单例模式的几种方式

学习设计模式有一段时间了，今天恰好用到了单例，而且单例模式比较常用，面试常考，比较重要。为了方便我不再一步一步详细的解释，下面只是给出几种常用的用法：

---

 

饿汉式，线程安全：

public class Singleton{
    //类加载时就初始化
    private static final Singleton instance = new Singleton();
    
    private Singleton(){}

    public static Singleton getInstance(){
        return instance;
    }
}

---

懒汉式，双重检测：

public class Singleton {
    private volatile static Singleton instance; //声明成 volatile
    private Singleton (){}

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

---

静态内部类，相对较优的方式：

public class Singleton {  
    private static class SingletonHolder {  
        private static final Singleton INSTANCE = new Singleton();  
    }  
    private Singleton (){}  
    public static final Singleton getInstance() {  
        return SingletonHolder.INSTANCE; 
    }  
}