Returns {@code true} if the apparent first queued thread, if one
exists, is waiting in exclusive mode. If this method returns
{@code true}, and the current thread is attempting to acquire in
shared mode (that is, this method is invoked from {@link
#tryAcquireShared}) then it is guaranteed that the current thread
is not the first queued thread. Used only as a heuristic in
ReentrantReadWriteLock.
Returns {@code true} if the apparent first queued thread, if one exists, is waiting in exclusive mode. If this method returns {@code true}, and the current thread is attempting to acquire in shared mode (that is, this method is invoked from {@link #tryAcquireShared}) then it is guaranteed that the current thread is not the first queued thread. Used only as a heuristic in ReentrantReadWriteLock.