.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 <https://www.cnblogs.com/edison0621/p/10854215.html>。

.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


<https://img2018.cnblogs.com/blog/533598/201905/533598-20190519150805787-510308491.png>


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。

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