ReentrantLock为啥会出现不公平的场景

 

 

    不公平锁产生的原因的是acquire在enqueue之前，这样后来的线程有可能会较在等待队列中的等待者之前获得锁。barging这个词可以理解为闯入，插队。不公平锁会导致某些线程总是不排队就获得锁，而同时排在队尾的线程总是出现饥饿状态。那么如何获取公平锁呢，Doug Lee页给出了方案，很简单在acquire的时候，首先检查当前线程是否在队列中（对首），如在队列中即可获得，如不在队列中，则乖乖得排在队尾睡大觉(parking)等着排在前面的线程叫醒它(unparking)。

     公平锁为啥会影响性能，从code上来看看公平锁仅仅是多了一项检查是否在队首会影响性能，如不是，那么又是在什么地方影响的？假如是闯入的线程，会排在队尾并睡觉(parking)等待前任节点唤醒，这样势必会比非公平锁添加很多paking和unparking的操作，我想这是影响公平锁性能的主要原因。

 

 

 

E文不好，不敢翻译，怕误了各位看官，还是直接贴原文吧

 

* <p><a name="barging">Because checks in acquire are invoked before

 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of

 * others that are blocked and queued.  However, you can, if desired,

 * define <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to

 * disable barging by internally invoking one or more of the inspection

 * methods, thereby providing a <em>fair</em> FIFO acquisition order.

 * In particular, most fair synchronizers can define <tt>tryAcquire</tt>

 * to return <tt>false</tt> if {@link #hasQueuedPredecessors} (a method

 * specifically designed to be used by fair synchronizers) returns

 * <tt>true</tt>.  Other variations are possible.

 *

 * <p>Throughput and scalability are generally highest for the

 * default barging (also known as <em>greedy</em>,

 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.

 * While this is not guaranteed to be fair or starvation-free, earlier

 * queued threads are allowed to recontend before later queued

 * threads, and each recontention has an unbiased chance to succeed

 * against incoming threads.  Also, while acquires do not

 * &quot;spin&quot; in the usual sense, they may perform multiple

 * invocations of <tt>tryAcquire</tt> interspersed with other

 * computations before blocking.  This gives most of the benefits of

 * spins when exclusive synchronization is only briefly held, without

 * most of the liabilities when it isn't. If so desired, you can

 * augment this by preceding calls to acquire methods with

 * "fast-path" checks, possibly prechecking {@link #hasContended}

 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer

 * is likely not to be contended.