“实践中的Java并发" -缓存的线程安全数字分解器(清单2.8)

问题描述

在以下代码中(复制自实践中的Java并发第2章，第2.5节，清单2.8):

In the following code (copied from Java Concurrency in Practice Chapter 2, section 2.5, Listing 2.8):

@ThreadSafe
public class CachedFactorizer implements Servlet {
    @GuardedBy("this") private BigInteger lastNumber;
    @GuardedBy("this") private BigInteger[] lastFactors;
    @GuardedBy("this") private long hits;
    @GuardedBy("this") private long cacheHits;

    public synchronized long getHits() { return hits; }

    public synchronized double getCacheHitRatio() {
        return (double) cacheHits / (double) hits;
    }

    public void service(ServletRequest req, ServletResponse resp) {
        BigInteger i = extractFromRequest(req);
        BigInteger[] factors = null;
        synchronized (this) {
            ++hits;
            if (i.equals(lastNumber)) {
                ++cacheHits;
                factors = lastFactors.clone(); // questionable line here
            }
        }
        if (factors == null) {
            factors = factor(i);
            synchronized (this) {
                lastNumber = i;
                lastFactors = factors.clone(); // and here
            }
        }
        encodeIntoResponse(resp, factors);
    }
}

为什么要克隆factors，lastFactors数组?不能简单地写为factors = lastFactors;和lastFactors = factors;吗?仅仅是因为factors是局部变量，然后将其传递给encodeIntoResponse，它可以对其进行修改?

why the factors, lastFactors arrays are cloned? Can't it be simply written as factors = lastFactors; and lastFactors = factors;? Just because the factors is a local variable and it is then passed to encodeIntoResponse, which can modify it?

希望问题很明确.谢谢.

Hope the question is clear. Thanks.

推荐答案

如果将factors = lastFactors.clone();更改为factors = lastFactors;，则factors和lastFactors都指向同一对象，factors为否更长，它将变为共享的可变状态.

If you change factors = lastFactors.clone(); to factors = lastFactors;, both factors and lastFactors point to the same object, factors is no longer a local variable, it becomes a shared mutable state.

想象一下，有三个请求，即请求A，B，C.请求A和B发送的数字为10，但是请求C发送的数字为20.如果发生以下执行顺序并且您进行了更改，则可能会出错. factors = lastFactors.clone();到factors = lastFactors;.

Imagine there are three requests, request A, B, C. The number sent by Request A and B is 10, but the number sent by request C is 20. Things can go wrong if the below execution order happens and you change factors = lastFactors.clone(); to factors = lastFactors;.

1. servlet服务器接收请求A，执行整个service方法，现在lastNumber是10，lastFactors是[1, 2, 5, 10].
2. servlet服务器同时接收请求B和C，首先处理了请求B，但在退出第一个synchronized块后(现在对于请求B，factors是[1, 2, 5, 10]，这是正确的)，处理请求C.
3. 对于请求C，将执行整个service方法，它将lastFactors从[1, 2, 5, 10]更改为[1, 2, 4, 5, 10, 20] ，因为这两个factors lastFactors都指向同一个对象，factors现在也为[1, 2, 4, 5, 10, 20]. 请求B的响应应该为[1, 2, 5, 10]，但现在为[1, 2, 4, 5, 10, 20] .

1. servlet server receives request A, the entire service method is executed, now lastNumber is 10, lastFactors is [1, 2, 5, 10].
2. servlet server receives both request B and C, request B is handled at first, but after exiting the first synchronized block (now for request B, factors is [1, 2, 5, 10], which is correct), request C is handled.
3. for request C, the entire service method is executed, it change lastFactors from [1, 2, 5, 10] to [1, 2, 4, 5, 10, 20], because both factors lastFactors point to the same object, factors is now [1, 2, 4, 5, 10, 20] too. The response of request B is supposed to be [1, 2, 5, 10], but is [1, 2, 4, 5, 10, 20] right now.

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