/WebApiClient

An open source project based on the HttpClient. You only need to define the c# interface and modify the related features to invoke the client library of the remote http interface asynchronously.

Primary LanguageC#MIT LicenseMIT

WebApiClientCore

WebApiClient.JIT的netcoreapp版本,集高性能高可扩展性于一体的声明式http客户端库,特别适用于微服务的restful资源请求,也适用于各种畸形http接口请求。

PackageReference

<PackageReference Include="WebApiClientCore" Version="1.0.0-beta*" />

Benchmark

使用MockResponseHandler消除真实http请求,原生HttpClient、WebApiClientCore和Refit的性能参考:

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.18362.836 (1903/May2019Update/19H1)
Intel Core i3-4150 CPU 3.50GHz (Haswell), 1 CPU, 4 logical and 2 physical cores
.NET Core SDK=3.1.202
  [Host]     : .NET Core 3.1.4 (CoreCLR 4.700.20.20201, CoreFX 4.700.20.22101), X64 RyuJIT
  DefaultJob : .NET Core 3.1.4 (CoreCLR 4.700.20.20201, CoreFX 4.700.20.22101), X64 RyuJIT
Method Mean Error StdDev
HttpClient_GetAsync 3.945 μs 0.2050 μs 0.5850 μs
WebApiClientCore_GetAsync 13.320 μs 0.2604 μs 0.3199 μs
Refit_GetAsync 43.503 μs 0.8489 μs 1.0426 μs
Method Mean Error StdDev
HttpClient_PostAsync 4.876 μs 0.0972 μs 0.2092 μs
WebApiClientCore_PostAsync 14.018 μs 0.1829 μs 0.2246 μs
Refit_PostAsync 46.512 μs 0.7885 μs 0.7376 μs

声明式接口定义

  • 支持Task、Task<>和ITask<>三种异步返回
  • 支持模型自动转换为Xml、Json、Form、和FormData共4种请求格式的内容
  • 支持HttpResponseMessage、byte[]、string和Stream原生类型返回内容
  • 支持原生HttpContent(比如StringContent)类型直接做为请求参数
  • 内置丰富的能满足各种环境的常用特性(ActionAttribute和ParameterAttribute)
  • 内置常用的FormDataFile等参数类型,同时支持自定义IApiParameter参数类型作为参数值
  • 支持用户自定义IApiActionAttribute、IApiParameterAttribue、IApiReturnAttribute和IApiFilterAttribute

1 Petstore接口例子

这个OpenApi文档在petstore.swagger.io,代码为使用WebApiClientCore.OpenApi.SourceGenerator工具将其OpenApi文档反向生成得到

namespace Petstore
{
    /// <summary>
    /// Everything about your Pets
    /// </summary>
    [LoggingFilter]
    [HttpHost("https://petstore.swagger.io/v2/")]
    public interface IPetApi : IHttpApi
    {
        /// <summary>
        /// Add a new pet to the store
        /// </summary>
        /// <param name="body">Pet object that needs to be added to the store</param>
        [HttpPost("pet")]
        Task AddPetAsync([Required] [JsonContent] Pet body, CancellationToken token = default);

        /// <summary>
        /// Update an existing pet
        /// </summary>
        /// <param name="body">Pet object that needs to be added to the store</param>
        [HttpPut("pet")]
        Task<HttpResponseMessage> UpdatePetAsync([Required] [JsonContent] Pet body, CancellationToken token = default);

        /// <summary>
        /// Finds Pets by status
        /// </summary>
        /// <param name="status">Status values that need to be considered for filter</param>
        /// <returns>successful operation</returns>
        [HttpGet("pet/findByStatus")]
        ITask<List<Pet>> FindPetsByStatusAsync([Required] IEnumerable<Anonymous> status);

        /// <summary>
        /// Finds Pets by tags
        /// </summary>
        /// <param name="tags">Tags to filter by</param>
        /// <returns>successful operation</returns>
        [Obsolete]
        [HttpGet("pet/findByTags")]
        ITask<List<Pet>> FindPetsByTagsAsync([Required] [PathQuery] IEnumerable<string> tags);

        /// <summary>
        /// Find pet by ID
        /// </summary>
        /// <param name="petId">ID of pet to return</param>
        /// <returns>successful operation</returns>
        [HttpGet("pet/{petId}")]
        ITask<Pet> GetPetByIdAsync([Required] long petId);

