一步步实现一个基本的缓存模块
注意后续代码及改进见后后文及github，文章上的并没有更新。
1. 前言
2. 请求级别缓存
2.1 多线程
3. 进程级别缓存
3.1 分区与计数
3.2 可空缓存值
3.3 封装与集成
4. 小结
1. 前言
- 面向读者：初、中级用户；
- 涉及知识：HttpContext、HttpRuime.Cache、DictionaryEntry、Unit Test等；
- 文章目的：这里的内容不会涉及 Memcached、Redies 等进程外缓存的使用，只针对包含WEB应用的常见场景，实现一个具有线程安全、分区、过期特性的缓存模块，略微提及DI等内容。
- jusfr 原创，转载请注明来自博客园。
2. 请求级别缓存
如果需要线程安全地存取数据，System.Collections.Concurrent 命名空间下的像 ConcurrentDictionary 等实现是首选；更复杂的特性像过期策略、文件依赖等就需要其他实现了。ASP.NET中的HttpContext.Current.Items 常常被用作自定义数据容器，注入工具像Unity、Autofac 等便借助自定义 HttpModule 将容器挂接在 HttpContext.Current 上以进行生命周期管理。
基本接口 ICacheProvider，请求级别的缓存从它定义，考虑到请求级别缓存的运用场景有限，故只定义有限特性；
1 public interface ICacheProvider { 2 Boolean TryGet<T>(String key, out T value); 3 T GetOrCreate<T>(String key, Func<T> function); 4 T GetOrCreate<T>(String key, Func<String, T> factory); 5 void Overwrite<T>(String key, T value); 6 void Expire(String key); 7 }
HttpContext.Current.Items 从 IDictionary 定义，存储 Object-Object 键值对，出于便利与直观，ICacheProvider 只接受String类型缓存键，故HttpContextCacheProvider内部使用 BuildCacheKey(String key) 方法生成真正缓存键以避免键值重复；
同时 HashTable 可以存储空引用作为缓存值，故 TryGet() 方法先进行 Contains() 判断存在与否，再进行类型判断，避免缓存键重复使用；
1 public class HttpContextCacheProvider : ICacheProvider { 2 protected virtual String BuildCacheKey(String key) { 3 return String.Concat("HttpContextCacheProvider_", key); 4 } 5 6 public Boolean TryGet<T>(String key, out T value) { 7 key = BuildCacheKey(key); 8 Boolean exist = false; 9 if (HttpContext.Current.Items.Contains(key)) { 10 exist = true; 11 Object entry = HttpContext.Current.Items[key]; 12 if (entry != null && !(entry is T)) { 13 throw new InvalidOperationException(String.Format("缓存项`[{0}]`类型错误, {1} or {2} ?", 14 key, entry.GetType().FullName, typeof(T).FullName)); 15 } 16 value = (T)entry; 17 } 18 else { 19 value = default(T); 20 } 21 return exist; 22 } 23 24 public T GetOrCreate<T>(String key, Func<T> function) { 25 T value; 26 if (TryGet(key, out value)) { 27 return value; 28 } 29 value = function(); 30 Overwrite(key, value); 31 return value; 32 } 33 34 public T GetOrCreate<T>(String key, Func<String, T> factory) { 35 T value; 36 if (TryGet(key, out value)) { 37 return value; 38 } 39 value = factory(key); 40 Overwrite(key, value); 41 return value; 42 } 43 44 public void Overwrite<T>(String key, T value) { 45 key = BuildCacheKey(key); 46 HttpContext.Current.Items[key] = value; 47 } 48 49 public void Expire(String key) { 50 key = BuildCacheKey(key); 51 HttpContext.Current.Items.Remove(key); 52 } 53 }
这里使用了 Func<T> 委托的运用，合并查询、判断和添加缓存项的操作以简化接口调用；如果用户期望不同类型缓存值可以存储到相同的 key 上，则需要重新定义 BuildCacheKey() 方法将缓存值类型作为参数参与生成缓存键，此时 Expire() 方法则同样需要了。测试用例：
