ArrayBlockingQueue的实现思路简单描述，ArrayBlockingQueue的底对于互斥访问使用的一个锁。细节参考源码take和put方法：
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.*;
import java.util.*; public class ArrayBlockingQueue<E> extends AbstractQueue<E> implements BlockingQueue<E>, java.io.Serializable { /** The queued items */
final Object[] items;//证明了java泛型是语法级别的泛型。 /** items index for next take, poll, peek or remove */
int takeIndex; /** items index for next put, offer, or add */
int putIndex; // 可以使用一个int变量计数原因是：ArrayBlockingQueue的实现只有一把锁，对count访问时候都是在锁的保护机制下实现互斥的。
/** Number of elements in the queue */
int count; /*
* Concurrency control uses the classic two-condition algorithm found in any
* textbook.
*/
// 使用一把锁
/** Main lock guarding all access */
final ReentrantLock lock; // 两个Condition对象
/** Condition for waiting takes */
private final Condition notEmpty;
/** Condition for waiting puts */
private final Condition notFull; /**
* Circularly decrement i.
*/
final int dec(int i) {
return ((i == 0) ? items.length : i) - 1;
} @SuppressWarnings("unchecked")
static <E> E cast(Object item) {
return (E) item;
} /**
* Returns item at index i.
*/
final E itemAt(int i) {
return this.<E>cast(items[i]);
} /**
* Throws NullPointerException if argument is null.
*
* @param v
* the element
*/
private static void checkNotNull(Object v) {
if (v == null)
throw new NullPointerException();
} /**
* Deletes item at position i. Utility for remove and iterator.remove. Call
* only when holding lock.
*/
void removeAt(int i) {
final Object[] items = this.items;
// if removing front item, just advance
if (i == takeIndex) {
items[takeIndex] = null;
takeIndex = inc(takeIndex);
} else {
// slide over all others up through putIndex.
for (;;) {
int nexti = inc(i);
if (nexti != putIndex) {
items[i] = items[nexti];
i = nexti;
} else {
items[i] = null;
putIndex = i;
break;
}
}
}
--count;
notFull.signal();
} /**
* Creates an {@code ArrayBlockingQueue} with the given (fixed) capacity and
* default access policy.
*
* @param capacity
* the capacity of this queue
* @throws IllegalArgumentException
* if {@code capacity < 1}
*/
public ArrayBlockingQueue(int capacity) {
this(capacity, false);
} /**
* Creates an {@code ArrayBlockingQueue} with the given (fixed) capacity and
* the specified access policy.
*
* @param capacity
* the capacity of this queue
* @param fair
* if {@code true} then queue accesses for threads blocked on
* insertion or removal, are processed in FIFO order; if
* {@code false} the access order is unspecified.
* @throws IllegalArgumentException
* if {@code capacity < 1}
*/
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();
} /**
* Creates an {@code ArrayBlockingQueue} with the given (fixed) capacity,
* the specified access policy and initially containing the elements of the
* given collection, added in traversal order of the collection's iterator.
*
* @param capacity
* the capacity of this queue
* @param fair
* if {@code true} then queue accesses for threads blocked on
* insertion or removal, are processed in FIFO order; if
* {@code false} the access order is unspecified.
* @param c
* the collection of elements to initially contain
* @throws IllegalArgumentException
* if {@code capacity} is less than {@code c.size()}, or less
* than 1.
* @throws NullPointerException
* if the specified collection or any of its elements are null
*/
public ArrayBlockingQueue(int capacity, boolean fair, Collection<? extends E> 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();
}
} /**
* Inserts the specified element at the tail of this queue if it is possible
* to do so immediately without exceeding the queue's capacity, returning
* {@code true} upon success and throwing an {@code IllegalStateException}
* if this queue is full.
*
* @param e
* the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws IllegalStateException
* if this queue is full
* @throws NullPointerException
* if the specified element is null
*/
public boolean add(E e) {
return super.add(e);
} /**
* Inserts the specified element at the tail of this queue if it is possible
* to do so immediately without exceeding the queue's capacity, returning
* {@code true} upon success and {@code false} if this queue is full. This
* method is generally preferable to method {@link #add}, which can fail to
* insert an element only by throwing an exception.
*
* @throws NullPointerException
* if the specified element is null
*/
public boolean offer(E e) {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
if (count == items.length)
return false;
else {
insert(e);
return true;
}
} finally {
lock.unlock();
}
} /**
* Inserts the specified element at the tail of this queue, waiting up to
* the specified wait time for space to become available if the queue is
* full.
*
* @throws InterruptedException
* {@inheritDoc}
* @throws NullPointerException
* {@inheritDoc}
*/
public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException { checkNotNull(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == items.length) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
insert(e);
return true;
} finally {
lock.unlock();
}
} public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (count == 0) ? null : extract();
} finally {
lock.unlock();
}
} public E poll(long timeout, TimeUnit unit) throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0) {
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
return extract();
} finally {
lock.unlock();
}
} public E peek() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (count == 0) ? null : itemAt(takeIndex);
} finally {
lock.unlock();
}
} // this doc comment is overridden to remove the reference to collections
// greater in size than Integer.MAX_VALUE
/**
* Returns the number of elements in this queue.