        /// <summary>
        /// Updates a pet in the store with form data
        /// </summary>
        /// <param name="petId">ID of pet that needs to be updated</param>
        /// <param name="name">Updated name of the pet</param>
        /// <param name="status">Updated status of the pet</param>
        [HttpPost("pet/{petId}")]
        Task UpdatePetWithFormAsync([Required] long petId, [FormFiled] string name, [FormFiled] string status);

        /// <summary>
        /// Deletes a pet
        /// </summary>
        /// <param name="api_key"></param>
        /// <param name="petId">Pet id to delete</param>
        [HttpDelete("pet/{petId}")]
        Task DeletePetAsync([Header("api_key")] string api_key, [Required] long petId);

        /// <summary>
        /// uploads an image
        /// </summary>
        /// <param name="petId">ID of pet to update</param>
        /// <param name="additionalMetadata">Additional data to pass to server</param>
        /// <param name="file">file to upload</param>
        /// <returns>successful operation</returns>
        [LoggingFilter(Enable = false)]
        [HttpPost("pet/{petId}/uploadImage")]
        ITask<ApiResponse> UploadFileAsync([Required] long petId, [FormDataText] string additionalMetadata, FormDataFile file);
    }
}

2 IOAuthClient接口例子

这个接口是在WebApiClientCore.Extensions.OAuths.IOAuthClient.cs代码中声明

using System;
using System.ComponentModel.DataAnnotations;
using System.Diagnostics.CodeAnalysis;
using System.Threading.Tasks;
using WebApiClientCore.Attributes;

namespace WebApiClientCore.Extensions.OAuths
{
    /// <summary>
    /// 定义Token客户端的接口
    /// </summary>
    [LoggingFilter]
    [XmlReturn(Enable = false)]
    [JsonReturn(EnsureMatchAcceptContentType = false, EnsureSuccessStatusCode = false)]
    public interface IOAuthClient
    {
        /// <summary>
        /// 以client_credentials授权方式获取token
        /// </summary>
        /// <param name="endpoint">token请求地址</param>
        /// <param name="credentials">身份信息</param>
        /// <returns></returns>
        [HttpPost]
        [FormField("grant_type", "client_credentials")]
        Task<TokenResult> RequestTokenAsync([Required, Uri] Uri endpoint, [Required, FormContent] ClientCredentials credentials);

        /// <summary>
        /// 以password授权方式获取token
        /// </summary>
        /// <param name="endpoint">token请求地址</param>
        /// <param name="credentials">身份信息</param>
        /// <returns></returns>
        [HttpPost]
        [FormField("grant_type", "password")]
        Task<TokenResult> RequestTokenAsync([Required, Uri] Uri endpoint, [Required, FormContent] PasswordCredentials credentials);

        /// <summary>
        /// 刷新token
        /// </summary>
        /// <param name="endpoint">token请求地址</param>
        /// <param name="credentials">身份信息</param>
        /// <returns></returns>
        [HttpPost]
        [FormField("grant_type", "refresh_token")]
        Task<TokenResult> RefreshTokenAsync([Required, Uri] Uri endpoint, [Required, FormContent] RefreshTokenCredentials credentials);
    }
}

编译时语法分析

WebApiClientCore.Analyzers提供编码时语法分析与提示。

比如[Header]特性,可以声明在Interface、Method和Parameter三个地方,但是必须使用正确的构造器,否则运行时会抛出异常。有了语法分析功能,在声明接口时就不会使用不当的语法。如果想让语法分析生效,你的接口必须继承空方法的IHttpApi接口。

/// <summary>
/// 你的接口,记得要实现IHttpApi
/// </summary>
public interface IYourApi : IHttpApi
{
    ...
}

接口注册与实例获取

1 接口服务注册

var services = new ServiceCollection();

services.AddHttpApi<IPetApi>(o =>
{
    o.UseParameterPropertyValidate = true;
    o.UseReturnValuePropertyValidate = false;
    o.KeyValueSerializeOptions.IgnoreNullValues = true;
    o.HttpHost = new Uri("http://localhost:6000/");
});

2 接口实例获取

public class MyService
{
    private readonly IpetApi petApi;
    
