在學習ReentrantLock時會不會有這樣的問題,爲併發包中的Lock啥要支持多個等待隊列?
爲啥ArrayBlockingQueue是一個鎖,兩個Condition?
ReentrantLock裏的sync變量(FairSync、NonfairSync都是繼承了Sync,Sync繼承了AbstractQueuedSynchronizer),AbstractQueuedSynchronizer的內部類ConditionObject實現了Condition接口
public class ConditionObject implements Condition, java.io.Serializable {
private static final long serialVersionUID = 1173984872572414699L;
/** First node of condition queue. */
private transient Node firstWaiter;
/** Last node of condition queue. */
private transient Node lastWaiter;
......
這裏的ArrayBlockingQueue源碼不是全部的,只是通過主要源碼分析阻塞隊列的消費者-生產者模型機制。
public class ArrayBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
/** The queued items */
final Object[] items;
//這裏通過takeIndex、putIndex下標將items數組做成環狀數組,
/** items index for next take, poll, peek or remove */
int takeIndex;
/** items index for next put, offer, or add */
int putIndex;
/** Number of elements in the queue */
int count;
/** Main lock guarding all access ,通過lock控制獲取元素、存放元素 */
final ReentrantLock lock;
/** Condition for waiting takes ,等待獲取元素條件*/
private final Condition notEmpty;
/** Condition for waiting puts ,等待存放元素條件*/
private final Condition notFull;
//從隊列中獲取一個元素
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
//如果數組中元素個數爲0,則該要從數組中獲取元素的消費者線程阻塞在notEmpty條件上
while (count == 0)
notEmpty.await();
//數組中的元素個數不爲0,則返回獲取元素
return dequeue();
} finally {
lock.unlock();
}
}
/**
* 提取元素,移動takeIndex下標,喚醒一個生成者線程,將生產的元素放入到數組中
* Extracts element at current take position, advances, and signals.
* Call only when holding lock.
*/
private E dequeue() {
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;
}
//向數組中存放一個生產出來的元素e
public void put(E e) throws InterruptedException {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
//如果數組已滿,則該生產者線程阻塞在notFull條件上
while (count == items.length)
notFull.await();
//向數組中插入元素
enqueue(e);
} finally {
lock.unlock();
}
}
private static void checkNotNull(Object v) {
if (v == null)
throw new NullPointerException();
}
/**
* 插入元素,移動putIndex下標,喚醒阻塞在notEmpty上的消費者線程
* Inserts element at current put position, advances, and signals.
* Call only when holding lock.
*/
private void enqueue(E x) {
final Object[] items = this.items;
items[putIndex] = x;
if (++putIndex == items.length)
putIndex = 0;
count++;
notEmpty.signal();
}
消費者線程是等待在notEmpty條件上,生產者是等待在notFull條件上,這樣就比較明確的知道等待在某個條件上的線程的功能是啥,能夠準確的喚醒需要某個功能(如生產元素、消費元素)的線程,讓其繼續工作。
爲啥ArrayBlockingQueue是一個鎖呢?因爲items數組、takeIndex、putIndex、count都是普通變量,線程從條件狀態喚醒後,必須獲得與Condition關聯的鎖,這樣線程進行這些變量的操作都是線程安全的。
/**
* Wakes up one waiting thread.
* 喚醒一個等待在Condition上的線程。該線程從等待方法返回前必須獲得與Condition關聯的鎖
*/
void signal();
如果使用synchronized來實現阻塞隊列的話,則需要使用兩個對象加鎖,一個鎖對象控制生產、一個鎖對象控制消費;