1 [TestClass] 2 public class HttpContextCacheProviderTest { 3 [TestInitialize] 4 public void Initialize() { 5 HttpContext.Current = new HttpContext(new HttpRequest(null, "http://localhost", null), new HttpResponse(null)); 6 } 7 8 [TestMethod] 9 public void NullValue() { 10 var key = "key-null"; 11 HttpContext.Current.Items.Add(key, null); 12 Assert.IsTrue(HttpContext.Current.Items.Contains(key)); 13 Assert.IsNull(HttpContext.Current.Items[key]); 14 } 15 16 [TestMethod] 17 public void ValueType() { 18 var key = "key-guid"; 19 ICacheProvider cache = new HttpContextCacheProvider(); 20 var id1 = Guid.NewGuid(); 21 var id2 = cache.GetOrCreate(key, () => id1); 22 Assert.AreEqual(id1, id2); 23 24 cache.Expire(key); 25 Guid id3; 26 var exist = cache.TryGet(key, out id3); 27 Assert.IsFalse(exist); 28 Assert.AreNotEqual(id1, id3); 29 Assert.AreEqual(id3, Guid.Empty); 30 } 31 }
引用类型测试用例忽略。
2.1 多线程
异步等情况下，HttpContext.Current并非无处不在，故异步等情况下 HttpContextCacheProvider 的使用可能抛出空引用异常，需要被处理，对此园友有过思考 ，这里贴上A大的方案 ，有需求的读者请按图索骥。
3. 进程级别缓存
HttpRuntime.Cache 定义在 System.Web.dll 中，System.Web 命名空间下，实际上是可以使用在非 Asp.Net 应用里的；另外 HttpContext 对象包含一个 Cache 属性，它们的关系可以阅读 HttpContext.Cache 和 HttpRuntime.Cache；
HttpRuntime.Cache 为 System.Web.Caching.Cache 类型，支持滑动/绝对时间过期策略、支持缓存优先级、缓存更新/过期回调、基于文件的缓存依赖项等，功能十分强大，这里借用少数特性来实现进程级别缓存，更多文档请自行检索。
从 ICacheProvider 定义 IHttpRuntimeCacheProvider，添加相对过期与绝对过期、添加批量的缓存过期接口 ExpireAll()；
1 public interface IHttpRuntimeCacheProvider : ICacheProvider { 2 T GetOrCreate<T>(String key, Func<T> function, TimeSpan slidingExpiration); 3 T GetOrCreate<T>(String key, Func<T> function, DateTime absoluteExpiration); 4 void Overwrite<T>(String key, T value, TimeSpan slidingExpiration); 5 void Overwrite<T>(String key, T value, DateTime absoluteExpiration); 6 void ExpireAll(); 7 }
System.Web.Caching.Cache 只继承 IEnumerable，内部使用 DictionaryEntry 存储Object-Object 键值对，但 HttpRuntime.Cache 只授受字符串类型缓存键及非空缓存值，关于空引用缓存值的问题，我们在3.2中讨论；
故 TryGet() 与 HttpContextCacheProvider.TryGet() 具有显著差异，前者需要拿出值来进行非空判断，后者则是使用 IDictionary.Contains() 方法；
除了 TryGet() 方法与过期过期参数外的差异外，接口实现与 HttpContextCacheProvider 类似；
1 public class HttpRuntimeCacheProvider : IHttpRuntimeCacheProvider { 2 private static readonly Object _sync = new Object(); 3 4 protected virtual String BuildCacheKey(String key) { 5 return String.Concat("HttpRuntimeCacheProvider_", key); 6 } 7 8 public Boolean TryGet<T>(String key, out T value) { 9 key = BuildCacheKey(key); 10 Boolean exist = false; 11 Object entry = HttpRuntime.Cache.Get(key); 12 if (entry != null) { 13 exist = true; 14 if (!(entry is T)) { 15 throw new InvalidOperationException(String.Format("缓存项[{0}]类型错误, {1} or {2} ?", 16 key, entry.GetType().FullName, typeof(T).FullName)); 17 } 18 value = (T)entry; 19 } 20 else { 21 value = default(T); 22 } 23 return exist; 24 } 25 26 public T GetOrCreate<T>(String key, Func<String, T> factory) { 27 T result; 28 if (TryGet<T>(key, out result)) { 29 return result; 30 } 31 result = factory(key); 32 Overwrite(key, result); 33 return result; 34 } 35 36 public T GetOrCreate<T>(String key, Func<T> function) { 37 T result; 38 if (TryGet<T>(key, out result)) { 39 return result; 40 } 41 result = function(); 42 Overwrite(key, result); 43 return result; 44 } 45 46 47 public T GetOrCreate<T>(String key, Func<T> function, TimeSpan slidingExpiration) { 48 T result; 49 if (TryGet<T>(key, out result)) { 