SafeCachingFactorizer.java

public class SafeCachingFactorizer implements Servlet {
    private static class FactorResult { BigInteger number; BigInteger[] factors; }

    private volatile FactorResult lastResult = null;

    public void service(ServletRequest req, ServletResponse resp) {
        BigInteger i = extractFromRequest(req);
        FactorResult result = this.lastResult;
        if (result == null || !i.equals(result.number)) {
            result = new FactorResult() { { number = i; factors = factor(i); } };
            this.lastResult = result;
        }
        encodeIntoResponse(resp, result.factors);
    }
}