*
* @return the number of elements in this queue
*/
public int size() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return count;
} finally {
lock.unlock();
}
} // this doc comment is a modified copy of the inherited doc comment,
// without the reference to unlimited queues.
/**
* Returns the number of additional elements that this queue can ideally (in
* the absence of memory or resource constraints) accept without blocking.
* This is always equal to the initial capacity of this queue less the
* current {@code size} of this queue.
*
* <p>
* Note that you <em>cannot</em> 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.
*/
public int remainingCapacity() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return items.length - count;
} finally {
lock.unlock();
}
} /**
* 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).
*
* <p>
* Removal of interior elements in circular array based queues is an
* intrinsically slow and disruptive operation, so should be undertaken only
* in exceptional circumstances, ideally only when the queue is known not to
* be accessible by other threads.
*
* @param o
* element to be removed from this queue, if present
* @return {@code true} if this queue changed as a result of the call
*/
public boolean remove(Object o) {
if (o == null)
return false;
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {
if (o.equals(items[i])) {
removeAt(i);
return true;
}
}
return false;
} finally {
lock.unlock();
}
} /**
* 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
*/
public boolean contains(Object o) {
if (o == null)
return false;
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
if (o.equals(items[i]))
return true;
return false;
} finally {
lock.unlock();
}
} /**
* Returns an array containing all of the elements in this queue, in proper
* sequence.
*
* <p>
* The returned array will be "safe" in that no references to it are
* maintained by this queue. (In other words, this method must allocate a
* new array). The caller is thus free to modify the returned array.
*
* <p>
* This method acts as bridge between array-based and collection-based APIs.
*
* @return an array containing all of the elements in this queue
*/
public Object[] toArray() {
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
final int count = this.count;
Object[] a = new Object[count];
for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
a[k] = items[i];
return a;
} finally {
lock.unlock();
}
} /**
* Returns an array containing all of the elements in this queue, in proper
* sequence; the runtime type of the returned array is that of the specified
* array. If the queue fits in the specified array, it is returned therein.
* Otherwise, a new array is allocated with the runtime type of the
* specified array and the size of this queue.
*
* <p>
* If this queue fits in the specified array with room to spare (i.e., the
* array has more elements than this queue), the element in the array
* immediately following the end of the queue is set to {@code null}.
*
* <p>
* Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs. Further, this method allows
* precise control over the runtime type of the output array, and may, under
* certain circumstances, be used to save allocation costs.
*
* <p>
* Suppose {@code x} is a queue known to contain only strings. The following
* code can be used to dump the queue into a newly allocated array of
* {@code String}:
*
* <pre>
* String[] y = x.toArray(new String[0]);
* </pre>
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a
* the array into which the elements of the queue are to be
* stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose
* @return an array containing all of the elements in this queue
* @throws ArrayStoreException
* if the runtime type of the specified array is not a supertype
* of the runtime type of every element in this queue
* @throws NullPointerException
* if the specified array is null
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
final int count = this.count;
final int len = a.length;
if (len < count)
a = (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), count);
for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
a[k] = (T) items[i];
if (len > count)
a[count] = null;
return a;
} finally {
lock.unlock();
}
} public String toString() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
int k = count;
if (k == 0)
return "[]"; StringBuilder sb = new StringBuilder();
sb.append('[');
for (int i = takeIndex;; i = inc(i)) {
Object e = items[i];
sb.append(e == this ? "(this Collection)" : e);
if (--k == 0)
return sb.append(']').toString();
sb.append(',').append(' ');
}
} finally {
lock.unlock();
}
} /**
* Atomically removes all of the elements from this queue. The queue will be
* empty after this call returns.
*/
public void clear() {
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
items[i] = null;
count = 0;
putIndex = 0;
takeIndex = 0;
notFull.signalAll();
} finally {
lock.unlock();
}
} /**
* @throws UnsupportedOperationException
* {@inheritDoc}
* @throws ClassCastException
* {@inheritDoc}
* @throws NullPointerException
* {@inheritDoc}
* @throws IllegalArgumentException
* {@inheritDoc}
*/
public int drainTo(Collection<? super E> c) {
checkNotNull(c);
if (c == this)
throw new IllegalArgumentException();
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
int i = takeIndex;
int n = 0;
int max = count;
while (n < max) {
c.add(this.<E>cast(items[i]));
items[i] = null;
i = inc(i);
++n;
}
if (n > 0) {
count = 0;
putIndex = 0;
takeIndex = 0;
notFull.signalAll();
}
return n;
} finally {
lock.unlock();
}
} /**
* @throws UnsupportedOperationException
* {@inheritDoc}
* @throws ClassCastException
* {@inheritDoc}
* @throws NullPointerException
* {@inheritDoc}
* @throws IllegalArgumentException
* {@inheritDoc}
*/
public int drainTo(Collection<? super E> c, int maxElements) {
checkNotNull(c);
if (c == this)
throw new IllegalArgumentException();
if (maxElements <= 0)
return 0;
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
int i = takeIndex;
int n = 0;
int max = (maxElements < count) ? maxElements : count;
while (n < max) {
c.add(this.<E>cast(items[i]));
items[i] = null;
i = inc(i);
++n;
}
if (n > 0) {
count -= n;
takeIndex = i;
notFull.signalAll();
}
return n;
} finally {
lock.unlock();
}
} /**
* Returns an iterator over the elements in this queue in proper sequence.
