多线程下的单例类

1.提前初始化，放弃延迟加载

public class Singleton1
{
    private static Singleton1 instance=new Singleton1();
    private Singleton1(){}
    public static Singleton1 getInstance()
    {
        return instance;
    }
}

2.volatile关键字禁止指令重排+双重检查锁定（DCL）

public class Singleton2
{
    private static volatile Singleton2 instance;
    private Singleton2(){}
    public static Singleton2 getInstance()
    {
        if (instance==null)
        {
            synchronized(Singleton2.class)
            {
                if (instance==null)
                {
                    instance=new Singleton2();
                }
            }
        }
        return instance;
    }
}

3.占位类来实现延迟加载

public class Singleton3
{
    private static class Holder
    {
        public static Singleton3 instance=new Singleton3();
    }
    private Singleton3(){}
    public static Singleton3 getInstance()
    {
        return Holder.instance;
    }
}