创建网站目录权限/简述优化搜索引擎的方法

先看类简介：

/*** 生产者可以等待消费者接收元素，当消息在应用传递时，生产者通过 transfer()等待消费者调用 take()/poll()来接收元素，很有用。在其他时候，通过put()可以不等待接收。* tryTransfer()/tryTransfer(Object) Non-blocking()/tryTransfer(Object,long,TimeUnit) time-out版本也可用。* 也可以通过hasWaitingConsumer()来查询是否有线程在等待元素，这与peek()操作相反。 * 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.** 就像其他的阻塞队列,TransferQueue也可能有容量限制.如果这样，尝试的传输操作可能首先阻塞对可用空间的等待，并且/或随后阻塞对消费者接收的等待。* 注意，在一个容量为零的队列中，例如{@link SynchronousQueue}， {@code put}和{@code transfer}实际上是同义的。 * <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*/

从类简介中可知两点：

1、生产者通过 transfer()等待消费者调用 take()/poll()来接收元素，如果消费者没有获取元素，那么当此入队操作就是一次失败的入队操作。

2、SynchronousQueue 是一个容量为0的BlockingQueue队列，并非是TransferQueue队列，但它的put()和TransferQueue中的transfer()操作是同义的。

看类源码实现：

public interface TransferQueue<E> extends BlockingQueue<E> {/*** 等待 consumer之后，立马开始传输一个文件。* Transfers the element to a waiting consumer immediately, if possible.** 更严格来说，如果存在一个用户通过take()或者定时poll(),已经在等待接收,可以立马进行元素传输。否认元素没有入队列，并返回false.* 如果被添加的元素，阻止其被添加到队列中，返回false。* <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);/*** 如果需要的话，给消费者传递一个元素。* Transfers the element to a consumer, waiting if necessary to do so.** 更严格来说，如果存在一个用户通过take()或者定时poll(),已经在等待接收,可以立马进行元素传输。否则一直等待直到元素被消费者接收。* <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;/*** 在超时之前，如果可能的话，给消费者传递一个元素* Transfers the element to a consumer if it is possible to do so* before the timeout elapses.** 在元素被传递之前，等待指定的时间，超时则返回false。* <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}* 等待时间的单位* @param unit a {@code TimeUnit} determining how to interpret the*        {@code timeout} parameter* 成功的话返回true，否则返回false，此时元素没有离开队列。* @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* 等待的时候被打断，抛出 InterruptedException异常，此时元素没有离开队列。* @throws InterruptedException if interrupted while waiting,*         in which case the element is not left enqueued* 如果指定的元素拒绝被插入队列，抛出 ClassCastException异常。* @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* 如果指定元素的部分属性，拒绝被插入队列，抛出 IllegalArgumentException异常。* @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;/*** 如果至少有一个消费者通过take()/定时poll()在等待接收元素，返回事务瞬时状态值(true)* 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();/*** 返回正在通过take/定时poll接收元素的消费者预估数量。返回值是瞬时状态的预估值，如果消费者完成或放弃等待，该值可能不准确。* 该值作为监控和探索可能比较有用，但是不能作为同步控制。该方法实现，明显比 hasWaitingConsumer()慢。* 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();
}

 tryTransfer()：会立马返回结果，如果成功，就返回true，失败返回false；

 transfer()：会等待元素被消费者取走

tryTransfer(E e, long timeout, TimeUnit unit)：在指定的时间内等待元素被消费者接收，否则返回false；

getWaitingConsumerCount()：该方法只是一个大概的估计值，作为监控比较有用，但是不能作为同步的依据，并且会比hasWaitingConsumer()慢。