DCL 单例模式设计为什么需要 volatile 修饰实例对象

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