因数分解,将最近的计算结果缓存起来
1、该Servlet在没有足够原子性保证的情况下对其最近计算结果进行缓存(不要这样做)
import java.math.BigInteger; import java.util.concurrent.atomic.*; import javax.servlet.*; import net.jcip.annotations.*; @NotThreadSafe public class UnsafeCachingFactorizer extends GenericServlet implements Servlet { private final AtomicReference<BigInteger> lastNumber = new AtomicReference<BigInteger>(); private final AtomicReference<BigInteger[]> lastFactors = new AtomicReference<BigInteger[]>(); public void service(ServletRequest req, ServletResponse resp) { BigInteger i = extractFromRequest(req); if (i.equals(lastNumber.get())) encodeIntoResponse(resp, lastFactors.get()); else { BigInteger[] factors = factor(i); lastNumber.set(i); lastFactors.set(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}; } }
上面会出现并发问题,虽然该类包含两个能进行原子操作的状态,但是两次状态修改操作之间,其他线程将发现不变性被破坏了
2、下面使用volatile类型来发布不可变对象
import java.math.BigInteger; import java.util.*; import net.jcip.annotations.*; @Immutable public class OneValueCache { private final BigInteger lastNumber; private final BigInteger[] lastFactors; public OneValueCache(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); } }
使用指向不可变容器对象的volatile类型引用以缓存最新的结果
import java.math.BigInteger; import javax.servlet.*; import net.jcip.annotations.*; @ThreadSafe public class VolatileCachedFactorizer extends GenericServlet 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); } 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}; } }