What

保證一個類只有一個實例，并提供它的全局唯一訪問點。

保證一個Class只有一個實體對象存在。
具體可以有很多種，只有保證全局唯一就可以

初始化就創(chuàng)建

public class Singleton {
    private Singleton() {
    }

    private static Singleton instance = new Singleton();

    public static Singleton getInstance()
    {
        return instance;
    }

}

lazy load

當(dāng)用的時候再創(chuàng)建。

public class SingletionLazy {
    private SingletionLazy() {}
    private static SingletionLazy instance = null;
    public static synchronized SingletionLazy getInstance()
    {
        if(instance == null)
            instance = new SingletionLazy();
        return instance;
    }
}

double check
兩次檢查null

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