如何进行方法签名缓存？

Question

我正在使用 .NET 和 C# 构建一个应用程序，我想通过使用属性/注释而不是方法中的显式代码来缓存一些结果。

我想要一个看起来有点像这样的方法签名：

[Cache, timeToLive=60]
String getName(string id, string location)

它应该根据输入生成哈希，并将其用作结果的密钥。 当然，会有一些配置文件告诉它如何实际放入内存缓存、本地字典或其他东西。

你知道这样的框架吗？

我什至对 Java 也感兴趣

Answer 1

通过 Microsoft Enterprise Library 中的 CacheHandler，您可以轻松实现此目的。
例如：

[CacheHandler(0, 30, 0)]
public Object GetData(Object input)
{
}

将使对该方法的所有调用缓存 30 分钟。所有调用都会根据输入数据和方法名称获取唯一的缓存键，因此如果您使用不同的输入调用该方法两次，则不会被缓存，但如果您在超时间隔内使用相同的输入调用它>1次，则该方法只执行一次。

我在微软的代码中添加了一些额外的功能：

我的修改版本如下所示：

using System;
using System.Diagnostics;
using System.IO;
using System.Reflection;
using System.Runtime.Remoting.Contexts;
using System.Text;
using System.Web;
using System.Web.Caching;
using System.Web.UI;
using Microsoft.Practices.EnterpriseLibrary.Common.Configuration;
using Microsoft.Practices.Unity.InterceptionExtension;


namespace Middleware.Cache
{
    /// <summary>
    /// An <see cref="ICallHandler"/> that implements caching of the return values of
    /// methods. This handler stores the return value in the ASP.NET cache or the Items object of the current request.
    /// </summary>
    [ConfigurationElementType(typeof (CacheHandler)), Synchronization]
    public class CacheHandler : ICallHandler
    {
        /// <summary>
        /// The default expiration time for the cached entries: 5 minutes
        /// </summary>
        public static readonly TimeSpan DefaultExpirationTime = new TimeSpan(0, 5, 0);

        private readonly object cachedData;

        private readonly DefaultCacheKeyGenerator keyGenerator;
        private readonly bool storeOnlyForThisRequest = true;
        private TimeSpan expirationTime;
        private GetNextHandlerDelegate getNext;
        private IMethodInvocation input;


        public CacheHandler(TimeSpan expirationTime, bool storeOnlyForThisRequest)
        {
            keyGenerator = new DefaultCacheKeyGenerator();
            this.expirationTime = expirationTime;
            this.storeOnlyForThisRequest = storeOnlyForThisRequest;
        }

        /// <summary>
        /// This constructor is used when we wrap cached data in a CacheHandler so that 
        /// we can reload the object after it has been removed from the cache.
        /// </summary>
        /// <param name="expirationTime"></param>
        /// <param name="storeOnlyForThisRequest"></param>
        /// <param name="input"></param>
        /// <param name="getNext"></param>
        /// <param name="cachedData"></param>
        public CacheHandler(TimeSpan expirationTime, bool storeOnlyForThisRequest,
                            IMethodInvocation input, GetNextHandlerDelegate getNext,
                            object cachedData)
            : this(expirationTime, storeOnlyForThisRequest)
        {
            this.input = input;
            this.getNext = getNext;
            this.cachedData = cachedData;
        }


        /// <summary>
        /// Gets or sets the expiration time for cache data.
        /// </summary>
        /// <value>The expiration time.</value>
        public TimeSpan ExpirationTime
        {
            get { return expirationTime; }
            set { expirationTime = value; }
        }

        #region ICallHandler Members

        /// <summary>
        /// Implements the caching behavior of this handler.
        /// </summary>
        /// <param name="input"><see cref="IMethodInvocation"/> object describing the current call.</param>
        /// <param name="getNext">delegate used to get the next handler in the current pipeline.</param>
        /// <returns>Return value from target method, or cached result if previous inputs have been seen.</returns>
        public IMethodReturn Invoke(IMethodInvocation input, GetNextHandlerDelegate getNext)
        {
            lock (input.MethodBase)
            {
                this.input = input;
                this.getNext = getNext;

                return loadUsingCache();
            }
        }

        public int Order
        {
            get { return 0; }
            set { }
        }

        #endregion

        private IMethodReturn loadUsingCache()
        {
            //We need to synchronize calls to the CacheHandler on method level
            //to prevent duplicate calls to methods that could be cached.
            lock (input.MethodBase)
            {
                if (TargetMethodReturnsVoid(input) || HttpContext.Current == null)
                {
                    return getNext()(input, getNext);
                }

                var inputs = new object[input.Inputs.Count];
                for (int i = 0; i < inputs.Length; ++i)
                {
                    inputs[i] = input.Inputs[i];
                }

                string cacheKey = keyGenerator.CreateCacheKey(input.MethodBase, inputs);
                object cachedResult = getCachedResult(cacheKey);

                if (cachedResult == null)
                {
                    var stopWatch = Stopwatch.StartNew();
                    var realReturn = getNext()(input, getNext);
                    stopWatch.Stop();
                    if (realReturn.Exception == null && realReturn.ReturnValue != null)
                    {
                        AddToCache(cacheKey, realReturn.ReturnValue);
                    }
                    return realReturn;
                }

                var cachedReturn = input.CreateMethodReturn(cachedResult, input.Arguments);

                return cachedReturn;
            }
        }

        private object getCachedResult(string cacheKey)
        {
            //When the method uses input that is not serializable 
            //we cannot create a cache key and can therefore not 
            //cache the data.
            if (cacheKey == null)
            {
                return null;
            }

            object cachedValue = !storeOnlyForThisRequest ? HttpRuntime.Cache.Get(cacheKey) : HttpContext.Current.Items[cacheKey];
            var cachedValueCast = cachedValue as CacheHandler;
            if (cachedValueCast != null)
            {
                //This is an object that is reloaded when it is being removed.
                //It is therefore wrapped in a CacheHandler-object and we must
                //unwrap it before returning it.
                return cachedValueCast.cachedData;
            }
            return cachedValue;
        }

        private static bool TargetMethodReturnsVoid(IMethodInvocation input)
        {
            var targetMethod = input.MethodBase as MethodInfo;
            return targetMethod != null && targetMethod.ReturnType == typeof (void);
        }

        private void AddToCache(string key, object valueToCache)
        {
            if (key == null)
            {
                //When the method uses input that is not serializable 
                //we cannot create a cache key and can therefore not 
                //cache the data.
                return;
            }

            if (!storeOnlyForThisRequest)
            {
                HttpRuntime.Cache.Insert(
                    key,
                    valueToCache,
                    null,
                    System.Web.Caching.Cache.NoAbsoluteExpiration,
                    expirationTime,
                    CacheItemPriority.Normal, null);
            }
            else
            {
                HttpContext.Current.Items[key] = valueToCache;
            }
        }
    }

    /// <summary>
    /// This interface describes classes that can be used to generate cache key strings
    /// for the <see cref="CacheHandler"/>.
    /// </summary>
    public interface ICacheKeyGenerator
    {
        /// <summary>
        /// Creates a cache key for the given method and set of input arguments.
        /// </summary>
        /// <param name="method">Method being called.</param>
        /// <param name="inputs">Input arguments.</param>
        /// <returns>A (hopefully) unique string to be used as a cache key.</returns>
        string CreateCacheKey(MethodBase method, object[] inputs);
    }

    /// <summary>
    /// The default <see cref="ICacheKeyGenerator"/> used by the <see cref="CacheHandler"/>.
    /// </summary>
    public class DefaultCacheKeyGenerator : ICacheKeyGenerator
    {
        private readonly LosFormatter serializer = new LosFormatter(false, "");

        #region ICacheKeyGenerator Members

        /// <summary>
        /// Create a cache key for the given method and set of input arguments.
        /// </summary>
        /// <param name="method">Method being called.</param>
        /// <param name="inputs">Input arguments.</param>
        /// <returns>A (hopefully) unique string to be used as a cache key.</returns>
        public string CreateCacheKey(MethodBase method, params object[] inputs)
        {
            try
            {
                var sb = new StringBuilder();

                if (method.DeclaringType != null)
                {
                    sb.Append(method.DeclaringType.FullName);
                }
                sb.Append(':');
                sb.Append(method.Name);

                TextWriter writer = new StringWriter(sb);

                if (inputs != null)
                {
                    foreach (var input in inputs)
                    {
                        sb.Append(':');
                        if (input != null)
                        {
                            //Diffrerent instances of DateTime which represents the same value
                            //sometimes serialize differently due to some internal variables which are different.
                            //We therefore serialize it using Ticks instead. instead.
                            var inputDateTime = input as DateTime?;
                            if (inputDateTime.HasValue)
                            {
                                sb.Append(inputDateTime.Value.Ticks);
                            }
                            else
                            {
                                //Serialize the input and write it to the key StringBuilder.
                                serializer.Serialize(writer, input);
                            }
                        }
                    }
                }

                return sb.ToString();
            }
            catch
            {
                //Something went wrong when generating the key (probably an input-value was not serializble.
                //Return a null key.
                return null;
            }
        }

        #endregion
    }
}

这段代码最值得赞扬的是微软。我们只添加了请求级别的缓存等内容，而不是跨请求（比您想象的更有用），并修复了一些错误（例如，相同的 DateTime 对象序列化为不同的值）。

Answer 2

要准确地执行您所描述的操作，即编写

public class MyClass {
  [Cache, timeToLive=60]
  string getName(string id, string location){
    return ExpensiveCall(id, location);
  }
}

// ...
MyClass c = new MyClass();
string name = c.getName("id", "location");
string name_again = c.getName("id", "location");

并仅调用一次昂贵的调用，而无需使用其他代码包装该类 (fx CacheHandlerc = new CacheHandler(new MyClass( ));) 你需要研究一个面向方面的编程框架。这些通常通过重写字节码来工作，因此您需要在编译过程中添加另一个步骤 - 但您在此过程中获得了很多能力。 AOP 框架有很多，但 PostSharp for .NET 和 AspectJ 是最受欢迎的。您可以轻松地谷歌如何使用它们来添加您想要的缓存方面。