BlockingQueue
阻塞队列接口继承自Queue接口，BlockingQueue接口提供了3个添加元素方法:
- add：添加元素到队列里，添加成功返回true，由于容量满了添加失败会抛出IllegalStateException异常
- offer：添加元素到队列里，添加成功返回true，添加失败返回false
- put：添加元素到队列里，如果容量满了会阻塞直到容量不满
3个删除方法: - poll：删除队列头部元素，如果队列为空，返回null。否则返回元素。
- remove：基于对象找到对应的元素，并删除。删除成功返回true，否则返回false
-
take：删除队列头部元素，如果队列为空，一直阻塞到队列有元素并删除
常用的阻塞队列具体类有ArrayBlockingQueue、LinkedBlockingQueue、LinkedBlockingDeque、PriorityBlockingQueue、DelayQueue、SynchronousQueue、LinkedTransferQueue等。
ArrayBlockingQueue
由数组实现的阻塞有界队列，FIFO。支持对等待的生产者线程和使用者线程进行排序的可选公平策略。
定义：
public class ArrayBlockingQueue extends AbstractQueue
implements BlockingQueue, java.io.Serializable {
/** 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;
/** Number of elements in the queue */
int count;
/** Main lock guarding all access */
//一把锁控制出列和入列
final ReentrantLock lock;
/** Condition for waiting takes */
private final Condition notEmpty;
/** Condition for waiting puts */
private final Condition notFull;
}
//fair为true表示公平锁
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();
}
LinkedBlockingQueue
由单向链表实现的无界阻塞队列（容量可选，默认为Integer.MAX_VALUE），内部入列和出列由不同的锁控制，队列大小定义为AtomicInteger 可随时可见。
定义
public class LinkedBlockingQueue extends AbstractQueue
implements BlockingQueue, java.io.Serializable {
/**
* Linked list node class
*/
static class Node {
E item;
/**
* One of:
* - the real successor Node
* - this Node, meaning the successor is head.next
* - null, meaning there is no successor (this is the last node)
*/
Node next;
Node(E x) { item = x; }
}
/** The capacity bound, or Integer.MAX_VALUE if none */
private final int capacity;
/** Current number of elements */
private final AtomicInteger count = new AtomicInteger();
/**
* Head of linked list.
* Invariant: head.item == null
*/
transient Node head;
/**
* Tail of linked list.
* Invariant: last.next == null
*/
private transient Node last;
/** Lock held by take, poll, etc */
private final ReentrantLock takeLock = new ReentrantLock();
/** Wait queue for waiting takes */
private final Condition notEmpty = takeLock.newCondition();
/** Lock held by put, offer, etc */
private final ReentrantLock putLock = new ReentrantLock();
/** Wait queue for waiting puts */
private final Condition notFull = putLock.newCondition();
}
LinkedBlockingDeque
由双向链表构成的可选容量队列，由一把锁控制入列和出列
定义
public class LinkedBlockingDeque
extends AbstractQueue
implements BlockingDeque, java.io.Serializable {
/** Doubly-linked list node class */
static final class Node {
/**
* The item, or null if this node has been removed.
*/
E item;
/**
* One of:
* - the real predecessor Node
* - this Node, meaning the predecessor is tail
* - null, meaning there is no predecessor
*/
Node prev;
/**
* One of:
* - the real successor Node
* - this Node, meaning the successor is head
* - null, meaning there is no successor
*/
Node next;
Node(E x) {
item = x;
}
}
/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
*/
transient Node first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
*/
transient Node last;
/** Number of items in the deque */
private transient int count;
/** Maximum number of items in the deque */
private final int capacity;
/** Main lock guarding all access */
final ReentrantLock lock = new ReentrantLock();
/** Condition for waiting takes */
private final Condition notEmpty = lock.newCondition();
/** Condition for waiting puts */
private final Condition notFull = lock.newCondition();
public LinkedBlockingDeque() {
this(Integer.MAX_VALUE);
}
public LinkedBlockingDeque(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
}
PriorityBlockingQueue
拥有优先级的阻塞队列，基于最小堆实现的有界队列。
public class PriorityBlockingQueue extends AbstractQueue
implements BlockingQueue, java.io.Serializable {
/**
* Default array capacity.
