Two threads can get inside of the if statement concurrently when instance is null.
当instance为null时，两个线程可以并发地进入if语句内部。
It is the statement that follows that tests the connection. For instance.
真正测试该连接的是该语句后面的那条语句。
Constructor call is valid only as the first statement in an instance constructor.
构造函数调用仅作为实例构造函数中的第一条语句有效。
Constructor call is valid only as the first statement in an instance constructor.
构造函数调用仅作为实例构造函数中的第一条语句有效。
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