This will save the old stack pointer and allocate stack memory atomically.
这会保存原来的堆栈指针，并自动分配堆栈内存。
The class of the object that will be updated atomically (in this case, Book).
将得到原子更新的对象的类(在本例中是Book)。
The class of the object that will be updated atomically (in this case, Book).
将得到原子更新的对象的类(在本例中是Book)。
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