It is implemented as a singleton class.
它是一个单实例类。
When copies of the singleton class run in multiple VMs, an instance is created for each machine.
当单例类的副本运行于多个虚拟机中时，每个虚拟机都会为其创建一个实例对象。
MockupDB, illustrated in Listing 3, is singleton class that simulates a database of customer orders.
清单3所示的MockupDB是个单体类，模拟客户订单的数据库。
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