    // 构造器注入IpetApi
    public MyService(IpetApi petApi)
    {
        tihs.petApi = petApi;
    }
}

接口选项与配置

每个接口的选项对应为HttpApiOptions<THttpApi>,除了Action配置,我们也可以使用Configuration配置结合一起使用,这部分内容为Microsoft.Extensions.Options范畴。

Action配置

services
    .ConfigureHttpApi<IpetApi>(Configuration.GetSection(nameof(IpetApi)))
    .ConfigureHttpApi<IpetApi>(o =>
    {
        // 符合国情的不标准时间格式,有些接口就是这么要求必须不标准
        o.JsonSerializeOptions.Converters.Add(new JsonLocalDateTimeConverter("yyyy-MM-dd HH:mm:ss"));
    });

appsettings.json的文件配置

{
  "IpetApi": {
    "HttpHost": "http://www.webappiclient.com/",
    "UseParameterPropertyValidate": false,
    "UseReturnValuePropertyValidate": false,
    "JsonSerializeOptions": {
      "IgnoreNullValues": true,
      "WriteIndented": false
    }
  }
}

Uri拼接规则

所有的Uri拼接都是通过Uri(Uri baseUri, Uri relativeUri)这个构造器生成。

/结尾的baseUri

  • http://a.com/ + b/c/d = http://a.com/b/c/d
  • http://a.com/path1/ + b/c/d = http://a.com/path1/b/c/d
  • http://a.com/path1/path2/ + b/c/d = http://a.com/path1/path2/b/c/d

不带/结尾的baseUri

  • http://a.com + b/c/d = http://a.com/b/c/d
  • http://a.com/path1 + b/c/d = http://a.com/b/c/d
  • http://a.com/path1/path2 + b/c/d = http://a.com/path1/b/c/d

事实上http://a.comhttp://a.com/是完全一样的,他们的path都是/,所以才会表现一样。为了避免低级错误的出现,请使用的标准baseUri书写方式,即使用/作为baseUri的结尾的第一种方式。

表单集合处理

按照OpenApi,一个集合在Uri的Query或表单中支持5种表述方式,分别是:

  • Csv // 逗号分隔
  • Ssv // 空格分隔
  • Tsv // 反斜杠分隔
  • Pipes // 竖线分隔
  • Multi // 多个同名键的键值对

对于 id = new string []{"001","002"} 这样的值,处理后分别是

  • id=001,002
  • id=001 002
  • id=001\002
  • id=001|002
  • id=001&id=002

默认的,PathQuryAttribute与FormContentAttribute使用了Multi处理方式,可以设置其CollectionFormat属性为其它值,比如:[FormContent(CollectionFormat = CollectionFormat.Csv)]

Accpet ContentType

这个用于控制客户端希望服务器返回什么样的内容格式,比如json或xml。

缺省配置值

缺省配置是[JsonReturn(0.01),XmlReturn(0.01)],对应的请求accept值是 Accept: application/json; q=0.01, application/xml; q=0.01

Json优先

在Interface或Method上显式地声明[JsonReturn],请求accept变为Accept: application/json, application/xml; q=0.01

禁用json

在Interface或Method上声明[JsonReturn(Enable = false)],请求变为Accept: application/xml; q=0.01

请求和响应日志

在整个Interface或某个Method上声明[LoggingFilter],即可把请求和响应的内容输出到LoggingFactory中。如果要排除某个Method不打印日志,在该Method上声明[LoggingFilter(Enable = false)],即可将本Method排除。

默认日志

[LoggingFilter]   
public interface IUserApi : IHttpApi
{
    [HttpGet("api/users/{account}")]
    ITask<HttpResponseMessage> GetAsync([Required]string account);  

    // 禁用日志
    [LoggingFilter(Enable =false)]
    [HttpPost("api/users/body")]
    Task<User> PostByJsonAsync([Required, JsonContent]User user, CancellationToken token = default);
}

自定义日志输出目标

class MyLogging : LoggingFilterAttribute
{
    protected override Task WriteLogAsync(ApiResponseContext context, LogMessage logMessage)
    {
        xxlogger.Log(logMessage.ToIndentedString(spaceCount: 4));
        return Task.CompletedTask;
    }
}

