在上文中《在Ocelot中使用自定义的中间件（一）》，我介绍了如何在Ocelot中使用自定义的中间件来修改下游服务的response body。今天，我们再扩展一下设计，让我们自己设计的中间件变得更为通用，使其能够应用在不同的Route上。比如，我们可以设计一个通用的替换response body的中间件，然后将其应用在多个Route上。

Ocelot的配置文件

我们可以将Ocelot的配置信息写在appsettings.json中，当然也可以将其放在单独的json文件里，然后通过ConfigureAppConfiguration的调用，将单独的json文件添加到配置系统中。无论如何，基于JSON文件的Ocelot配置都是可以加入我们自定义的内容的，基于数据库的或者其它存储的配置文件信息或许扩展起来并不方便，因此，使用JSON文件作为配置源还是一个不错的选择。比如，我们可以在ReRoute的某个配置中添加以下内容：

{

"DownstreamPathTemplate": "/api/themes",

"DownstreamScheme": "http",

"DownstreamHostAndPorts": [

{

"Host": "localhost",

"Port": 5010

}

],

"UpstreamPathTemplate": "/themes-api/themes",

"UpstreamHttpMethod": [ "Get" ],

"CustomMiddlewares": [

{

"Name": "themeCssMinUrlReplacer",

"Enabled": true,

"Config": {

"replacementTemplate": "/themes-api/theme-css/{name}"

}

]

}

然后就需要有一个方法能够解析这部分配置内容。为了方便处理，可以增加以下配置Model，专门存放CustomMiddlewares下的配置信息：

public class CustomMiddlewareConfiguration

{

public string DownstreamPathTemplate { get; set; }

public string UpstreamPathTemplate { get; set; }

public int ReRouteConfigurationIndex { get; set; }

public string Name { get; set; }

public bool Enabled { get; set; }

public Dictionary<string, object> Config { get; set; }

}

然后定义下面的扩展方法，用以从IConfiguration对象中解析出所有的CustomMiddleware的配置信息：

public static IEnumerable<CustomMiddlewareConfiguration> GetCustomMiddlewareConfigurations(this IConfiguration config)

{

var reRoutesConfigSection = config.GetSection("ReRoutes");

if (reRoutesConfigSection.Exists())

{

var reRoutesConfigList = reRoutesConfigSection.GetChildren();

for (var idx = 0; idx < reRoutesConfigList.Count(); idx++)

{

var reRouteConfigSection = reRoutesConfigList.ElementAt(idx);

var upstreamPathTemplate = reRouteConfigSection.GetSection("UpstreamPathTemplate").Value;

var downstreamPathTemplate = reRouteConfigSection.GetSection("DownstreamPathTemplate").Value;

var customMidwareConfigSection = reRouteConfigSection.GetSection("CustomMiddlewares");

if (customMidwareConfigSection.Exists())

{

var customMidwareConfigList = customMidwareConfigSection.GetChildren();

foreach (var customMidwareConfig in customMidwareConfigList)

{

var customMiddlewareConfiguration = customMidwareConfig.Get<CustomMiddlewareConfiguration>();

customMiddlewareConfiguration.UpstreamPathTemplate = upstreamPathTemplate;

customMiddlewareConfiguration.DownstreamPathTemplate = downstreamPathTemplate;

customMiddlewareConfiguration.ReRouteConfigurationIndex = idx;

yield return customMiddlewareConfiguration;

}

yield break;

}

CustomMiddleware基类

为了提高程序员的开发体验，我们引入CustomMiddleware基类，在Invoke方法中，CustomMiddleware对象会读取所有的CustomMiddleware配置信息，并找到属于当前ReRoute的CustomMiddleware配置信息，从而决定当前的CustomMiddleware是否应该被执行。相关代码如下：

public abstract class CustomMiddleware : OcelotMiddleware

{

#region Private Fields

private readonly ICustomMiddlewareConfigurationManager customMiddlewareConfigurationManager;

private readonly OcelotRequestDelegate next;

#endregion Private Fields

#region Protected Constructors

protected CustomMiddleware(OcelotRequestDelegate next,

ICustomMiddlewareConfigurationManager customMiddlewareConfigurationManager,

IOcelotLogger logger) : base(logger)

{

this.next = next;

this.customMiddlewareConfigurationManager = customMiddlewareConfigurationManager;

