书中添加了2个计数器
一个是命中计数器，一个是缓存命中计数器（如果新的参数和上次一样，则直接获取）

    private BigInteger lastNumber;//上次的因子
    private BigInteger[] lastFactors;//上次的因式分解的结果
    private long hits;//访问次数
    private long cacheHits;//缓存命中次数
    private BigInteger N;//全局变量
    public synchronized long getHits(){ return hits; }
    public synchronized double getCacheHitRadio(){ return cacheHits / hits; }
    public void service(BigInteger x){
        BigInteger[] factors = null;
        synchronized(this){
            //可变的 共享变量（计数器）写入      需要考虑并发
            //而且分解过程和记录计数器过程不干扰  可以拆分
            ++hits;//记录
            if(x.equals(lastNumber)){
                ++cacheHits;
                factors = lastFactors.clone();//缓存上次结果
            }
        }
        if(factors == null){
            //如果没有本次 x 因式分解的记录    factor即求解过程
            factors = factor(x);
            synchronized(this){
                lastNumber = x;
                lastFactors = factors.clone();
            }
        }
        for(BigInteger fac:factors){

            System.err.println(fac);
        }
    }


    BigInteger[] factor(BigInteger num) {
        //因数分解过程
        BigInteger[] factors = new BigInteger[3];
        BigInteger min = new BigInteger("2");
        int n = 0;
        while(num.compareTo(min) == 1){
            System.out.println("NUM"+num +" MIN: "+min+"MOD: "+num.mod(min));
            if(num.mod(min).compareTo(new BigInteger("0")) == 0){
                //num 不被整除且  num>0
                factors[n] = min;
                n++;
                num = num.divide(min);
                min = new BigInteger("2");
            }else{
                min = min.add(new BigInteger("1"));
            }
        }
        if(num.compareTo(min) == 0){
            factors[n] = min;
        }
        return factors;
    }