如何实现以命中次数为值的Guava Cache？

Question

我正在尝试实现一个缓存，它将统计过去5分钟内登录尝试的次数，在我的代码中，我想检查用户尝试的次数是否超过了MAX_ATTEMPTS。

在我为"Guava Cache“在线找到的所有代码示例中，使用load方法从其他来源获取值或使用某种方法计算它，我如何在每次缓存命中时递增它？

static LoadingCache<String, Integer> cache = CacheBuilder.newBuilder()
    .maximumSize(100000)
    .expireAfterAccess(5, TimeUnit.MINUTES)
    .build(
            new CacheLoader<String, Integer>() {
                public Integerload(String user) {
                       return ????;
                }
            }
    );

稍后在运行时，我想检查一下：

if(cache.getIfPresent(user) != null && cache.get(user) > MAX_ATTEMPTS)

如果满足以下条件，还会递增它：

if(cache.getIfPresent(user) != null && cache.get(user) <= MAX_ATTEMPTS)

Answer 1

@Oren你的解决方案不是线程安全的，因为你是在Cache之外的值上操作。最好使用Cache#asMap()视图，并在ConcurrentMap#compute(K, BiFunction)方法中更改值：

forgetPasswordCache.asMap().compute(email, (cachedEmail, currentCount) -> {
  if (currentCount != null && currentCount >= RESET_PASSWORD_MAX_ATTEMPTS) {
    logger.error("User with id: " + user.getId() + " and email: " + email +
         " has reached the maximum number of reset password attempts, the mail will not be sent");
    return null;
  }

  if (currentCount == null) {
    return 1;
  } else {
    return currentCount + 1;
  }
});

Answer 2

您可以尝试改编以下代码片段：

public class Demo {
    public static void main(String[] x) {
        CacheLoader<String, AtomicInteger> initialValueLoader = new CacheLoader<String, AtomicInteger>() {
            @Override
            public AtomicInteger load(String key) {
                // do not care of the key. everybody starts with 0 login attempts.
                return new AtomicInteger(0);
            }
        };

        LoadingCache<String, AtomicInteger> c = CacheBuilder
            .newBuilder()
            .maximumSize(100000)
            .expireAfterAccess(2, TimeUnit.SECONDS)
            .build(initialValueLoader);

        String user = "bob";

        try {
            // when the user tries to login, increment the attemps by one
            Verify.verify(c.get(user).incrementAndGet() == 1);

            // next call will get one
            Verify.verify(c.get(user).get() == 1);
        } catch (ExecutionException e) {
            throw new RuntimeException("oups: " + e, e);
        }

        // simulate time
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
        }

        // after some time, the cache is cleared
        try {
            Verify.verify(c.get(user).get() == 0);
        } catch (ExecutionException e) {
            throw new RuntimeException("oups: " + e, e);
        }

        System.out.println("done.");
    }
}

Answer 3

通过仅使用Guava的缓存功能解决了问题(不使用加载器)

缓存如下所示：

public class ForgetPasswordAttemptsCache {

    private static final Cache<String, Integer> forgetPasswordCache = CacheBuilder.newBuilder()
            .expireAfterAccess(24, TimeUnit.HOURS)
            .build();

    private ForgetPasswordAttemptsCache(){
    }

    public static Cache<String, Integer> getInstance() {
        return forgetPasswordCache;
    }

}

以及它的用法：

final String email = user.getEmail();
Integer currentCount = ForgetPasswordAttemptsCache.getInstance().getIfPresent(email);

if (currentCount != null && currentCount >= RESET_PASSWORD_MAX_ATTEMPTS) {
    logger.error("User with id: " + user.getId() + " and email: " + email +
            " has reached the maximum number of reset password attempts, the mail will not be sent");
    return;
}

if (currentCount == null) {
    ForgetPasswordAttemptsCache.getInstance().put(email, new Integer(1));
} else {
    ForgetPasswordAttemptsCache.getInstance().put(email, new Integer(currentCount + 1));
}

感谢spi提供的解决方案。(现在我正在寻找一种方法来测试它，而不需要24小时运行的测试。)