在做App时遇到这么一个需求：要求App在启动的时候检测当前运行设备是否为虚拟设备，如果为虚拟设备则App停止运行并退出。通过网络上搜集的资料，总结了以下检测方法。

Double-Checked Locking is widely cited and used as an efficient method for implementing lazy initialization in a multithreaded environment.

Unfortunately, it will not work reliably in a platform independent way when implemented in Java, without additional synchronization. When implemented in other languages, such as C++, it depends on the memory model of the processor, the reorderings performed by the compiler and the interaction between the compiler and the synchronization library. Since none of these are specified in a language such as C++, little can be said about the situations in which it will work. Explicit memory barriers can be used to make it work in C++, but these barriers are not available in Java.

概览

先来看下官网对module.exports和exports的描述：

The module.exports object is created by the Module system. Sometimes this is not acceptable; many want their module to be an instance of some class. To do this, assign the desired export object to module.exports. Note that assigning the desired object to exports will simply rebind the local exports variable, which is probably not what you want to do.

The exports variable is available within a module’s file-level scope, and is assigned the value of module.exports before the module is evaluated.

It allows a shortcut, so that module.exports.f = … can be written more succinctly as exports.f = …. However, be aware that like any variable, if a new value is assigned to exports, it is no longer bound to module.exports

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