单例的几种写法

1.双重校验锁

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

2.内部类实现

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

3.枚举

public enum Singleton {
    INSTANCE;
}