java非公平锁知识点实例详解

 /**
     * 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;
        }
    }

/**
     * 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;
    }