Java常用阻塞隊列
ArrayBlockingQueue
內部由一個固定長度的數組來實現阻塞隊列
/** The queued items */
final Object[] items;
/** items index for next take, poll, peek or remove */
int takeIndex;
/** items index for next put, offer, or add */
int putIndex;
public ArrayBlockingQueue(int capacity, boolean fair) {
if (capacity <= 0)
throw new IllegalArgumentException();
/** 定長數組 */
this.items = new Object[capacity];
lock = new ReentrantLock(fair);
notEmpty = lock.newCondition();
notFull = lock.newCondition();
}
提供了兩個入隊操作方法,offer()和put()
offer方法不會阻塞,但有返回值,如果隊列滿了,那麼直接返回false,否則插入數據並返回true。
/**
* 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 {
enqueue(e);
return true;
}
} finally {
lock.unlock();
}
}
put()會在隊列滿了的時候會阻塞生產者線程,知道有消費者線程消費後將其喚醒。
public void put(E e) throws InterruptedException {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == items.length)
notFull.await();
enqueue(e);
} finally {
lock.unlock();
}
}
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(); // 出隊後喚醒生產者線程
return x;
}
LinkedBlockingQueue
基於鏈表的阻塞隊列,同ArrayListBlockingQueue類似,其內部也維持着一個數據緩衝隊列(該隊列由一個鏈表構成),當生產者往隊列中放入一個數據時,隊列會從生產者手中獲取數據,並緩存在隊列內部,而生產者立即返回;只有當隊列緩衝區達到最大值緩存容量時,纔會阻塞生產者隊列,直到消費者從隊列中消費掉一份數據,生產者線程會被喚醒,反之對於消費者這端的處理也基於同樣的原理。
需要注意的是,如果構造一個LinkedBlockingQueue對象,而沒有指定其容量大小,LinkedBlockingQueue會默認一個類似無限大小的容量(Integer.MAX_VALUE),這樣的話,如果生產者的速度一旦大於消費者的速度,也許還沒有等到隊列滿阻塞產生,系統內存就有可能已被消耗殆盡了。
/**
* Creates a {@code LinkedBlockingQueue} with a capacity of
* {@link Integer#MAX_VALUE}.
*/
public LinkedBlockingQueue() {
this(Integer.MAX_VALUE);
}
/**
* Creates a {@code LinkedBlockingQueue} with the given (fixed) capacity.
*
* @param capacity the capacity of this queue
* @throws IllegalArgumentException if {@code capacity} is not greater
* than zero
*/
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
last = head = new Node<E>(null);
}
使用 BlockingQueue 實現生產者消費者問題
public class ProducerConsumer {
private static BlockingQueue<String> queue = new ArrayBlockingQueue<>(5);
private static class Producer extends Thread {
@Override
public void run() {
try {
queue.put("product");
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.print("produce..");
}
}
private static class Consumer extends Thread {
@Override
public void run() {
try {
String product = queue.take();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.print("consume..");
}
}
}
public static void main(String[] args) {
for (int i = 0; i < 2; i++) {
Producer producer = new Producer();
producer.start();
}
for (int i = 0; i < 5; i++) {
Consumer consumer = new Consumer();
consumer.start();
}
for (int i = 0; i < 3; i++) {
Producer producer = new Producer();
producer.start();
}
}
output:
produce..produce..consume..consume..produce..consume..produce..consume..produce..consume..