.NET Core 3.0之深入源码理解Configuration(二)
.NET Core 3.0之深入源码理解Configuration(二)
文件型配置基本内容
上一篇文章讨论了Configuration的几个核心对象，本文继续讨论Configuration中关于文件型配置的相关内容。相比较而言，文件型配置的使用场景更加广泛，用户自定义配置扩展也可以基于文件型配置进行扩展。如果需要查看上一篇文章，可以点击移步。
.NET Core文件型配置中我们提供了三种主要的实现，分别是JSON、XML、INI，请查看下图
由图可知，这三种配置的实现方式是一样的，当然了其他的配置比如命令行配置、环境变量配置等也是大同小异，理解了改配置类型的实现方式，后面我们再扩展基于Consul或者ZK的实现，就非常简单了。
文件型配置的抽象扩展
文件型配置的抽象扩展位于Microsoft.Extensions.Configuration.FileExtensions组件中，该扩展是一个基础实现。不过其命名空间是Microsoft.Extensions.Configuration，而Micros oft.Extensions.Configuration扩建本身又是整个.NET Core Configuration的基础实现。将File扩展独立于外部，体验了.NET Core的模块化设计。
FileConfigurationSource
Configuration.FileExtensions组件中，FileConfigurationSource是继承于IConfigurationSource的一个抽象类，包含了一个IConfigurationProvider类型的抽象方法，如下所示
1: /// <summary> 2: /// Builds the <see cref="IConfigurationProvider"/> for this source. 3: /// </summary> 4: /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param> 5: /// <returns>A <see cref="IConfigurationProvider"/></returns> 6: public abstract IConfigurationProvider Build(IConfigurationBuilder builder);该抽象类中还包括了几个比较重要的参数，分别用于配置性行为、文件内容访问以及异常处理。
string Path：文件的路径
bool Optional：标识加载的文件是否是可选的
bool ReloadOnChange：如果文件发生修改，是否重新加载配置源
int ReloadDelay：加载延迟，单位是毫秒，默认是250毫秒
IFileProvider FileProvider：用于获取文件内容
Action<FileLoadExceptionContext> OnLoadException：文件加载异常处理
该类对FileProvider有特殊处理，就是如果没有提供FileProvider实例，则会基于绝对路径，在最近的现有目录中创建物理文件提供程序。源码如下，
1: /// <summary> 2: /// If no file provider has been set, for absolute Path, this will creates a physical file provider 3: /// for the nearest existing directory. 4: /// </summary> 5: public void ResolveFileProvider() 6: { 7: if (FileProvider == null && 8: !string.IsNullOrEmpty(Path) && 9: System.IO.Path.IsPathRooted(Path)) 10: { 11: var directory = System.IO.Path.GetDirectoryName(Path);
12: var pathToFile = System.IO.Path.GetFileName(Path); 13: while (!string.IsNullOrEmpty(directory) && !Directory.Exists(directory)) 14: { 15: pathToFile = System.IO.Path.Combine(System.IO.Path.GetFileName(directory), pathToFile); 16: directory = System.IO.Path.GetDirectoryName(directory); 17: } 18: if (Directory.Exists(directory)) 19: { 20: FileProvider = new PhysicalFileProvider(directory); 21: Path = pathToFile; 22: } 23: } 24: }FileConfigurationProvider
该类是继承于ConfigurationProvider的抽象类，是从文件系统加载配置的基类，同时还继承了IDisposable，其抽象方法是Load方法，用于从当前的Provider中以Stream方式加载数据
1: /// <summary> 2: /// Loads this provider's data from a stream. 3: /// </summary> 4: /// <param name="stream">The stream to read.</param> 5: public abstract void Load(Stream stream);该类还重写了ConfigurationProvider的Load方法，并对文件加载中的异常做了处理，Data属性在前文有提到过，此处不再做其他说明。方法源码如下所示：
1: private void Load(bool reload) 2: { 3: var file = Source.FileProvider?.GetFileInfo(Source.Path); 4: if (file == null || !file.Exists) 5: { 6: if (Source.Optional || reload) // Always optional on reload 7: { 8: Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase); 9: } 10: else 11: { 12: var error = new StringBuilder($"The configuration file '{Source.Path}' was not found and is not optional."); 13: if (!string.IsNullOrEmpty(file?.PhysicalPath)) 14: { 15: error.Append($" The physical path is '{file.PhysicalPath}'."); 16: } 17: HandleException(new FileNotFoundException(error.ToString())); 18: } 19: } 20: else 21: { 22: // Always create new Data on reload to drop old keys 23: if (reload) 24: { 25: Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase); 26: } 27: using (var stream = file.CreateReadStream()) 28: { 29: try 30: { 31: Load(stream); 32: } 33: catch (Exception e) 34: { 35: HandleException(e); 36: } 37: } 38: } 39: // REVIEW: Should we raise this in the base as well / instead?，通过注释，我们可以知道OnReload()方法可能会在新版中发生变化 40: OnReload(); 41: } 42: 43: /// <summary> 44: /// Loads the contents of the file at <see cref="Path"/>. 45: /// </summary> 46: /// <exception cref="FileNotFoundException">If Optional is <c>false</c> on the source and a 47: /// file does not exist at specified Path.</exception> 48: public override void Load() 49: { 50: Load(reload: false); 51: }另外它还有一个特殊方法，就是参数类型为FileConfigurationSource的构造函数，其主要功能是监控文件，并在FileConfigurationSource.ReloadDelay设置的时间里重新加载文件并返回一个IDisposable类型的值，以下是该构造函数的源码：
1: /// <summary> 2: /// Initializes a new instance with the specified source. 3: /// </summary> 4: /// <param name="source">The source settings.</param> 5: public FileConfigurationProvider(FileConfigurationSource source) 6: { 7: if (source == null) 8: { 9: throw new ArgumentNullException(nameof(source)); 10: } 11: Source = source; 12: 13: if (Source.ReloadOnChange && Source.FileProvider != null) 14: { 15: _changeTokenRegistration = ChangeToken.OnChange( 16: () => Source.FileProvider.Watch(Source.Path), 17: () => { 18: Thread.Sleep(Source.ReloadDelay); 19: Load(reload: true); 20: }); 21: } 22: }FileConfigurationExtensions
