如何以及为什么此代码是线程安全的..？

Question

这是我的代码..

@immutable // This is not a standard annotation .Only for Showing that behavior of Class 
class OneValueCached{
    private final BigInteger lastNumber;
    private final BigInteger[] lastFactors;
    public OneValueCached(BigInteger i,BigInteger[] factors){
        lastNumber=i;
        lastFactors=Arrays.copyOf(factors, factors.length);
    }

    public BigInteger[] getFactors(BigInteger i){
        if(lastNumber==null || !lastNumber.equals(i))
            return null;
        else 
            return Arrays.copyOf(lastFactors, lastFactors.length);
    }
}

@threadSafe // This is not a standard annotation .Only for Showing that behavior of Class 
public class VolatileCachedFactorizer implements Servlet{
    private volatile OneValueCached cache=new OneValueCached(null, null);

    public void service(ServletRequest req, ServletResponce resp){
        BigInteger i= extractFromRequest(req);
        BigInteger[] factors=cache.getFactors(i);
        if(factors==null){   // ---> line 1
            factors=factor(i);  // --> line 2
            cache=new OneValueCached(i, factors);
        }

        encodeIntoResponse(resp,factors);
    }
}

为什么类VolatileCachedFactorizer是根据Book 的线程，但我的观点是..
1. @ 第 1 行 如果 2 个线程在该点同时到来第 1 个thread检查条件并发现factor=null并且第 2 个thread也在第 1 次thread 挂起后同时检查相同的条件在第2行，发现 factor=null
并且两者都将创建新OneValueCached对象然后这段代码是线程安全的..根据本书这是线程安全的..

谢谢

Answer 1

它是线程安全的，因为lastNumber和之间永远不会存在不一致lastFactors，这可能会导致不正确的因式分解。它不保证会发生最小数量的因式分解：OneValueCached可以多次创建，但这仍然是线程安全的。