Note that when the second thread enters the synchronized block, it does not check to see if instance is non-null.
注意：当第二个线程进入synchronized块时，它并没有检查instance是否非null。
2
It was found that the code length distribution was closely correlated with the shift symmetry properties of synchronized coding and was characterized by sequencing and non-integral power of 2.
研究发现：其码长分布与同步字的移位对称数密切相关，具有连续和非2的整数次幂等特点；
3
If a method only operates on independently synchronized objects, local variables, or non-volatile data members, such as those initialized during construction, then synchronization is not required.