The DefaultDAO class stores an instance of itself (as a Singleton) in the static variable instance, allowing access to that instance through the getInstance() method.

DefaultDAO 类在静态变量 instance 中存储了它自己的一个实例（存储为 Singleton），并允许通过 getInstance() 方法访问该实例。

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This line of code creates a Singleton object and initializes the variable instance to refer to this object.

此行代码创建了一个Singleton对象并初始化变量instance来引用此对象。

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The problem with this line of code is that the variable instance can become non-null before the body of the Singleton constructor executes.

这行代码的问题是：在Singleton构造函数体执行之前，变量instance可能成为非null的。

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