如何在多线程环境中设置值时消除双重检查

public class Singleton {
    private volatile static Singleton instance;
    
    private Singleton() {}
    
    public static Singleton getInstance() {
        if (instance == null) {  // 非同步的检查
            synchronized(Singleton.class) {
                if (instance == null) {  // 同步块内的检查
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}