對《JCP》"3.4.2"的理解

This chapter explain publishing immutable objects using "Volatile".

The example codes are 

class OneValueCache {
    private final BigInteger lastNumer;
    private final BigInterger[] lastFactors;

    public OneValueCahche(BigInterger i, BigInteger[] factors){
        lastNumber  = i;
        lastFactors = factors; 
    }

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

public class VolatileCachedFactorizer implements Servlet {
    private volatile OneValueCache cache = new OneValueCache(null,null);
    public void service(ServletRequest req, ServletResponse resp){
        BigInteger i = extractFromRequest(req);
        BigInteger [] factors = cache.getFactors(i);
        if(factors == null){
            factors = factor(i);
            cache = new OneValueCache(i,factors);
        }
    }
    encodeIntoResponse(resp,factors);
}

The explanation why it is safety is the following.

The cache-related operations cannot interfere with each other because One-ValueCache is immutable and

the cache field is accessed only once in each of the relevant code paths. 

I try to understand "the cache field is accessed only once in each of the relevant code paths".

Change the codes. See the next. The class "VolatileCachedFactorizer" is not safety again.

class OneValueCache {
    private final BigInteger lastNumber;
    private final BigInterger[] lastFactors;

    public OneValueCahche(BigInterger i, BigInteger[] factors){
        lastNumber  = i;
        lastFactors = factors;  
    }

    public BigInteger[] getFactors(){
        //if(lastNumber == null || !lastNumber.equals(i)){

       if(lastNumber == null){
            return null;
        }else{
            return Arrays.copyOf(lastFactors,lastFactors.length);
        }
    }

   public boolean isIntegerSame(BigInteger i){

       return (i==lastNumber);

   }
}

public class VolatileCachedFactorizer implements Servlet {
    private volatile OneValueCache cache = new OneValueCache(null,null);
    public void service(ServletRequest req, ServletResponse resp){
        BigInteger i = extractFromRequest(req);

        
        //BigInteger [] factors = cache.getFactors(i);

        // In the code path, "cache" is accessed twice. so it is not thread-safety.

      // The next case is possible. firstly, "cache.isIntegerSame(i)" return true;

      // before executing "cache.getFactors()", another thread update "cache",

      // "lastNumber" and "lastFactors" are changed. Then execute "cache.getFactors()",

      // get a wrong "factors". It is a typical thread-unsafety condition "Check-Then-Act".

        factors = cache.isIntegerSame(i) ? cache.getFactors() : null;
        if(factors == null){
            factors = factor(i);
            cache = new OneValueCache(i,factors);
        }       

    }
    encodeIntoResponse(resp,factors);
}