茴香豆系列之单例的几种写法

1. 懒汉式

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

2. 饿汉式

public class Singleton {
    private static Singleton instance = new Singleton();
    private Singleton(){}
    public static Singleton getInstance(){
        return instance;
    }
}

3. 静态内部类

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

4. 枚举式

public enum Singleton {
    instance;
    public void somthing(){
    }
}

5. 锁两遍

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