Java阻塞队列-BlockingQueue介绍及实现原理
阻塞队列是对普通队列的一种扩展，在普通队列功能上增加了一些额外功能。
普通队列的功能可以参照java的Queue接口
public interface Queue extends Collection {
/**
* Inserts the specified element into this queue if it is possible to do so
* immediately without violating capacity restrictions, returning
* {@code true} upon success and throwing an {@code IllegalStateException}
* if no space is currently available.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null and
* this queue does not permit null elements
* @throws IllegalArgumentException if some property of this element
* prevents it from being added to this queue
*/
boolean add(E e);
/**
* Inserts the specified element into this queue if it is possible to do
* so immediately without violating capacity restrictions.
* When using a capacity-restricted queue, this method is generally
* preferable to {@link #add}, which can fail to insert an element only
* by throwing an exception.
*
* @param e the element to add
* @return {@code true} if the element was added to this queue, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null and
* this queue does not permit null elements
* @throws IllegalArgumentException if some property of this element
* prevents it from being added to this queue
*/
boolean offer(E e);
/**
* Retrieves and removes the head of this queue. This method differs
* from {@link #poll poll} only in that it throws an exception if this
* queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
E remove();
/**
* Retrieves and removes the head of this queue,
* or returns {@code null} if this queue is empty.
*
* @return the head of this queue, or {@code null} if this queue is empty
*/
E poll();
/**
* Retrieves, but does not remove, the head of this queue. This method
* differs from {@link #peek peek} only in that it throws an exception
* if this queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
E element();
/**
* Retrieves, but does not remove, the head of this queue,
* or returns {@code null} if this queue is empty.
*
* @return the head of this queue, or {@code null} if this queue is empty
*/
E peek();
} 其实主要就是入队、出队操作，入队和出队都有两种方法，add()/offer()，remove()/poll()，返回队头元素（只返回，不出队)也有两个方法element()/peek()，两组方法的功能相同，不同的是对于一些特殊情况的处理是返回特殊值还是抛出异常，比如队列已经满了的情况下调用入队操作，add()会抛出异常，offer()会返回false。具体可以看下面表：
| Throws exception | Returns special value | |
| Insert | add(e) |
offer(e) |
| Remove | remove() |
poll() |
| Examine | element() |
peek() |
阻塞队列接口（BlockingQueue）继承自普通队列
public interface BlockingQueue extends Queue {
/**
* Inserts the specified element into this queue if it is possible to do
* so immediately without violating capacity restrictions, returning
* {@code true} upon success and throwing an
* {@code IllegalStateException} if no space is currently available.
* When using a capacity-restricted queue, it is generally preferable to
* use {@link #offer(Object) offer}.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean add(E e);
/**
* Inserts the specified element into this queue if it is possible to do
* so immediately without violating capacity restrictions, returning
* {@code true} upon success and {@code false} if no space is currently
* available. When using a capacity-restricted queue, this method is
* generally preferable to {@link #add}, which can fail to insert an
* element only by throwing an exception.
*
* @param e the element to add
* @return {@code true} if the element was added to this queue, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean offer(E e);
/**
* Inserts the specified element into this queue, waiting if necessary
* for space to become available.
*
* @param e the element to add
* @throws InterruptedException if interrupted while waiting
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
void put(E e) throws InterruptedException;
/**
* Inserts the specified element into this queue, waiting up to the
* specified wait time if necessary for space to become available.
*
* @param e the element to add
* @param timeout how long to wait before giving up, in units of
* {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the
* {@code timeout} parameter
* @return {@code true} if successful, or {@code false} if
* the specified waiting time elapses before space is available
* @throws InterruptedException if interrupted while waiting
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Retrieves and removes the head of this queue, waiting if necessary
* until an element becomes available.
*
* @return the head of this queue
* @throws InterruptedException if interrupted while waiting
*/
E take() throws InterruptedException;
/**
* Retrieves and removes the head of this queue, waiting up to the
* specified wait time if necessary for an element to become available.
*
* @param timeout how long to wait before giving up, in units of
* {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the
* {@code timeout} parameter
* @return the head of this queue, or {@code null} if the
* specified waiting time elapses before an element is available
* @throws InterruptedException if interrupted while waiting
*/
E poll(long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Returns the number of additional elements that this queue can ideally
* (in the absence of memory or resource constraints) accept without
* blocking, or {@code Integer.MAX_VALUE} if there is no intrinsic
* limit.
*
* Note that you cannot always tell if an attempt to insert
* an element will succeed by inspecting {@code remainingCapacity}
* because it may be the case that another thread is about to
* insert or remove an element.