该类是一个静态类，其提供了的多个扩展方法，主要基于
- IConfigurationBuilder
- IFileProvider
- Action<FileLoadExceptionContext>
包括主要用于设置或获取IFileProvider对象，前文有介绍过，是存储于字典之中，需要注意的是，在Get的时候如果字典中并不存在IFileProvider对象，则会实例化一个PhysicalFileProvider对象出来，该类位于Microsoft.Extensions.FileProviders.PhysicalFileProvider
1: /// <summary> 2: /// Sets the default <see cref="IFileProvider"/> to be used for file-based providers. 3: /// </summary> 4: /// <param name="builder">The <see cref="IConfigurationBuilder"/> to add to.</param> 5: /// <param name="fileProvider">The default file provider instance.</param> 6: /// <returns>The <see cref="IConfigurationBuilder"/>.</returns> 7: public static IConfigurationBuilder SetFileProvider(this IConfigurationBuilder builder, IFileProvider fileProvider) 8: { 9: if (builder == null) 10: { 11: throw new ArgumentNullException(nameof(builder)); 12: } 13: 14: builder.Properties[FileProviderKey] = fileProvider ?? throw new ArgumentNullException(nameof(fileProvider)); 15: return builder; 16: } 17: 18: /// <summary> 19: /// Gets the default <see cref="IFileProvider"/> to be used for file-based providers. 20: /// </summary> 21: /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param> 22: /// <returns>The <see cref="IConfigurationBuilder"/>.</returns> 23: public static IFileProvider GetFileProvider(this IConfigurationBuilder builder) 24: { 25: if (builder == null) 26: { 27: throw new ArgumentNullException(nameof(builder)); 28: } 29: 30: if (builder.Properties.TryGetValue(FileProviderKey, out object provider)) 31: { 32: return provider as IFileProvider; 33: } 34: 35: return new PhysicalFileProvider(AppContext.BaseDirectory ?? string.Empty); 36: }为指定路径的物理文件设置文件型Provider，该方法同样基于PhysicalFileProvider，并返回IConfigurationBuilder对象
1: /// <summary> 2: /// Sets the FileProvider for file-based providers to a PhysicalFileProvider with the base path. 3: /// </summary> 4: /// <param name="builder">The <see cref="IConfigurationBuilder"/> to add to.</param> 5: /// <param name="basePath">The absolute path of file-based providers.</param> 6: /// <returns>The <see cref="IConfigurationBuilder"/>.</returns> 7: public static IConfigurationBuilder SetBasePath(this IConfigurationBuilder builder, string basePath) 8: { 9: if (builder == null) 10: { 11: throw new ArgumentNullException(nameof(builder)); 12: } 13: 14: if (basePath == null) 15: { 16: throw new ArgumentNullException(nameof(basePath)); 17: } 18: 19: return builder.SetFileProvider(new PhysicalFileProvider(basePath)); 20: }以及异常处理，可以看到其异常处理也会存放于字典中，如果字典中找不到，就会返回空，这个地方如果直接使用，需要注意空指针问题。
1: /// <summary> 2: /// Sets a default action to be invoked for file-based providers when an error occurs. 3: /// </summary> 4: /// <param name="builder">The <see cref="IConfigurationBuilder"/> to add to.</param> 5: /// <param name="handler">The Action to be invoked on a file load exception.</param> 6: /// <returns>The <see cref="IConfigurationBuilder"/>.</returns> 7: public static IConfigurationBuilder SetFileLoadExceptionHandler(this IConfigurationBuilder builder, Action<FileLoadExceptionContext> handler) 8: { 9: if (builder == null) 10: { 11: throw new ArgumentNullException(nameof(builder)); 12: } 13: 14: builder.Properties[FileLoadExceptionHandlerKey] = handler; 15: return builder; 16: } 17: 18: /// <summary> 19: /// Gets the default <see cref="IFileProvider"/> to be used for file-based providers. 20: /// </summary> 21: /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param> 22: /// <returns>The <see cref="IConfigurationBuilder"/>.</returns> 23: public static Action<FileLoadExceptionContext> GetFileLoadExceptionHandler(this IConfigurationBuilder builder) 24: { 25: if (builder == null) 26: { 27: throw new ArgumentNullException(nameof(builder)); 28: } 29: 30: if (builder.Properties.TryGetValue(FileLoadExceptionHandlerKey, out object handler)) 31: { 32: return handler as Action<FileLoadExceptionContext>; 33: } 34: return null;
35: }该类还有两个静态私有变量，指定了文件Provider的Key以及文件加载异常处理Key。
1: private static string FileProviderKey = "FileProvider"; 2: private static string FileLoadExceptionHandlerKey = "FileLoadExceptionHandler";总结
文件型配置还依赖于.NET Core的其他组件Microsoft.Extensions.FileProviders和Microsoft.Extensions.Primitives。
FileProviders组件提供了文件处理的一般方法，Primitives组件提供了监控机制，同时还包括两个比较重要的结构体StringValues和StringSegment，本文暂时不做讨论，有兴趣的朋友，可以自行查看该组件源码。