}

#endregion Protected Constructors

#region Public Methods

public async Task Invoke(DownstreamContext context)

{

var customMiddlewareConfigurations = from cmc in this

.customMiddlewareConfigurationManager

.GetCustomMiddlewareConfigurations()

where cmc.DownstreamPathTemplate == context

.DownstreamReRoute

.DownstreamPathTemplate

.Value &&

cmc.UpstreamPathTemplate == context

.DownstreamReRoute

.UpstreamPathTemplate

.OriginalValue

select cmc;

var thisMiddlewareName = this.GetType().GetCustomAttribute<CustomMiddlewareAttribute>(false)?.Name;

var customMiddlewareConfiguration = customMiddlewareConfigurations.FirstOrDefault(x => x.Name == thisMiddlewareName);

if (customMiddlewareConfiguration?.Enabled ?? false)

{

await this.DoInvoke(context, customMiddlewareConfiguration);

}

await this.next(context);

}

#endregion Public Methods

#region Protected Methods

protected abstract Task DoInvoke(DownstreamContext context, CustomMiddlewareConfiguration configuration);

#endregion Protected Methods

}

接下来就简单了，只需要让自定义的Ocelot中间件继承于CustomMiddleware基类就行了，当然，为了解耦类型名称与中间件名称，使用一个自定义的CustomMiddlewareAttribute：

[CustomMiddleware("themeCssMinUrlReplacer")]

public class ThemeCssMinUrlReplacer : CustomMiddleware

{

private readonly Regex regex = new Regex(@"\w+://[a-zA-Z0-9]+(\:\d+)?/themes/(?<theme_name>[a-zA-Z0-9_]+)/bootstrap.min.css");

public ThemeCssMinUrlReplacer(OcelotRequestDelegate next,

ICustomMiddlewareConfigurationManager customMiddlewareConfigurationManager,

IOcelotLoggerFactory loggerFactory)

: base(next, customMiddlewareConfigurationManager, loggerFactory.CreateLogger<ThemeCssMinUrlReplacer>())

{

}

protected override async Task DoInvoke(DownstreamContext context, CustomMiddlewareConfiguration configuration)

{

var downstreamResponseString = await context.DownstreamResponse.Content.ReadAsStringAsync();

var downstreamResponseJson = JObject.Parse(downstreamResponseString);

var themesArray = (JArray)downstreamResponseJson["themes"];

foreach(var token in themesArray)

{

var cssMinToken = token["cssMin"];

var cssMinValue = cssMinToken.Value<string>();

if (regex.IsMatch(cssMinValue))

{

var themeName = regex.Match(cssMinValue).Groups["theme_name"].Value;

var replacementTemplate = configuration.Config["replacementTemplate"].ToString();

var replacement = $"{context.HttpContext.Request.Scheme}://{context.HttpContext.Request.Host}{replacementTemplate}"

.Replace("{name}", themeName);

cssMinToken.Replace(replacement);

}

context.DownstreamResponse = new DownstreamResponse(

new StringContent(downstreamResponseJson.ToString(Formatting.None), Encoding.UTF8, "application/json"),

context.DownstreamResponse.StatusCode, context.DownstreamResponse.Headers, context.DownstreamResponse.ReasonPhrase);

}

自定义中间件的注册

在上文介绍的BuildCustomOcelotPipeline扩展方法中，加入以下几行，就完成所有自定义中间件的注册：

var customMiddlewareTypes = from type in typeof(Startup).Assembly.GetTypes()

where type.BaseType == typeof(CustomMiddleware) &&

type.IsDefined(typeof(CustomMiddlewareAttribute), false)

select type;

foreach (var customMiddlewareType in customMiddlewareTypes)

{

builder.UseMiddleware(customMiddlewareType);

}

当然，app.UseOcelot的调用要调整为：

1	`app.UseOcelot((b, c) => b.BuildCustomOcelotPipeline(c).Build()).Wait();`

运行

重新运行API网关，得到结果跟之前的一样。所不同的是，我们可以将ThemeCssMinUrlReplacer在其它的ReRoute配置上重用了。

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标签

ocelot

.Net Framework

在Ocelot中使用自定义的中间件（二）

Ocelot的配置文件

CustomMiddleware基类

自定义中间件的注册

运行