* The elements will be returned in order from first (head) to last (tail).
*
* <p>
* The returned {@code Iterator} is a "weakly consistent" iterator that will
* never throw {@link java.util.ConcurrentModificationException
* ConcurrentModificationException}, and guarantees to traverse elements as
* they existed upon construction of the iterator, and may (but is not
* guaranteed to) reflect any modifications subsequent to construction.
*
* @return an iterator over the elements in this queue in proper sequence
*/
public Iterator<E> iterator() {
return new Itr();
} /**
* 重点分析方法:
*
* @return
* @throws InterruptedException
*/
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
//解决伪唤醒
while (count == 0)
notEmpty.await();//如果缓冲区为空的话，则阻塞
return extract();
} finally {
//释放锁
lock.unlock();
}
} /**
* Extracts element at current take position, advances, and signals. Call
* only when holding lock.
*/
private E extract() {
final Object[] items = this.items;
//takeIndexe为消费数据
E x = this.<E>cast(items[takeIndex]);
items[takeIndex] = null;
// 计算下一个消费数据的位置
takeIndex = inc(takeIndex);
--count;
//通知生产者生产数据
notFull.signal();
return x;
} // Internal helper methods
/**
* Circularly increment i.
*/
final int inc(int i) {
return (++i == items.length) ? 0 : i;
} /**
* 重点分析方法:
*
* Inserts the specified element at the tail of this queue, waiting for
* space to become available if the queue is full.
*
* @throws InterruptedException
* {@inheritDoc}
* @throws NullPointerException
* {@inheritDoc}
*/
public void put(E e) throws InterruptedException {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
//解决伪唤醒
while (count == items.length)
notFull.await();//如果容器满了则阻塞此处
insert(e);
} finally {
// 释放锁
lock.unlock();
}
} /**
* Inserts element at current put position, advances, and signals. Call only
* when holding lock.
*/
private void insert(E x) {
items[putIndex] = x;
putIndex = inc(putIndex);
++count;
//唤醒消费者进行消费
notEmpty.signal();
} /**
* Iterator for ArrayBlockingQueue. To maintain weak consistency with
* respect to puts and takes, we (1) read ahead one slot, so as to not
* report hasNext true but then not have an element to return -- however we
* later recheck this slot to use the most current value; (2) ensure that
* each array slot is traversed at most once (by tracking "remaining"
* elements); (3) skip over null slots, which can occur if takes race ahead
* of iterators. However, for circular array-based queues, we cannot rely on
* any well established definition of what it means to be weakly consistent
* with respect to interior removes since these may require slot overwrites
* in the process of sliding elements to cover gaps. So we settle for
* resiliency, operating on established apparent nexts, which may miss some
* elements that have moved between calls to next.
*/
private class Itr implements Iterator<E> {
private int remaining; // Number of elements yet to be returned
private int nextIndex; // Index of element to be returned by next
private E nextItem; // Element to be returned by next call to next
private E lastItem; // Element returned by last call to next
private int lastRet; // Index of last element returned, or -1 if none Itr() {
final ReentrantLock lock = ArrayBlockingQueue.this.lock;
lock.lock();
try {
lastRet = -1;
if ((remaining = count) > 0)
nextItem = itemAt(nextIndex = takeIndex);
} finally {
lock.unlock();
}
} public boolean hasNext() {
return remaining > 0;
} public E next() {
final ReentrantLock lock = ArrayBlockingQueue.this.lock;
lock.lock();
try {
if (remaining <= 0)
throw new NoSuchElementException();
lastRet = nextIndex;
E x = itemAt(nextIndex); // check for fresher value
if (x == null) {
x = nextItem; // we are forced to report old value
lastItem = null; // but ensure remove fails
} else
lastItem = x;
while (--remaining > 0 && // skip over nulls
(nextItem = itemAt(nextIndex = inc(nextIndex))) == null)
;
return x;
} finally {
lock.unlock();
}
} public void remove() {
final ReentrantLock lock = ArrayBlockingQueue.this.lock;
lock.lock();
try {
int i = lastRet;
if (i == -1)
throw new IllegalStateException();
lastRet = -1;
E x = lastItem;
lastItem = null;
// only remove if item still at index
if (x != null && x == items[i]) {
boolean removingHead = (i == takeIndex);
removeAt(i);
if (!removingHead)
nextIndex = dec(nextIndex);
}
} finally {
lock.unlock();
}
} } }