实现一个简单的单例模式

public class Singleton {

    private Singleton() {

    }

    private volatile static Singleton instance = null;

    public static Singleton getInstance() {
        if(instance == null) {
            synchronized(this) {
                if(instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}