A singleton is simply a class that is instantiated exactly once. Singletons typically represent a system component that is intrinsically unique, such as the window manager or file system. Making a class a singleton can make it difficult to test its clients, as it’s impossible to substitute a mock implementation for a singleton unless it implements an interface that serves as its type.

表示不理解啊。。

单例模式：单例模式确保某一个类只有一个实例，而且自行实例化并向整个系统提供这个实例。这个类称为单例类。

singleton如何向整个系统提供实例？

class  A {
    private static A a=null;
    private A(){
        ...
}
    private static A getInstance(){
        a=new A();
        
        return a;   
}
}

调用的时候 A a= getInstance()；获得实例

单列模式最寓意在于，程序运行期间，值允许有该类的一个实例存在
private Test(){}   为了确保只存在一个，所以用private
synchorzied 同步，主要为了避免多线程产生多个实例

是synchronized ，嘿嘿，还有应该判断一下，但里是不是已经存在了啊。

祝好