单利模式

饿汉式

   //创建对象
    private static Model model = new Model();

    //私有构造
    private Model(){

    }
    //获取单列对象
    public static Model getInstance(){
        return model;
    }

懒汉式，也是常用的形式

双重检查[推荐用]

public 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;
    }
}