一：阻塞队列
阻塞队列的用法：
阻塞队列的根接口为BlockingQueue，实现这个接口的阻塞队列有很多。
阻塞队列也分为有界和无界的区分。判断有界和无界主要是根据初始化的时候队列的大小是否初始化，如果没有指定队列的大小就是无界队列，否则就是有界队列。
1：数组阻塞队列ArrayBlockingQueue
ArrayBlockingQueue是一个有界的阻塞队列，其内部实现是将对象放到一个数组里，队列内部是以一个先进先出的顺序进行储存，队列中的头元素在所有元素之中是放入时间最久的那个，而尾元素则是最短的那个。
/**
* 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();
}
}通过源码可以看到创建ArrayBlockingQueue队列的时候必须指定队列的容量，也就是说这是一个有界队列。
2：链表阻塞队列LinkedBlockingQueue
LinkedBlockingQueue内部以一个链式结构对其元素进行储存，内部以先进先出的顺序对元素进行储存。队列中的头元素在所有元素之中是放入时间最近的那个，而尾元素则是最短的那个。
/**
* 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);
}
/**
* Creates a {@code LinkedBlockingQueue} with a capacity of
* {@link Integer#MAX_VALUE}, initially containing the elements of the
* given collection,
* added in traversal order of the collection's iterator.
*
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection or any
* of its elements are null
*/
public LinkedBlockingQueue(Collection<? extends E> c) {
this(Integer.MAX_VALUE);
final ReentrantLock putLock = this.putLock;
putLock.lock(); // Never contended, but necessary for visibility
try {
int n = 0;
for (E e : c) {
if (e == null)
throw new NullPointerException();
if (n == capacity)
throw new IllegalStateException("Queue full");
enqueue(new Node<E>(e));
++n;
}
count.set(n);
} finally {
putLock.unlock();
}
}通过源码可以看到LinkedBlockingQueue初始化的时候如果不指定队列的大小，则容量的大小会默认为Integer.MAX_VALUE，当然我们也可以指定一个队列的大小，本质上这个阻塞队列叫有界队列。
3：延时阻塞队列DelayQueue
DelayQueue对元素进行持有直到一个特定的延迟到期。注入其中的元素必须实现java.util.concurrent.Delayed接口。
/**
* Returns the remaining delay associated with this object, in the
* given time unit.
*
* @param unit the time unit
* @return the remaining delay; zero or negative values indicate
* that the delay has already elapsed
*/
long getDelay(TimeUnit unit);根据返回的时间是0或者负值，延迟将被认为过期，该元素将会在DelayQueue的下一次take被调用的时候被释放掉。
/**
* Retrieves and removes the head of this queue, waiting if necessary
* until an element with an expired delay is available on this queue.
*
* @return the head of this queue
* @throws InterruptedException {@inheritDoc}
*/
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
for (;;) {
E first = q.peek();
if (first == null)
available.await();
else {
long delay = first.getDelay(NANOSECONDS);
if (delay <= 0)
return q.poll();
first = null; // don't retain ref while waiting
if (leader != null)
available.await();
else {
Thread thisThread = Thread.currentThread();
leader = thisThread;
try {
available.awaitNanos(delay);
} finally {
if (leader == thisThread)
leader = null;
}
}
}
}
} finally {
if (leader == null && q.peek() != null)
available.signal();
lock.unlock();
}
}通过源码分析可以看到延迟队列是一个无界队列。
/**
* Creates a new {@code DelayQueue} that is initially empty.
*/
public DelayQueue() {}
/**
* Creates a {@code DelayQueue} initially containing the elements of the
* given collection of {@link Delayed} instances.
*
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection or any
* of its elements are null
*/
public DelayQueue(Collection<? extends E> c) {
this.addAll(c);
}在下单之后半个小时之内支付完成的延时操作，可以通过延时队列进行实现。
4：优先阻塞队列PriorityBlockingQueue
PriorityBlockingQueue是一个无界队列，插入的元素必须实现java.util.Comparator接口，因此该队列中的元素的排序就取决于你自己的Comparable实现。当compare()==0的元素并不强制特定行为。
/**
* Default array capacity.
*/
private static final int DEFAULT_INITIAL_CAPACITY = 11;
/**
* Creates a {@code PriorityBlockingQueue} with the default
* initial capacity (11) that orders its elements according to
* their {@linkplain Comparable natural ordering}.
