Interface - java.util.concurrent.TransferQueue
Created by : Mr Dk.
2020 / 10 / 22 13:44
Nanjing, Jiangsu, China
Definition
继承自 BlockingQueue。BlockingQueue 的出队和入队需要上锁后进行操作，而 TransferQueue 会额外维护正在等待资源的线程，当资源可用时，资源将不需要入队而是直接被转移给等待资源的线程。
/**
* A {@link BlockingQueue} in which producers may wait for consumers
* to receive elements. A {@code TransferQueue} may be useful for
* example in message passing applications in which producers
* sometimes (using method {@link #transfer}) await receipt of
* elements by consumers invoking {@code take} or {@code poll}, while
* at other times enqueue elements (via method {@code put}) without
* waiting for receipt.
* {@linkplain #tryTransfer(Object) Non-blocking} and
* {@linkplain #tryTransfer(Object,long,TimeUnit) time-out} versions of
* {@code tryTransfer} are also available.
* A {@code TransferQueue} may also be queried, via {@link
* #hasWaitingConsumer}, whether there are any threads waiting for
* items, which is a converse analogy to a {@code peek} operation.
*
* <p>Like other blocking queues, a {@code TransferQueue} may be
* capacity bounded. If so, an attempted transfer operation may
* initially block waiting for available space, and/or subsequently
* block waiting for reception by a consumer. Note that in a queue
* with zero capacity, such as {@link SynchronousQueue}, {@code put}
* and {@code transfer} are effectively synonymous.
*
* <p>This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.7
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
public interface TransferQueue<E> extends BlockingQueue<E> {
}
Transfer
非阻塞地尝试将元素直接转移给一个正在等待中的消费者。如果没有等待中的消费者，那么不进行入队操作，直接返回 false。
/**
* Transfers the element to a waiting consumer immediately, if possible.
*
* <p>More precisely, transfers the specified element immediately
* if there exists a consumer already waiting to receive it (in
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
* otherwise returning {@code false} without enqueuing the element.
*
* @param e the element to transfer
* @return {@code true} if the element was transferred, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean tryTransfer(E e);
阻塞定时版本，在超时后如果还没有被消费则返回 false。
/**
* Transfers the element to a consumer if it is possible to do so
* before the timeout elapses.
*
* <p>More precisely, transfers the specified element immediately
* if there exists a consumer already waiting to receive it (in
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
* else waits until the element is received by a consumer,
* returning {@code false} if the specified wait time elapses
* before the element can be transferred.
*
* @param e the element to transfer
* @param timeout how long to wait before giving up, in units of
* {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the
* {@code timeout} parameter
* @return {@code true} if successful, or {@code false} if
* the specified waiting time elapses before completion,
* in which case the element is not left enqueued
* @throws InterruptedException if interrupted while waiting,
* in which case the element is not left enqueued
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean tryTransfer(E e, long timeout, TimeUnit unit)
throws InterruptedException;
阻塞版本。将一直阻塞直到被消费者消费。
/**
* Transfers the element to a consumer, waiting if necessary to do so.
*
* <p>More precisely, transfers the specified element immediately
* if there exists a consumer already waiting to receive it (in
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
* else waits until the element is received by a consumer.
*
* @param e the element to transfer
* @throws InterruptedException if interrupted while waiting,
* in which case the element is not left enqueued
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
void transfer(E e) throws InterruptedException;
Consumer
返回是否有正在等待的消费者：
/**
* Returns {@code true} if there is at least one consumer waiting
* to receive an element via {@link #take} or
* timed {@link #poll(long,TimeUnit) poll}.
* The return value represents a momentary state of affairs.
*
* @return {@code true} if there is at least one waiting consumer
*/
boolean hasWaitingConsumer();
返回正在等待的消费者个数：
/**
* Returns an estimate of the number of consumers waiting to
* receive elements via {@link #take} or timed
* {@link #poll(long,TimeUnit) poll}. The return value is an
* approximation of a momentary state of affairs, that may be
* inaccurate if consumers have completed or given up waiting.
* The value may be useful for monitoring and heuristics, but
* not for synchronization control. Implementations of this
* method are likely to be noticeably slower than those for
* {@link #hasWaitingConsumer}.
*
* @return the number of consumers waiting to receive elements
*/
int getWaitingConsumerCount();