50 return result; 51 } 52 result = function(); 53 Overwrite(key, result, slidingExpiration); 54 return result; 55 } 56 57 public T GetOrCreate<T>(String key, Func<T> function, DateTime absoluteExpiration) { 58 T result; 59 if (TryGet<T>(key, out result)) { 60 return result; 61 } 62 result = function(); 63 Overwrite(key, result, absoluteExpiration); 64 return result; 65 } 66 67 public void Overwrite<T>(String key, T value) { 68 HttpRuntime.Cache.Insert(BuildCacheKey(key), value); 69 } 70 71 //slidingExpiration 时间内无访问则过期 72 public void Overwrite<T>(String key, T value, TimeSpan slidingExpiration) { 73 HttpRuntime.Cache.Insert(BuildCacheKey(key), value, null, 74 Cache.NoAbsoluteExpiration, slidingExpiration); 75 } 76 77 //absoluteExpiration 绝对时间过期 78 public void Overwrite<T>(String key, T value, DateTime absoluteExpiration) { 79 HttpRuntime.Cache.Insert(BuildCacheKey(key), value, null, 80 absoluteExpiration, Cache.NoSlidingExpiration); 81 } 82 83 public void Expire(String key) { 84 HttpRuntime.Cache.Remove(BuildCacheKey(key)); 85 } 86 87 public void ExpireAll() { 88 lock (_sync) { 89 var entries = HttpRuntime.Cache.OfType<DictionaryEntry>() 90 .Where(entry => (entry.Key is String) && ((String)entry.Key).StartsWith("HttpRuntimeCacheProvider_")); 91 foreach (var entry in entries) { 92 HttpRuntime.Cache.Remove((String)entry.Key); 93 } 94 } 95 } 96 }
测试用例与 HttpContextCacheProviderTest 类似，这里贴出缓存过期的测试：
1 public class HttpRuntimeCacheProviderTest { 2 [TestMethod] 3 public void GetOrCreateWithAbsoluteExpirationTest() { 4 var key = Guid.NewGuid().ToString(); 5 var val = Guid.NewGuid(); 6 7 IHttpRuntimeCacheProvider cacheProvider = new HttpRuntimeCacheProvider(); 8 var result = cacheProvider.GetOrCreate<Guid>(key, () => val, DateTime.UtcNow.AddSeconds(2D)); 9 Assert.AreEqual(result, val); 10 11 var exist = cacheProvider.TryGet<Guid>(key, out val); 12 Assert.IsTrue(exist); 13 Assert.AreEqual(result, val); 14 15 Thread.Sleep(2000); 16 exist = cacheProvider.TryGet<Guid>(key, out val); 17 Assert.IsFalse(exist); 18 Assert.AreEqual(val, Guid.Empty); 19 } 20 21 [TestMethod] 22 public void ExpireAllTest() { 23 var key = Guid.NewGuid().ToString(); 24 var val = Guid.NewGuid(); 25 26 IHttpRuntimeCacheProvider cacheProvider = new HttpRuntimeCacheProvider(); 27 var result = cacheProvider.GetOrCreate<Guid>(key, () => val); 28 Assert.AreEqual(result, val); 29 30 cacheProvider.ExpireAll(); 31 Guid val2; 32 var exist = cacheProvider.TryGet<Guid>(key, out val2); 33 Assert.IsFalse(exist); 34 Assert.AreEqual(val2, Guid.Empty); 35 } 36 }
3.1 分区与计数
缓存分区是常见需求，缓存用户A、用户B的认证信息可以拿用户标识作为缓存键，但每个用户分别有一整套包含授权的其他数据时，为创建以用户分区的缓存应该是更好的选择；
常规的想法是为缓存添加类似 `Region` 或 `Partition`的参数，个人觉得这不是很好的实践，因为接口被修改，同时过多的参数非常让人困惑；
读者可能对前文中 BuildCacheKey() 方法被 protected virtual 修饰觉得很奇怪，是的，个人觉得定义新的接口，配合从缓存Key的生成算法作文章来分区貌似比较巧妙，也迎合依赖注册被被广泛使用的现状；
分区的进程级别缓存定义，只需多出一个属性：
1 public interface IHttpRuntimeRegionCacheProvider : IHttpRuntimeCacheProvider { 2 String Region { get; } 3 }
