共享鎖的乙個實現類就是訊號量semaphore，semaphores一般用於對某種訪問資源的限制，乙個訊號量相當於持有一些許可（permits），執行緒可以呼叫semaphore物件的acquire()方法獲取乙個許可，呼叫release()來歸還乙個許可。
同樣semaphore有公平和非公平兩種模式。預設就是非公平模式。
先看下aqs的acquireshared()方法
/**
* acquires in shared mode, ignoring interrupts. implemented by
* first invoking at least once ,
* returning on success. otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking until success.
** @param arg the acquire argument. this value is conveyed to
* but is otherwise uninterpreted
* and can represent anything you like.
*/public final void acquireshared(int arg)
/**
* acquires in shared uninterruptible mode.
* @param arg the acquire argument
*/private void doacquireshared(int arg)
}// 判斷狀態，尋找安全點，進入waiting狀態，等著被unpark()或interrupt()
if (shouldparkafte***iledacquire(p, node) &&
parkandcheckinterrupt())
interrupted = true;
}} finally
}
/**
* sets head of queue, and checks if successor may be waiting
* in shared mode, if so propagating if either propagate > 0 or
* propagate status was set.
** @param node the node
* @param propagate the return value from a tryacquireshared
*/private void setheadandpropagate(node node, int propagate)
}
tryacquireshared()嘗試獲取資源，成功則直接返回，否則進入流程2。
doacquireshared()會將當前執行緒加入等待佇列尾部休息，直到其他執行緒釋放資源喚醒自己。它還會嘗試著讓喚醒傳遞到後面的節點。
其實跟acquire()的流程大同小異，只不過多了個自己拿到資源後，還會去喚醒後繼隊友的操作（這才是共享嘛）。
接下來看下releaseshared()
/**
* releases in shared mode. implemented by unblocking one or more
* threads if returns true.
** @param arg the release argument. this value is conveyed to
* but is otherwise uninterpreted
* and can represent anything you like.
* @return the value returned from
*/public final boolean releaseshared(int arg)
return false;
}
/**
* release action for shared mode -- signal successor and ensure
* propagation. (note: for exclusive mode, release just amounts
* to calling unparksuccessor of head if it needs signal.)
*/private void doreleaseshared()
//如果後繼節點暫時不需要喚醒，則把當前節點狀態設定為propagate確保以後可以傳遞下去
else if (ws == 0 &&
!compareandsetwaitstatus(h, 0, node.propagate))
continue;
}//如果頭結點沒有發生變化，表示設定完成，退出迴圈
//如果頭結點發生變化，比如說其他執行緒獲取到了鎖，為了使自己的喚醒動作可以傳遞，必須進行重試
if (h == head)
break;
}}
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