Question

我正在使用this cache manager, taken from bbarry on github。

我不知道缓存数据的实际存储位置和方式。我见过的大多数缓存使用MemoryCache，但我在任何地方都没有看到这一点。

我只能认为ConcurrentDictionary正在存储缓存的数据，但如果是这样，我就不明白为什么这是readonly的设置。

public static class Cache<T> {
    static readonly ConcurrentDictionary<string, T> Dictionary = new ConcurrentDictionary<string, T>();

    //by making this a tuple with the generic constraint, there will be one per cache type; the first parameter will always be the default value
    static readonly ConcurrentDictionary<string, Tuple<T, DateTime, TimeSpan>> Removals = new ConcurrentDictionary<string, Tuple<T, DateTime, TimeSpan>>();


    public static T GetOrAdd(string key, Func<string, T> creator) { return GetOrAdd(key, creator, null, null); }


    public static T GetOrAdd(string key, Func<string, T> creator, DateTime absoluteExpiration) { return GetOrAdd(key, creator, absoluteExpiration, null); }


    public static T GetOrAdd(string key, Func<string, T> creator, TimeSpan slidingExpiration) { return GetOrAdd(key, creator, null, slidingExpiration); }


    public static bool TryGetValue(string key, out T value) {
        Tuple<T, DateTime, TimeSpan> when;
        if (Removals.TryGetValue(key, out when) && when.Item3 != TimeSpan.Zero) {
            Remove(key, Tuple.Create(default(T), DateTime.Now.Add(when.Item3), when.Item3));
        }
        return Dictionary.TryGetValue(key, out value); 
    }


    public static bool Expire(string key, out T value) { return Dictionary.TryRemove(key, out value); }


    public static void Expire(string key) {
        T value;
        Dictionary.TryRemove(key, out value);
    }

    static T GetOrAdd(string key, Func<string, T> creator, DateTime? absoluteExpiration, TimeSpan? slidingExpiration) {
        if (key == null) {
            throw new ArgumentNullException("key");
        }
        Tuple<T, DateTime, TimeSpan> when;
        var updateRemoval = Removals.TryGetValue(key, out when) && when.Item3 != TimeSpan.Zero;
        var v = Dictionary.GetOrAdd(key, creator);
        if (absoluteExpiration == null && slidingExpiration == null && !updateRemoval) {
            return v;
        }
        if (absoluteExpiration != null || slidingExpiration != null) {
            var expiration = (TimeSpan)(slidingExpiration ?? (absoluteExpiration - DateTime.Now));
            when = Tuple.Create(default(T), DateTime.Now.Add(expiration), expiration);
        } else {
            when = Tuple.Create(default(T), DateTime.Now.Add(when.Item3), when.Item3);
        }
        if (absoluteExpiration != null) {
            Removals.TryAdd(key, Tuple.Create(default(T), (DateTime)absoluteExpiration, TimeSpan.Zero));
        } else {
            Removals.AddOrUpdate(key, when, (a, b) => when);
        }

        Remove(key, when);
        return v;
    }

    static void Remove(string key, Tuple<T, DateTime, TimeSpan> then) {
        System.Threading.Tasks.Task.Delay(then.Item3).ContinueWith(task => {
            Tuple<T, DateTime, TimeSpan> when;
            if (!Removals.TryGetValue(key, out when) || when.Item2 >= DateTime.Now)
                return;
            T v;
            Dictionary.TryRemove(key, out v);
            Removals.TryRemove(key, out when);
        });
    }
}

Answer 1

readonly修饰符并不意味着您无法修改标有该标记的集合（在这种情况下为ConcurrentDictionary<string, T> Dictionary）。

这只意味着您无法重新分配其价值。所以在这个例子中你不能做

Dictionary = new ConcurrentDictionary<string, T>();

一次。 readonly（上面链接）的文档列出了可以分配标记为readonly的字段的可能位置。在构造函数或静态构造函数或字段声明中。

您的问题的答案是肯定的，它是在这种情况下存储数据的ConcurrentDictionary。（GetOrAdd方法）。

这个缓存管理器如何存储缓存的数据？

1 个答案: