[缓存TryGetValue与Get

Question

这就是我实现CacheManager的方式。我面临的问题是TryGetValue将始终在RemoveFromCache函数中返回null。在令牌之一过期后调用此函数，因此我试图从缓存中的列表中清除该令牌，而GetAllTokens返回所有令牌的完整列表。 AddTokenToCache工作正常。

它是ASPNET-Core 3.0上的WebAPI

CacheManager.cs

public class CacheManager : ICacheManager
{
    private IMemoryCache _cache;

    public CacheManager(IMemoryCache cache) {
        _cache = cache;

    }        
    public void AddTokenToCache(string appName, string tokenString)
    {
        List<Token> tokens = new List<Token>();

        //save this token against the application record in-memory
        if (!_cache.TryGetValue(CacheHelper.CacheKey_Tokens, out tokens))
        {
            if (tokens == null)
                tokens = new List<Token>();

        }

        tokens.Add(new Token
        {
            AppName = appName,
            GeneratedAt = DateTime.Now,
            TokenId = tokenString
        });

        // Set cache options.
        var cacheEntryOptions = new MemoryCacheEntryOptions()
        ;//    .SetSlidingExpiration(TimeSpan.FromSeconds(180)); //3 minutes

        _cache.Set(CacheHelper.CacheKey_Tokens, tokens, cacheEntryOptions);
    }

    public List<Token> GetAllTokens()
    {
        return _cache.Get<List<Token>>(CacheHelper.CacheKey_Tokens);
    }

    public bool RemoveFromCache(string tokenId)
    {

        List<Token> tokens = new List<Token>();            

        //remove this token from memory            
        if (!_cache.TryGetValue(CacheHelper.CacheKey_Tokens, out tokens)) {
            return false;
        }
        else
        {
            if (tokens != null && tokens.Count > 0)
            {
                //_logger.LogInfo("Processing token");
                //trimming quotations from the string
                tokenId = tokenId.Substring(1, tokenId.Length - 2);
                int index = tokens.FindIndex(t => t.TokenId == tokenId);

                if (index >= 0)
                    tokens.RemoveAt(index);

                var cacheEntryOptions = new MemoryCacheEntryOptions();
                _cache.Set(CacheHelper.CacheKey_Tokens, tokens, cacheEntryOptions);

                return true;
            }                
        }

        return false;
    }
}

我的通话顺序是：

• AddTokenToCache（令牌已成功添加到缓存）
• GetAllToken（显示令牌已添加到缓存）
• AddTokenToCache（令牌已成功添加到缓存）
• GetAllToken（显示两个令牌都已添加到缓存）
• 调用RemoveFromCache的Fire TokenExpired事件（令牌为空）
• GetAllToken（显示两个令牌都已添加到缓存）

Startup.cs

public void ConfigureServices(IServiceCollection services)
    { 
        services.AddSingleton<ILoggerManager, LoggerManager>();

        services.AddMemoryCache();
        services.AddDbContext<GEContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
        services.AddControllers();
        services.AddRazorPages();
        services.AddSingleton<ICacheManager, CacheManager>();

        RegisterHandlerforTokenExpiredEvent(services);
        //other code removed for brevity
    }
public void RegisterHandlerforTokenExpiredEvent(IServiceCollection services)
    {
        var sp = services.BuildServiceProvider();

        var jwtManager = sp.GetService<IJWTAuthenticationManager>(); //publisher            
        var cacheManager = sp.GetService<ICacheManager>(); //subscriber

        jwtManager.TokenExpired += cacheManager.OnTokenExpired;            
    }

Answer 1

这是因为您通过ServiceProvider建立了另一个services.BuildServiceProvider()：

public void RegisterHandlerforTokenExpiredEvent(IServiceCollection services)

{
    var sp = services.BuildServiceProvider();  // this is a different service provider from the default one built by ASP.NET Core itself.

    var jwtManager = sp.GetService<IJWTAuthenticationManager>(); //publisher            
    var cacheManager = sp.GetService<ICacheManager>(); //subscriber
    // it doesn't work because the cacheManager is not the same instance that you use in the controllers
    jwtManager.TokenExpired += cacheManager.OnTokenExpired;            
}

结果，您获得的ICacheManager实例是您在Controllers / Other Services中注入的NOt the same singleton。换句话说，您将有两个不同的ICacheManager实例！

作为一个黄金法则，DO NOT在您的应用程序层代码中通过services.BuildServiceProvider()生成ServiceProvider的另一个副本，除非您确定它适合您。

如何修复

1. 代替构建服务提供商的另一个副本然后获取另一个实例，您应该始终使用IoC而不是服务定位器模式。

2. 似乎您的JWTAuthenticationManager是单例，并且您想在启动时绑定事件处理程序。如果是这种情况，您可以注册一个HostedService。

   public class MyHostedService : IHostedService
   {
       private readonly IJWTAuthenticationManager _jWTAuthManager;
       private readonly ICacheManager _cacheManager;
   
       // suppose your IJWTAuthenticationManager  is a singleton service
       public MyHostedService(IJWTAuthenticationManager jWTAuthManager, ICacheManager cacheManager)
       {
           this._jWTAuthManager = jWTAuthManager;
           this._cacheManager = cacheManager;
       }
   
       public Task StartAsync(CancellationToken cancellationToken)
       {
           this._jWTAuthManager.TokenExpired += this._cacheManager.OnTokenExpired;
           return Task.CompletedTask;
       }
   
       public Task StopAsync(CancellationToken cancellationToken)
       {
           this._jWTAuthManager.TokenExpired -= this._cacheManager.OnTokenExpired;
           return Task.CompletedTask;
       }
   }
   

   并在启动中注册此服务：

   services.AddHostedService<MyHostedService>();