SynchronizedFactorizer.java

package ch21concurrent.examples;

import java.math.BigInteger;
import javax.servlet.*;

import ch21concurrent.annotations.*;

/**
 * SynchronizedFactorizer
 * <p/>
 * Servlet that caches last result, but with unnacceptably poor concurrency
 *
 * @author Brian Goetz and Tim Peierls
 */

@ThreadSafe
public class SynchronizedFactorizer extends GenericServlet implements Servlet {
    @GuardedBy("this")
    private BigInteger lastNumber;
    @GuardedBy("this")
    private BigInteger[] lastFactors;

    public synchronized void service(ServletRequest req,
                                     ServletResponse resp) {
        BigInteger i = extractFromRequest(req);
        if (i.equals(lastNumber))
            encodeIntoResponse(resp, lastFactors);
        else {
            BigInteger[] factors = factor(i);
            lastNumber = i;
            lastFactors = factors;
            encodeIntoResponse(resp, factors);
        }
    }

    void encodeIntoResponse(ServletResponse resp, BigInteger[] factors) {
    }

    BigInteger extractFromRequest(ServletRequest req) {
        return new BigInteger("7");
    }

    BigInteger[] factor(BigInteger i) {
        // Doesn't really factor
        return new BigInteger[]{i};
    }
}