*/
private static final int DEFAULT_INITIAL_CAPACITY = 11;
/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
private transient Object[] queue;
/**
* The number of elements in the priority queue.
*/
private transient int size;
/**
* The comparator, or null if priority queue uses elements'
* natural ordering.
*/
private transient Comparator comparator;
/**
* Lock used for all public operations
*/
private final ReentrantLock lock;
/**
* Condition for blocking when empty
*/
private final Condition notEmpty;
/**
* Spinlock for allocation, acquired via CAS.扩容时需要
*/
private transient volatile int allocationSpinLock;
/**
* A plain PriorityQueue used only for serialization,
* to maintain compatibility with previous versions
* of this class. Non-null only during serialization/deserialization.
*/
private PriorityQueue q;
public PriorityBlockingQueue() {
//11
this(DEFAULT_INITIAL_CAPACITY, null);
}
public PriorityBlockingQueue(int initialCapacity) {
this(initialCapacity, null);
}
public PriorityBlockingQueue(int initialCapacity,
Comparator comparator) {
if (initialCapacity < 1)
throw new IllegalArgumentException();
this.lock = new ReentrantLock();
this.notEmpty = lock.newCondition();
this.comparator = comparator;
this.queue = new Object[initialCapacity];
}
}
DelayQueue
DelayQueue是一个支持延时操作的阻塞队列。列头的元素是最先“到期”的元素，如果队列里面没有元素到期，是不能从列头获取元素的，哪怕有元素也不行。也就是说只有在延迟期满时才能够从队列中去元素。
它主要运用于如下场景：
缓存系统的设计：缓存是有一定的时效性的，可以用DelayQueue保存缓存的有效期，然后利用一个线程查询DelayQueue，如果取到元素就证明该缓存已经失效了。
定时任务的调度：DelayQueue保存当天将要执行的任务和执行时间，一旦取到元素（任务），就执行该任务。
DelayQueue实现的关键主要有如下几个：
- 可重入锁ReentrantLock
- 用于阻塞和通知的Condition对象
- 根据Delay时间排序的优先级队列：PriorityQueue
- 用于优化阻塞通知的线程元素leader
定义
public class DelayQueue extends AbstractQueue
implements BlockingQueue {
private final transient ReentrantLock lock = new ReentrantLock();
private final PriorityQueue q = new PriorityQueue();
private Thread leader = null;
private final Condition available = lock.newCondition();
public DelayQueue() {}
Delayed 接口
public interface Delayed extends Comparable {
/**
* Returns the remaining delay associated with this object, in the
* given time unit.
*/
long getDelay(TimeUnit unit);
}
SynchronousQueue
作为BlockingQueue中的一员，SynchronousQueue与其他BlockingQueue有着不同特性：
- SynchronousQueue没有容量。与其他BlockingQueue不同，SynchronousQueue是一个不存储元素的BlockingQueue。每一个put操作必须要等待一个take操作，否则不能继续添加元素，反之亦然。
- 因为没有容量，所以对应 peek, contains, clear, isEmpty … 等方法其实是无效的。例如clear是不执行任何操作的，contains始终返回false,peek始终返回null。
- SynchronousQueue分为公平和非公平，默认情况下采用非公平性访问策略，当然也可以通过构造函数来设置为公平性访问策略（为true即可）。
- 若使用 TransferQueue, 则队列中永远会存在一个 dummy node（这点后面详细阐述）。
SynchronousQueue非常适合做交换工作，生产者的线程和消费者的线程同步以传递某些信息、事件或者任务。
LinkedTransferQueue
基于链表的FIFO无界阻塞队列,不会锁住整个队列。
LinkedTransferQueue采用一种预占模式。什么意思呢:
有就直接拿走，没有就占着这个位置直到拿到或者超时或者中断。即消费者线程到队列中取元素时，如果发现队列为空，则会生成一个null节点，然后park住等待生产者。后面如果生产者线程入队时发现有一个null元素节点，这时生产者就不会入列了，直接将元素填充到该节点上，唤醒该节点的线程，被唤醒的消费者线程拿东西走人。