请求条件性重试

使用ITask<>异步声明,就有Retry的扩展,Retry的条件可以为捕获到某种Exception或响应模型符合某种条件。

var result = await youApi.GetModelAsync(id: "id001")
    .Retry(maxCount: 3)
    .WhenCatch<HttpRequestException>()
    .WhenResult(r => r.ErrorCode > 0);

异常和异常处理

请求一个接口,不管出现何种异常,最终都抛出HttpRequestException,HttpRequestException的内部异常为实际具体异常,之所以设计为内部异常,是为了完好的保存内部异常的堆栈信息。

WebApiClient内部的很多异常都基于ApiException这个抽象异常,也就是很多情况下,抛出的异常都是内为某个ApiException的HttpRequestException。

try
{
    var model = await api.GetAsync();
}
catch (HttpRequestException ex) when (ex.InnerException is ApiInvalidConfigException configException)
{
    // 请求配置异常
}
catch (HttpRequestException ex) when (ex.InnerException is ApiResponseStatusException statusException)
{
    // 响应状态码异常
}
catch (HttpRequestException ex) when (ex.InnerException is ApiException apiException)
{
    // 抽象的api异常
}
catch (HttpRequestException ex) when (ex.InnerException is SocketException socketException)
{
    // socket连接层异常
}
catch (HttpRequestException ex)
{
    // 请求异常
}
catch (Exception ex)
{
    // 异常
}

响应内容缓存

配置CacheAttribute特性的Method会将本次的响应内容缓存起来,下一次如果符合预期条件的话,就不会再请求到远程服务器,而是从IResponseCacheProvider获取缓存内容,开发者可以自己实现ResponseCacheProvider。

声明缓存特性

// 缓存一分钟
[Cache(60 * 1000)]
[HttpGet("api/users/{account}")]
ITask<HttpResponseMessage> GetAsync([Required]string account);

自定义缓存提供者

public class RedisResponseCacheProvider : IResponseCacheProvider
{
    public string Name => nameof(RedisResponseCacheProvider);

    public Task<ResponseCacheResult> GetAsync(string key)
    {
        throw new NotImplementedException();
    }

    public Task SetAsync(string key, ResponseCacheEntry entry, TimeSpan expiration)
    {
        throw new NotImplementedException();
    }
}

// 注册RedisResponseCacheProvider
var services = new ServiceCollection();
services.AddSingleton<IResponseCacheProvider, RedisResponseCacheProvider>();

非模型请求

有时候我们未必需要强模型,假设我们已经有原始的form文本内容,或原始的json文本内容,甚至是System.Net.Http.HttpContent对象,只需要把这些原始内请求到远程远程器。

原始文本

[HttpPost]
Task PostAsync([RawStringContent("txt/plain")] string text);

[HttpPost]
Task PostAsync(StringContent text);

原始json

[HttpPost]
Task PostAsync([RawJsonContent] string json);

原始xml

[HttpPost]
Task PostAsync([RawXmlContent] string xml);

原始表单内容

[HttpPost]
Task PostAsync([RawFormContent] string form);

自定义无特性的参数类型

在某些极限情况下,比如人脸比对的接口,我们输入模型与传输模型未必是对等的:

服务端要求的json模型

{
    "image1" : "图片1的base64",
    "image2" : "图片2的base64"
}

客户端期望的业务模型

class FaceModel
{
    public Bitmap Image1 {get; set;}
    public Bitmap Image2 {get; set;}
}

我们希望构造模型实例时传入Bitmap对象,但传输的时候变成Bitmap的base64值,所以我们要改造FaceModel,让它实现IApiParameter接口:

class FaceModel : IApiParameter
{
    public Bitmap Image1 { get; set; }

    public Bitmap Image2 { get; set; }


    public Task OnRequestAsync(ApiParameterContext context)
    {
        var image1 = GetImageBase64(this.Image1);
        var image2 = GetImageBase64(this.Image2);
        var model = new { image1, image2 };

        var options = context.HttpContext.Options.JsonSerializeOptions;
        var json = System.Text.Json.JsonSerializer.Serialize(model, options);
        context.HttpContext.RequestMessage.Content = new StringContent(json, Encoding.UTF8, "application/json");
        return Task.CompletedTask;
    }

    private static string GetImageBase64(Bitmap image)
    {
        using var stream = new MemoryStream();
        image.Save(stream, System.Drawing.Imaging.ImageFormat.Jpeg);
        return Convert.ToBase64String(stream.ToArray());
    }
}

