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};
}
}