分区的缓存实现，先为 IHttpRuntimeCacheProvider 添加计数，然后重构HttpRuntimeCacheProvider，提取出过滤算法，接着重写 BuildCacheKey() 方法的实现，使不同分区的生成不同的缓存键，缓存项操作方法无须修改；
1 public interface IHttpRuntimeCacheProvider : ICacheProvider { 2 ... 3 Int32 Count { get; } 4 } 5 6 public class HttpRuntimeCacheProvider : IHttpRuntimeCacheProvider { 7 ... 8 protected virtual Boolean Hit(DictionaryEntry entry) { 9 return (entry.Key is String) && ((String)entry.Key).StartsWith("HttpRuntimeCacheProvider_"); 10 } 11 12 public void ExpireAll() { 13 lock (_sync) { 14 var entries = HttpRuntime.Cache.OfType<DictionaryEntry>().Where(Hit); 15 foreach (var entry in entries) { 16 HttpRuntime.Cache.Remove((String)entry.Key); 17 } 18 } 19 } 20 21 public Int32 Count { 22 get { 23 lock (_sync) { 24 return HttpRuntime.Cache.OfType<DictionaryEntry>().Where(Hit).Count(); 25 } 26 } 27 } 28 } 29 30 public class HttpRuntimeRegionCacheProvider : HttpRuntimeCacheProvider, IHttpRuntimeRegionCacheProvider { 31 private String _prefix; 32 public virtual String Region { get; private set; } 33 34 private String GetPrifix() { 35 if (_prefix == null) { 36 _prefix = String.Concat("HttpRuntimeRegionCacheProvider_", Region, "_"); 37 } 38 return _prefix; 39 } 40 41 public HttpRuntimeRegionCacheProvider(String region) { 42 Region = region; 43 } 44 45 protected override String BuildCacheKey(String key) { 46 //Region 为空将被当作 String.Empty 处理 47 return String.Concat(GetPrifix(), base.BuildCacheKey(key)); 48 } 49 50 protected override Boolean Hit(DictionaryEntry entry) { 51 return (entry.Key is String) && ((String)entry.Key).StartsWith(GetPrifix()); 52 } 53 }
测试用例示例了两个分区缓存对相同 key 的操作：
1 [TestClass] 2 public class HttpRuntimeRegionCacheProviderTest { 3 [TestMethod] 4 public void ValueType() { 5 var key = "key-guid"; 6 IHttpRuntimeCacheProvider cache1 = new HttpRuntimeRegionCacheProvider("Region1"); 7 var id1 = cache1.GetOrCreate(key, Guid.NewGuid); 8 9 IHttpRuntimeCacheProvider cache2 = new HttpRuntimeRegionCacheProvider("Region2"); 10 var id2 = cache2.GetOrCreate(key, Guid.NewGuid); 11 Assert.AreNotEqual(id1, id2); 12 13 cache1.ExpireAll(); 14 Assert.AreEqual(cache1.Count, 0); 15 Assert.AreEqual(cache2.Count, 1); 16 } 17 }
至此一个基本的缓存模块已经完成；
3.2 可空缓存值
前文提及过，HttpRuntime.Cache 不授受空引用作为缓存值，与 HttpContext.Current.Items表现不同，另一方面实际需求中，空值作为字典的值仍然是有意义，此处给出一个支持空缓存值的实现；
HttpRuntime.Cache 断然是不能把 null 存入的，查看 HttpRuntimeCacheProvider.TryGet() 方法，可知 HttpRuntime.Cache.Get() 获取的总是 Object 类型，思路可以这样展开：
1) 添加缓存时进行判断，如果非空，常规处理，否则把用一个特定的自定义对象存入；
2) 取出缓存时进行判断，如果为特定的自定义对象，返回 null；
为 HttpRuntimeCacheProvider 的构造函数添加可选参数，TryGet() 加入 null 判断逻辑；添加方法 BuildCacheEntry()，替换空的缓存值为 _nullEntry，其他方法不变；
