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java非公平鎖知識(shí)點(diǎn)實(shí)例詳解

更新時(shí)間：2021年10月21日 08:33:15 作者：小妮淺淺

在本篇文章里小編給大家整理了一篇關(guān)于java非公平鎖知識(shí)點(diǎn)實(shí)例詳解，有興趣的朋友們可以學(xué)習(xí)參考下。

1、非公平鎖不能保證鎖的獲取是按照請(qǐng)求鎖的順序進(jìn)行的。這可能會(huì)導(dǎo)致某個(gè)或某些線程永遠(yuǎn)得不到鎖。

2、CPU喚醒線程的費(fèi)用可以降低，整體吞吐效率會(huì)很高。但是可能會(huì)有線程長(zhǎng)時(shí)間甚至永遠(yuǎn)得不到鎖，導(dǎo)致餓死。

實(shí)例

 /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
 
        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }
 
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }
 
    /**
     * Sync object for fair locks
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;
 
        final void lock() {
            acquire(1);
        }
 
        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

知識(shí)點(diǎn)擴(kuò)展：

非公平鎖，顧名思義，各個(gè)線程獲取到鎖的順序，不一定和它們申請(qǐng)的先后順序一致，有可能后來(lái)的線程，反而先獲取到了鎖。

在實(shí)現(xiàn)上，公平鎖在進(jìn)行l(wèi)ock時(shí)，首先會(huì)進(jìn)行tryAcquire()操作。在tryAcquire中，會(huì)判斷等待隊(duì)列中是否已經(jīng)有別的線程在等待了。如果隊(duì)列中已經(jīng)有別的線程了，則tryAcquire失敗，則將自己加入隊(duì)列。如果隊(duì)列中沒(méi)有別的線程，則進(jìn)行獲取鎖的操作。

/**
     * Fair version of tryAcquire. Don't grant access unless
     * recursive call or no waiters or is first.
     **/
    protected final boolean tryAcquire(int acquires) {
      final Thread current = Thread.currentThread();
      int c = getState();
      if (c == 0) {
        if (!hasQueuedPredecessors() &&
          compareAndSetState(0, acquires)) {
          setExclusiveOwnerThread(current);
          return true;
        }
      }
      else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0)
          throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
      }
      return false;
    }

非公平鎖，在進(jìn)行l(wèi)ock時(shí)，會(huì)直接嘗試進(jìn)行加鎖，如果成功，則獲取到鎖，如果失敗，則進(jìn)行和公平鎖相同的動(dòng)作。

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