最后,我们在使用改进后的FaceModel来请求

public interface IFaceApi
{
    [HttpPost("/somePath")]
    Task<HttpResponseMessage> PostAsync(FaceModel faces);
}

自定义请求内容与响应内容解析

除了常见的xml或json响应内容要反序列化为强类型结果模型,你可能会遇到其它的二进制协议响应内容,比如google的ProtoBuf二进制内容。

1 编写相关自定义特性

自定义请求内容处理特性
public class ProtobufContentAttribute : HttpContentAttribute
{
    public string ContentType { get; set; } = "application/x-protobuf";

    protected override Task SetHttpContentAsync(ApiParameterContext context)
    {
        var stream = new MemoryStream();
        if (context.ParameterValue != null)
        {
            Serializer.NonGeneric.Serialize(stream, context.ParameterValue);
            stream.Position = 0L;
        }

        var content = new StreamContent(stream);
        content.Headers.ContentType = new MediaTypeHeaderValue(this.ContentType);
        context.HttpContext.RequestMessage.Content = content;
        return Task.CompletedTask;
    }
}
自定义响应内容解析特性
public class ProtobufReturnAttribute : ApiReturnAttribute
{
    public ProtobufReturnAttribute(string acceptContentType = "application/x-protobuf")
        : base(new MediaTypeWithQualityHeaderValue(acceptContentType))
    {
    }

    public override async Task SetResultAsync(ApiResponseContext context)
    {
        if (context.ApiAction.Return.DataType.IsRawType == false)
        {
            var stream = await context.HttpContext.ResponseMessage.Content.ReadAsStreamAsync();
            context.Result = Serializer.NonGeneric.Deserialize(context.ApiAction.Return.DataType.Type, stream);
        }
    }
}

2 应用相关自定义特性

[ProtobufReturn]
public interface IProtobufApi
{
    [HttpPut("/users/{id}")]
    Task<User> UpdateAsync([Required, PathQuery] string id, [ProtobufContent] User user);
}

适配畸形接口

在实际应用场景中,常常会遇到一些设计不标准的畸形接口,主要是早期还没有restful概念时期的接口,我们要区分分析这些接口,包装为好友的客户端调用接口。

不好友的参数名别名

例如服务器要求一个Query参数的名字为field-Name,这个是c#关键字或变量命名不允许的,我们可以使用[AliasAsAttribute]来达到这个要求:

[HttpGet("api/users/{account}")]
ITask<HttpResponseMessage> GetAsync([Required]string account, [AliasAs("field-Name")] string fieldName);

然后最终请求uri变为api/users/account1?field-name=fileName1

Form的某个字段为json文本

字段
field1 abc
field2 {"name":"jName","data":{"data1":"jData1"}}

对应强类型模型是

class Model
{
    public string Filed1 {get; set;}
    public string Field2 {get; set;}
}

我们在构建这个Model的实例时,不得不使用json序列化将field2的实例得到json文本,然后赋值给field2这个string属性,工作量大而且没有约束性。

依托于JsonString<>这个类型,现在只要我们把Field2结构声明为强类型模型,然后包装为JsonString<>类型,最后为HttpApiOptions添加JsonStringTypeConverter即可。

class Model
{
    public string Filed1 {get; set;}
    public JsonString<Field2Info> Field2 {get; set;}
}

class Field2Info
{
    public string Name {get; set;}
    public Field2Data data {get; set;}
}

class Field2Data
{
    public string data1 {get; set;}
}


// 添加转换器 
services
    .AddHttpApi<IMyApi>(o =>
    {
        o.HttpHost = new Uri("http://localhost:6000/");
        o.KeyValueSerializeOptions.Converters.Add(JsonStringTypeConverter.Default);
    }); 

响应未指明ContentType

明明响应的内容肉眼看上是json内容,但服务响应头里没有ContentType告诉客户端这内容是json,这好比客户端使用Form或json提交时就不在请求头告诉服务器内容格式是什么,而是让服务器猜测一样的道理。

解决办法是在Interface或Method声明[JsonReturn]特性,并设置其EnsureMatchAcceptContentType属性为false,表示ContentType不是期望值匹配也要处理。