1 public class HttpRuntimeCacheProvider : IHttpRuntimeCacheProvider { 2 private static readonly Object _sync = new Object(); 3 private static readonly Object _nullEntry = new Object(); 4 private Boolean _supportNull; 5 6 public HttpRuntimeCacheProvider(Boolean supportNull = false) { 7 _supportNull = supportNull; 8 } 9 10 protected virtual String BuildCacheKey(String key) { 11 return String.Concat("HttpRuntimeCacheProvider_", key); 12 } 13 14 protected virtual Object BuildCacheEntry<T>(T value) { 15 Object entry = value; 16 if (value == null) { 17 if (_supportNull) { 18 entry = _nullEntry; 19 } 20 else { 21 throw new InvalidOperationException(String.Format("Null cache item not supported, try ctor with paramter 'supportNull = true' ")); 22 } 23 } 24 return entry; 25 } 26 27 public Boolean TryGet<T>(String key, out T value) { 28 Object entry = HttpRuntime.Cache.Get(BuildCacheKey(key)); 29 Boolean exist = false; 30 if (entry != null) { 31 exist = true; 32 if (!(entry is T)) { 33 if (_supportNull && !(entry == _nullEntry)) { 34 throw new InvalidOperationException(String.Format("缓存项`[{0}]`类型错误, {1} or {2} ?", 35 key, entry.GetType().FullName, typeof(T).FullName)); 36 } 37 value = (T)((Object)null); 38 } 39 else { 40 value = (T)entry; 41 } 42 } 43 else { 44 value = default(T); 45 } 46 return exist; 47 } 48 49 public T GetOrCreate<T>(String key, Func<String, T> factory) { 50 T value; 51 if (TryGet<T>(key, out value)) { 52 return value; 53 } 54 value = factory(key); 55 Overwrite(key, value); 56 return value; 57 } 58 59 public T GetOrCreate<T>(String key, Func<T> function) { 60 T value; 61 if (TryGet<T>(key, out value)) { 62 return value; 63 } 64 value = function(); 65 Overwrite(key, value); 66 return value; 67 } 68 69 public T GetOrCreate<T>(String key, Func<T> function, TimeSpan slidingExpiration) { 70 T value; 71 if (TryGet<T>(key, out value)) { 72 return value; 73 } 74 value = function(); 75 Overwrite(key, value, slidingExpiration); 76 return value; 77 } 78 79 public T GetOrCreate<T>(String key, Func<T> function, DateTime absoluteExpiration) { 80 T value; 81 if (TryGet<T>(key, out value)) { 82 return value; 83 } 84 value = function(); 85 Overwrite(key, value, absoluteExpiration); 86 return value; 87 } 88 89 public void Overwrite<T>(String key, T value) { 90 HttpRuntime.Cache.Insert(BuildCacheKey(key), BuildCacheEntry<T>(value)); 91 } 92 93 //slidingExpiration 时间内无访问则过期 94 public void Overwrite<T>(String key, T value, TimeSpan slidingExpiration) { 95 HttpRuntime.Cache.Insert(BuildCacheKey(key), BuildCacheEntry<T>(value), null, 96 Cache.NoAbsoluteExpiration, slidingExpiration); 97 } 98 99 //absoluteExpiration 时过期 100 public void Overwrite<T>(String key, T value, DateTime absoluteExpiration) { 101 HttpRuntime.Cache.Insert(BuildCacheKey(key), BuildCacheEntry<T>(value), null, 102 absoluteExpiration, Cache.NoSlidingExpiration); 103 } 104 105 public void Expire(String key) { 106 HttpRuntime.Cache.Remove(BuildCacheKey(key)); 107 } 108 109 protected virtual Boolean Hit(DictionaryEntry entry) { 110 return (entry.Key is String) && ((String)entry.Key).StartsWith("HttpRuntimeCacheProvider_"); 111 } 112 113 public void ExpireAll() { 114 lock (_sync) { 115 var entries = HttpRuntime.Cache.OfType<DictionaryEntry>().Where(Hit); 116 foreach (var entry in entries) { 117 HttpRuntime.Cache.Remove((String)entry.Key); 118 } 119 } 120 } 121 122 public Int32 Count { 123 get { 124 lock (_sync) { 125 return HttpRuntime.Cache.OfType<DictionaryEntry>().Where(Hit).Count(); 126 } 127 } 128 } 129 }
