1.提前初始化，放棄延遲加載

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

2.volatile關(guān)鍵字禁止指令重排+雙重檢查鎖定（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.占位類來實現(xiàn)延遲加載

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