，返回false
else {
insert(e);//插入数组
return true;
}
} finally {
lock.unlock();
}
}
private void insert(E x) {
items[putIndex] = x;
putIndex = inc(putIndex);//索引加1
++count;
notEmpty.signal();//唤醒写线程
}
4. SynchronousQueue
同步Queue,属于线程安全的BlockingQueue的一种,此队列设计的理念类似于"单工模式",对于每个put/offer操作,必须等待一个take/poll操作,类似于我们的现实生活中的"火把传递":一个火把传递地他人,需要2个人"触手可及"才行. 因为这种策略,最终导致队列中并没有一个真正的元素;这是一种pipleline思路的基于queue的"操作传递".
void put(E o):向队列提交一个元素,阻塞直到其他线程take或者poll此元素.
boolean offer(E o):向队列中提交一个元素,如果此时有其他线程正在被take阻塞(即其他线程已准备接收)或者"碰巧"有poll操作,那么将返回true,否则返回false.
E take():获取并删除一个元素,阻塞直到有其他线程offer/put.
boolean poll():获取并删除一个元素,如果此时有其他线程正在被put阻塞(即其他线程提交元素正等待被接收)或者"碰巧"有offer操作,那么将返回true,否则返回false.
E peek():总会返回null,硬编码.（转）