*/
public PriorityBlockingQueue() {
this(DEFAULT_INITIAL_CAPACITY, null);
}
/**
* Creates a {@code PriorityBlockingQueue} with the specified
* initial capacity that orders its elements according to their
* {@linkplain Comparable natural ordering}.
*
* @param initialCapacity the initial capacity for this priority queue
* @throws IllegalArgumentException if {@code initialCapacity} is less
* than 1
*/
public PriorityBlockingQueue(int initialCapacity) {
this(initialCapacity, null);
}
/**
* Creates a {@code PriorityBlockingQueue} with the specified initial
* capacity that orders its elements according to the specified
* comparator.
*
* @param initialCapacity the initial capacity for this priority queue
* @param comparator the comparator that will be used to order this
* priority queue. If {@code null}, the {@linkplain Comparable
* natural ordering} of the elements will be used.
* @throws IllegalArgumentException if {@code initialCapacity} is less
* than 1
*/
public PriorityBlockingQueue(int initialCapacity,
Comparator<? super E> comparator) {
if (initialCapacity < 1)
throw new IllegalArgumentException();
this.lock = new ReentrantLock();
this.notEmpty = lock.newCondition();
this.comparator = comparator;
this.queue = new Object[initialCapacity];
}根据源码可以看到如果没有指定队列中的容量大小，则会默认创建一个容量为11的队列。这也不能说明就是一个有界队列，
/**
* Tries to grow array to accommodate at least one more element
* (but normally expand by about 50%), giving up (allowing retry)
* on contention (which we expect to be rare). Call only while
* holding lock.
*
* @param array the heap array
* @param oldCap the length of the array
*/
private void tryGrow(Object[] array, int oldCap) {
lock.unlock(); // must release and then re-acquire main lock
Object[] newArray = null;
if (allocationSpinLock == 0 &&
UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
0, 1)) {
try {
int newCap = oldCap + ((oldCap < 64) ?
(oldCap + 2) : // grow faster if small
(oldCap >> 1));
if (newCap - MAX_ARRAY_SIZE > 0) { // possible overflow
int minCap = oldCap + 1;
if (minCap < 0 || minCap > MAX_ARRAY_SIZE)
throw new OutOfMemoryError();
newCap = MAX_ARRAY_SIZE;
}
if (newCap > oldCap && queue == array)
newArray = new Object[newCap];
} finally {
allocationSpinLock = 0;
}
}
if (newArray == null) // back off if another thread is allocating
Thread.yield();
lock.lock();
if (newArray != null && queue == array) {
queue = newArray;
System.arraycopy(array, 0, newArray, 0, oldCap);
}
}根据源码的tryGrow()方法，可以看到优先队列中有一个扩容机制，所以优先队列其实是一个无界队列。
5：同步阻塞队列SynchronousQueue
SynchronousQueue是一个特殊的队列，它的内部只能容纳单个元素，如果该队列已有一元素的话，试图向队列中插入一个新元素的线程将会阻塞，直到另一个线程将该元素从队列中抽走，同样，如果该队列为空，试图向队列中抽取一个元素的线程将会阻塞，直到另一个线程向队列中插入了一条新的元素。
6：阻塞双端队列BlockingDeque
阻塞双端队列的用法：
7：链表双端阻塞队列LinkedBlockingDeque
LinkedBlockingDeque实现了BlockingDeque接口，是一个双端队列，两端都可以抽取数据，和链表阻塞队列一样也是一个有界队列。
/**
* Creates a {@code LinkedBlockingDeque} with a capacity of
* {@link Integer#MAX_VALUE}.
*/
public LinkedBlockingDeque() {
this(Integer.MAX_VALUE);
}
/**
* Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
*
* @param capacity the capacity of this deque
* @throws IllegalArgumentException if {@code capacity} is less than 1
*/
public LinkedBlockingDeque(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
}
/**
* Creates a {@code LinkedBlockingDeque} with a capacity of
* {@link Integer#MAX_VALUE}, initially containing the elements of
* the given collection, added in traversal order of the
* collection's iterator.
*
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection or any
* of its elements are null
*/
public LinkedBlockingDeque(Collection<? extends E> c) {
this(Integer.MAX_VALUE);
final ReentrantLock lock = this.lock;
lock.lock(); // Never contended, but necessary for visibility
try {
for (E e : c) {
if (e == null)
throw new NullPointerException();
if (!linkLast(new Node<E>(e)))
throw new IllegalStateException("Deque full");
}
} finally {
lock.unlock();
}
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