*
* @return the remaining capacity
*/
int remainingCapacity();
/**
* Removes a single instance of the specified element from this queue,
* if it is present. More formally, removes an element {@code e} such
* that {@code o.equals(e)}, if this queue contains one or more such
* elements.
* Returns {@code true} if this queue contained the specified element
* (or equivalently, if this queue changed as a result of the call).
*
* @param o element to be removed from this queue, if present
* @return {@code true} if this queue changed as a result of the call
* @throws ClassCastException if the class of the specified element
* is incompatible with this queue
* (optional)
* @throws NullPointerException if the specified element is null
* (optional)
*/
boolean remove(Object o);
/**
* Returns {@code true} if this queue contains the specified element.
* More formally, returns {@code true} if and only if this queue contains
* at least one element {@code e} such that {@code o.equals(e)}.
*
* @param o object to be checked for containment in this queue
* @return {@code true} if this queue contains the specified element
* @throws ClassCastException if the class of the specified element
* is incompatible with this queue
* (optional)
* @throws NullPointerException if the specified element is null
* (optional)
*/
public boolean contains(Object o);
/**
* Removes all available elements from this queue and adds them
* to the given collection. This operation may be more
* efficient than repeatedly polling this queue. A failure
* encountered while attempting to add elements to
* collection {@code c} may result in elements being in neither,
* either or both collections when the associated exception is
* thrown. Attempts to drain a queue to itself result in
* {@code IllegalArgumentException}. Further, the behavior of
* this operation is undefined if the specified collection is
* modified while the operation is in progress.
*
* @param c the collection to transfer elements into
* @return the number of elements transferred
* @throws UnsupportedOperationException if addition of elements
* is not supported by the specified collection
* @throws ClassCastException if the class of an element of this queue
* prevents it from being added to the specified collection
* @throws NullPointerException if the specified collection is null
* @throws IllegalArgumentException if the specified collection is this
* queue, or some property of an element of this queue prevents
* it from being added to the specified collection
*/
int drainTo(Collection c);
/**
* Removes at most the given number of available elements from
* this queue and adds them to the given collection. A failure
* encountered while attempting to add elements to
* collection {@code c} may result in elements being in neither,
* either or both collections when the associated exception is
* thrown. Attempts to drain a queue to itself result in
* {@code IllegalArgumentException}. Further, the behavior of
* this operation is undefined if the specified collection is
* modified while the operation is in progress.
*
* @param c the collection to transfer elements into
* @param maxElements the maximum number of elements to transfer
* @return the number of elements transferred
* @throws UnsupportedOperationException if addition of elements
* is not supported by the specified collection
* @throws ClassCastException if the class of an element of this queue
* prevents it from being added to the specified collection
* @throws NullPointerException if the specified collection is null
* @throws IllegalArgumentException if the specified collection is this
* queue, or some property of an element of this queue prevents
* it from being added to the specified collection
*/
int drainTo(Collection c, int maxElements);
}
其实主要在Queue基础上增加了阻塞的入队(put())和出队(take())操作，即当队列已满时调用put()入队时，当前线程会阻塞，直到队列有空间时才会继续入队；当队列为空时，调用take()出队操作时，当前线程会阻塞，直到队列中有元素时才会继续出队。这就是阻塞队列的核心。
阻塞队列实现原理
阻塞队列有很多实现类，根据存储结构不同有ArrayBlockingQueue，LinkedBlockingQueue，SynchronousQueue，这里主要以LinkedBlockingQueue为例来介绍，线程池Executors类里FixedThreadPool和SingledThreadPool的作业队列都用的是LinkedBlockingQueue。
/**
* Inserts the specified element at the tail of this queue, waiting if
* necessary for space to become available.
*
* @throws InterruptedException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
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 {
/*
* 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.
*/
//====================①
while (count.get() == capacity) {
notFull.await();
}
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
} 主要看上边代码①处，当前元素个数等于队列容量时，调用notFull.await()方法，当前线程阻塞在这里，知道有地方调用notFull.signal()方法，当前现在才被唤醒。
出队操作类似有个notEmpty.await()方法，当队列为空时阻塞出队线程
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;
}while (count.get() == 0) {
notEmpty.await();
}那么notFull.await()，notEmpty.await()，notEmpty.signal()是什么东西？有点儿像object类的wait()，notify()，看代码：
/** 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();他们是JDK1.5引入的Condition类，和ReentrantLock锁配合实现syncronized语法的类，比syncronized语法更灵活，某些场景效率更高，具体区别和实现原理后续文章再写。