Finally, there's not currently space in the mark word to support both an identity hashCode() value as well as the thread ID needed for the biased locking encoding. Given that, you can avoid biased locking on a per-object basis by calling System.identityHashCode(o). If the object is already biased, assigning an identity hashCode will result in revocation, otherwise, the assignment of a hashCode() will make the object ineligible for subsequent biased locking. This property is an artifact of our current implementation, of course.отсюда
Вот так-то вот -- либо identityHashCode, либо biased locking -- вы бы что выбрали?