[JsonReturn(EnsureMatchAcceptContentType = false)] 
public interface IJsonResponseApi : IHttpApi
{
}

类签名参数或token参数

例如每个请求的url额外的动态添加一个叫sign的参数,这个sign可能和请求参数值有关联,每次都需要计算。

我们可以自定义ApiFilterAttribute来实现自己的sign功能,然后把自定义Filter声明到Interface或Method即可

class SignFilterAttribute : ApiFilterAttribute
{
    public override Task OnRequestAsync(ApiRequestContext context)
    {
        var sign = DateTime.Now.Ticks.ToString();
        context.HttpContext.RequestMessage.AddUrlQuery("sign", sign);
        return Task.CompletedTask;
    }
}

[SignFilter]
public interface ISignedApi 
{
    ...
}

HttpMessageHandler配置

Http代理配置

services
    .AddHttpApi<IMyApi>(o =>
    {
        o.HttpHost = new Uri("http://localhost:6000/");
    })
    .ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler
    {
        UseProxy = true,
        Proxy = new WebProxy
        {
            Address = new Uri("http://proxy.com"),
            Credentials = new NetworkCredential
            {
                UserName = "useranme",
                Password = "pasword"
            }
        }
    });

客户端证书配置

有些服务器为了限制客户端的连接,开启了https双向验证,只允许它执有它颁发的证书的客户端进行连接

services
    .AddHttpApi<IMyApi>(o =>
    {
        o.HttpHost = new Uri("http://localhost:6000/");
    })
    .ConfigurePrimaryHttpMessageHandler(() =>
    {
        var handler = new HttpClientHandler();
        handler.ClientCertificates.Add(yourCert);
        return handler;
    });

维持CookieContainer不变

如果请求的接口不幸使用了Cookie保存身份信息机制,那么就要考虑维持CookieContainer实例不要跟随HttpMessageHandler的生命周期,默认的HttpMessageHandler最短只有2分钟的生命周期。

var cookieContainer = new CookieContainer();
services
    .AddHttpApi<IMyApi>(o =>
    {
        o.HttpHost = new Uri("http://localhost:6000/");
    })
    .ConfigurePrimaryHttpMessageHandler(() =>
    {
        var handler = new HttpClientHandler();
        handler.CookieContainer = cookieContainer;
        return handler;
    });

OAuths&Token

使用WebApiClientCore.Extensions.OAuths扩展,轻松支持token的获取、刷新与应用

1 注册相应类型的TokenProvider

// 为接口注册与配置token提者选项
services.AddClientCredentialsTokenProvider<IMyApi>(o =>
{
    o.Endpoint = new Uri("http://localhost:6000/api/tokens");
    o.Credentials.Client_id = "clientId";
    o.Credentials.Client_secret = "xxyyzz";
});

2 声明对应的Token特性

/// <summary>
/// 用户操作接口
/// </summary>
[ClientCredentialsToken]
public interface IMyApi
{
    ...
}

3 其它操作

清空Token,未过期的token也强制刷新

var providers = serviceProvider.GetServices<ITokenProvider>();
foreach(var item in providers)
{
    // 强制清除token以支持下次获取到新的token
    item.ClearToken();
}

自定义Token应用,得到token值,怎么用自己说了算

class MyTokenAttribute : ClientCredentialsTokenAttribute
{
    protected override void UseTokenResult(ApiRequestContext context, TokenResult tokenResult)
    {
        context.HttpContext.RequestMessage.Headers.TryAddWithoutValidation("xxx-header", tokenResult.Access_token);
    }
}

/// <summary>
/// 用户操作接口
/// </summary>
[MyToken]
public interface IMyApi
{
    ...
}

生态融合

Microsoft.Extensions.Http支持收入各种第三方的HttpMessageHandler来build出一种安全的HttpClient,同时支持将此HttpClient实例包装为强类型服务的目标服务类型注册功能。

Polly

Microsoft.Extensions.Http.Polly项目依托于Polly,将Polly策略实现到System.Net.Http.DelegatingHandler,其handler可以为HttpClient提供重试、降级和断路等功能。

WebApiClientCore

WebApiClientCore可以将Microsoft.Extensions.Http创建出来的HttpClient实例包装为声明式接口的代理实例,使开发者从面向命令式的编程模式直达声明式的AOP编程。