Java之BlockingQueue
再看BlockingQueue之前，首先得了解什么是Queue，以解了解Queue的逻辑结构还有特性，了解ReentrantLock可重入锁是什么。
BlockingQueue是阻塞队列，多应用于多线程开发，线程池。
这里讲解基本的操作，如添加、删除、返回队首元素、清除队列方式 | 抛出异常 | 有返回值，不抛出异常 | 阻塞等待 | 超时等待 |
添加 | add() | offer() | pull() | offer() |
移除 | remove() | poll() | take() | poll() |
检测队首元素 | element() | peak() |
第一种add() remove() element() 继承自AbstractQueue.java/**
* 抛出异常 add remove
*/
public static void test1(){
//队列的大小
ArrayBlockingQueue blockingQueue = new ArrayBlockingQueue(3);//3代表容量
System.out.println(blockingQueue.add("a"));
System.out.println(blockingQueue.add("b"));
System.out.println(blockingQueue.add("c"));
//IllegalStateException: Queue full 抛出异常！
System.out.println(blockingQueue.add("d"));
System.out.println(blockingQueue.element());//查看队首元素是谁
System.out.println(blockingQueue.remove());
System.out.println(blockingQueue.remove());
System.out.println(blockingQueue.remove());
System.out.println(blockingQueue.remove());
//NoSuchElementException 抛出异常!
}首先看一下ArrayBlockingQueue()的源码，会发现ArrayBlockingQueue()源码在ArrayBlockingQueue.java下
//ArrayBlockingQueue.java
public ArrayBlockingQueue(int capacity) {
this(capacity, false);
}继续点开this
//ArrayBlockingQueue.java
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();
}这就是底层的代码，应该都看得懂吧，多看看源码，还是非常有帮助的
add()方法点开源码我们发现add()方法来自于ArrayBlockingQueue.java
//ArrayBlockingQueue.java
public boolean add(E e) {
return super.add(e);
}继续点super后面的add(e)，会发现ArrayBlockingQueue.java里面的add()方法继承于AbstractQueue.java
//AbstractQueue.java
public boolean add(E e) {
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}看清楚，这里注意，当队列满的时候，会抛出IllegalStateException(“Queue full”)异常
还有源码中，add()方法的实现基于offer()方法。
remove()方法同理看一下源码
//AbstractQueue.java
public E remove() {
E x = poll();
if (x != null)
return x;
else
throw new NoSuchElementException();
}当队列为空时，继续移除会抛出NoSuchElementException()异常，remove()方法实现基于poll()方法
element()方法//AbstractQueue.java
public E element() {
E x = peek();
if (x != null)
return x;
else
throw new NoSuchElementException();
}有关于offer()、poll()、peek()方法下面会讲解到
第二种offer()、poll()、peek()方法/**
* 有返回值，没有异常 offer poll
*/
public static void test2(){
ArrayBlockingQueue blockingQueue = new ArrayBlockingQueue(2);
System.out.println(blockingQueue.offer("a"));
System.out.println(blockingQueue.offer("b"));
//System.out.println(blockingQueue.offer("c"));
System.out.println(blockingQueue.peek());
System.out.println(blockingQueue.poll());//false 不抛出异常！
System.out.println(blockingQueue.poll());
System.out.println(blockingQueue.poll());
//System.out.println(blockingQueue.poll());//null 返回null
}直接看方法的源码吧，方便理解
offer()方法//ArrayBlockingQueue.java
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();
}
}可以看到这里加入了可重入锁，保证了线程安全
如果对队列如何存不了解的话，可以点开enqueue(e)
//ArrayBlockingQueue.java
private void enqueue(E x) {
// assert lock.getHoldCount() == 1;
// assert items[putIndex] == null;
final Object[] items = this.items;
items[putIndex] = x;
if (++putIndex == items.length)
putIndex = 0;
count++;
notEmpty.signal();
}自己理解一下
poll()方法//ArrayBlockingQueue.java
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (count == 0) ? null : dequeue();
} finally {
lock.unlock();
}
}dequeue()队列取源码
//ArrayBlockingQueue.java
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;
}peek()方法//ArrayBlockingQueue.java
public E peek() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return itemAt(takeIndex); // null when queue is empty
} finally {
lock.unlock();
}
}第三种offer(E e, long timeout, TimeUnit unit)，poll(long timeout, TimeUnit unit)/**
* 等待，阻塞（等待超时）
*/
public static void test3() throws InterruptedException{
ArrayBlockingQueue blockingQueue = new ArrayBlockingQueue(3);
blockingQueue.offer("a");
blockingQueue.offer("b");
blockingQueue.offer("c");
blockingQueue.offer("d",2, TimeUnit.SECONDS);
System.out.println(blockingQueue.poll());
System.out.println(blockingQueue.poll());
System.out.println(blockingQueue.poll());
System.out.println(blockingQueue.poll(3, TimeUnit.SECONDS));
}这里直接上源码，就是多了参数
//ArrayBlockingQueue.java
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);
}
enqueue(e);
return true;
} finally {
lock.unlock();
}
}//ArrayBlockingQueue.java
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();
}
}这里可能对TimeUnit不了解，以及toNanos方法，这里讲一下
//TimeUnit.java部分代码
public enum TimeUnit {
/**
* Time unit representing one thousandth of a microsecond
*/
NANOSECONDS {
public long toNanos(long d) { return d; }
public long toMicros(long d) { return d/(C1/C0); }
public long toMillis(long d) { return d/(C2/C0); }
public long toSeconds(long d) { return d/(C3/C0); }
public long toMinutes(long d) { return d/(C4/C0); }
public long toHours(long d) { return d/(C5/C0); }
public long toDays(long d) { return d/(C6/C0); }
public long convert(long d, TimeUnit u) { return u.toNanos(d); }
int excessNanos(long d, long m) { return (int)(d - (m*C2)); }
},这里可以看到toNanos方法，可能还是不太了解但是翻到TimeUnit.java的最后会看到这么一个方法
//TimeUnit.java
public void sleep(long timeout) throws InterruptedException {
if (timeout > 0) {
long ms = toMillis(timeout);
int ns = excessNanos(timeout, ms);
Thread.sleep(ms, ns);
}
}是不是很熟悉，如果知道Thread.sleep(),方法的就会发现，这里怎么也有sleep()方法，其实是Thread.sleep()方法的包装，这里将月、日、时、分、秒封装成方法，提供了更强的可读性，建议使用TimeUnit
第四种pu()t take()/**
* 等待，阻塞（一直阻塞） put take
*/
public static void test4() throws InterruptedException {
ArrayBlockingQueue blockingQueue = new ArrayBlockingQueue(3);
blockingQueue.put("a");
blockingQueue.put("b");
blockingQueue.put("c");
//blockingQueue.put("d");
System.out.println(blockingQueue.take());
System.out.println(blockingQueue.take());
System.out.println(blockingQueue.take());
System.out.println(blockingQueue.take());
}直接上源码，对照offer()和poll()看
//ArrayBlockingQueue.java
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();
}
}//ArrayBlockingQueue.java
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0)
notEmpty.await();
return dequeue();
} finally {
lock.unlock();
}
}请大家注意对比一下两个的返回值
有什么问题的话，请大家赐教
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