在上一節,介紹了ArrayBlockingQueue的添加元素的方法,本節,結合源碼給大家介紹一下獲取元素的方法。
獲取元素的方法有下述幾種
E poll() 立刻返回,如果隊列爲空,返回null
E take() 如果隊列不爲空,返回隊首元素,否則阻塞到隊列不爲空
E poll(long timeout, TimeUnit unit) 等待timeout 時間的poll
E peek() 獲得隊首的元素,並不將這個元素彈出
分別看一下這幾個方法的源碼
1.E poll()
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (count == 0) ? null : dequeue();
} finally {
lock.unlock();
}
}
此方法先獲得lock,如果lock獲得成功,會先去判斷count 的值,如果這個值爲0,那就說明此隊列爲空,立刻返回null
2.E take()
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0)
notEmpty.await();
return dequeue();
} finally {
lock.unlock();
}
}
take 方法和poll方法的區別就在於,如果count爲0,那麼在notEmpty上等待,直到被喚醒,這裏有個需要遵循的原則,await/wait方法調用的時候,一定要確保本線程持有依賴的狀態(這裏邊爲count)的鎖,並且被喚醒之後,依舊要去檢查依賴條件的狀態,這是《JAVA 併發編程實戰》推薦的使用方式,原因此處不再多說。
3.E poll(long timeout, TimeUnit unit)
我們看到此方法傳遞了一個超時時間,也就是在條件隊列上等待相應的時間,如果等待超過這個時間,那就返回空。
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 dequeue();
} finally {
lock.unlock();
}
}
public E peek() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return itemAt(takeIndex); // null when queue is empty
} finally {
lock.unlock();
}
}
5. 再來分析一下出隊的方法,這個方法是幾個獲取方法的核心
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;
if (++takeIndex == items.length)
takeIndex = 0;
count--;
if (itrs != null)
itrs.elementDequeued();
notFull.signal();
return x;
}
看這個方法,感覺不難,只是將takeIndex的內容取出來,並將該位置的元素設置爲空,然後檢查一下隊列是不是空了,然後喚醒在notFull條件等待的線程,最後操作一下迭代器
6.remove ,
/**
* 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 {
if (count > 0) {
final int putIndex = this.putIndex;
int i = takeIndex;
do {
if (o.equals(items[i])) {
removeAt(i);
return true;
}
if (++i == items.length)
i = 0;
} while (i != putIndex);
}
return false;
} finally {
lock.unlock();
}
}
這個remove方法,首先判斷對列是否有元素,然後從takeIndex 開始調用equals 方法,判斷是否是相同的元素,如果相同,會去調removeAt(i);
我們再看removeAt這個方法,ArrayBlockingQueue支持刪除任意位置上的元素
void removeAt(final int removeIndex) {
// assert lock.getHoldCount() == 1;
// assert items[removeIndex] != null;
// assert removeIndex >= 0 && removeIndex < items.length;
final Object[] items = this.items;
if (removeIndex == takeIndex) {
// removing front item; just advance
items[takeIndex] = null;
if (++takeIndex == items.length)
takeIndex = 0;
count--;
if (itrs != null)
itrs.elementDequeued();
} else {
// an "interior" remove
// slide over all others up through putIndex.
final int putIndex = this.putIndex;
for (int i = removeIndex;;) {
int next = i + 1;
if (next == items.length)
next = 0;
if (next != putIndex) {
items[i] = items[next];
i = next;
} else {
items[i] = null;
this.putIndex = i;
break;
}
}
count--;
if (itrs != null)
itrs.removedAt(removeIndex);
}
notFull.signal();
}
這個方法,首先判斷了需要刪除的元素是否是takeIndex所指向的元素,如果是,只需要將這個元素設置爲null,並修改一下takeIndex 就好,但是如果不是,就需要將後面的元素全都往前移動一個位置,並且,迭代器也將固定位置的元素刪除。
看到現在,貌似沒有發現迭代器有什麼明顯的作用,暫且先認爲他是這個ArrayBlockingQueue的一個鏈表實現的副本,等下節我們再研究