单例模式

/**
 * 单例模式：懒汉式
 */
class Singleton{
    private static volatile Singleton singleton = null;
    private Singleton(){

    }
    public static Singleton getInstance(){
        if(singleton == null){
            synchronized (Singleton.class){
                if (singleton == null){
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }