This means that an object is initialized with a value and not reassigned.
这意味着用一个值初始化对象,以后不再重新赋值。
Because it is not, thread 2 returns the instance reference to a fully constructed, but partially initialized, Singleton object.
因为实例不为null,线程2将instance引用返回给一个构造完整但部分初始化了的Singleton对象。
The Discussions object has already been initialized, and can not be reinitialized.
讨论对象已经初始化,并且不能重新初始化。
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