LockSupport

LockSupport.unpark(thread1);

会给当前指定的线程，发放一个许可，底层是给一个 count + 1 的操作，默认 count 为 0

LockSupport.park();

当前线程遇到 park 方法首先会检查 count 是否为 0，如果 count 为 0，则进行阻塞操作，等待许可，一旦发现许可，便可以解阻塞

java.util.concurrent.locks.Condition 类下的 await() 和 signal() 的实现就是用 LockSupport 来实现的

public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    Node node = addConditionWaiter();
    int savedState = fullyRelease(node);
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        LockSupport.park(this);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null) // clean up if cancelled
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
}

final boolean transferForSignal(Node node) {
    /*
         * If cannot change waitStatus, the node has been cancelled.
         */
    if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
        return false;
    /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
    Node p = enq(node);
    int ws = p.waitStatus;
    if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
        LockSupport.unpark(node.thread);
    return true;
}