然后是分区缓存需要修改构造函数：
1 public HttpRuntimeRegionCacheProvider(String region) 2 : base(false) { 3 Region = region; 4 } 5 6 public HttpRuntimeRegionCacheProvider(String region, Boolean supportNull) 7 : base(supportNull) { 8 Region = region; 9 } 10 ... 11 }
测试用例：
1 [TestClass] 2 public class HttpRuntimeCacheProviderTest { 3 [TestMethod] 4 public void NullCacheErrorTest() { 5 var key = "key-null"; 6 Person person = null; 7 IHttpRuntimeCacheProvider cacheProvider = new HttpRuntimeCacheProvider(false); 8 try { 9 cacheProvider.GetOrCreate<Person>(key, () => person); //error 10 Assert.Fail(); 11 } 12 catch (Exception ex) { 13 Assert.IsTrue(ex is InvalidOperationException); 14 } 15 16 Person person2; 17 var exist = cacheProvider.TryGet(key, out person2); 18 Assert.IsFalse(exist); 19 Assert.AreEqual(person2, null); 20 } 21 22 [TestMethod] 23 public void NullableCacheTest() { 24 var key = "key-nullable"; 25 Person person = null; 26 IHttpRuntimeCacheProvider cacheProvider = new HttpRuntimeCacheProvider(true); 27 cacheProvider.GetOrCreate<Person>(key, () => person); 28 Person person2; 29 var exist = cacheProvider.TryGet(key, out person2); 30 Assert.IsTrue(exist); 31 Assert.AreEqual(person2, null); 32 } 33 34 class Person { 35 public Int32 Id { get; set; } 36 public String Name { get; set; } 37 } 38 }
3.3 封装与集成
多数情况下我们不需要暴露实现和手动创建上文所提各种 CacheProvider，实践中它们被 internal 修饰，再配合工厂类使用：
1 public static class CacheProviderFacotry { 2 public static ICacheProvider GetHttpContextCache() { 3 return new HttpContextCacheProvider(); 4 } 5 6 public static IHttpRuntimeCacheProvider GetHttpRuntimeCache(Boolean supportNull = false) { 7 return new HttpRuntimeCacheProvider(supportNull); 8 } 9 10 public static IHttpRuntimeRegionCacheProvider GetHttpRuntimeRegionCache(String region, Boolean supportNull = false) { 11 return new HttpRuntimeRegionCacheProvider(region, supportNull); 12 } 13 14 public static IHttpRuntimeRegionCacheProvider Region(this IHttpRuntimeCacheProvider runtimeCacheProvider, String region, Boolean supportNull = false) { 15 return GetHttpRuntimeRegionCache(region, supportNull); 16 } 17 }
然后在依赖注入中的声明如下，这里是 Autofac 下的组件注册：
1 ... 2 //请求级别缓存, 使用 HttpContext.Current.Items 作为容器 3 builder.Register(ctx => CacheProviderFacotry.GetHttpContextCache()).As<ICacheProvider>().InstancePerLifetimeScope(); 4 //进程级别缓存, 使用 HttpRuntime.Cache 作为容器 5 builder.RegisterInstance(CacheProviderFacotry.GetHttpRuntimeCache()).As<IRuntimeCacheProvider>().ExternallyOwned(); 6 //进程级别且隔离的缓存, 若出于key算法唯一考虑而希望加入上下文件信息, 则仍然需要 CacheModule 类的实现 7 builder.Register(ctx => CacheProviderFacotry.GetHttpRuntimeRegionCache(/*... 分区依据 ...*/)) 8 .As<IRuntimeRegionCacheProvider>().InstancePerLifetimeScope(); 9 ...
4. 小结
本文简单探讨了一个具有线程安全、分区、过期特性缓存模块的实现过程，只使用了HttpRuntime.Cache的有限特性，有更多需求的同学可以自行扩展；见解有限，谬误之处还请园友指正。
园友Jusfr 原创，转载请注明来自博客园 。
注意后续代码及改进见后后文及github，文章上的并没有更新。
