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为什么提建造者模式？在阅读.NET Core源码时，时常碰到IHostBuilder，IConfigurationBuilder，ILoggerBuilder等诸如此类带Builder名称的类/接口，起初专研时那是一头愣。知识不够，勤奋来凑，在了解到Builder模式后终于理解，明白这些Builder类是用来构建相对应类的对象，用完即毁别无他用。理解建造者模式，有助于阅读源码时发现核心接口/类，将文件分类，直指堡垒。详细建造者模式可参阅此篇文章：磁悬浮快线
在.NET Core中读取配置是通过IConfiguration接口，它存在于Microsoft.Extensions.Configuration.Abstractions项目中，如下图：
IConfiguration：配置访问接口
IConfigurationProvider：配置提供者接口
IConfigurationSource：配置源接口
IConfigurationRoot：配置根接口，继承IConfiguration，维护着IConfigurationProvider集合及重新加载配置
IConfigurationBuilder：IConfigurationRoot接口实例的构造者接口
1.服务容器中IConfiguration实例注册（ConfigurationRoot）
/// <summary>
/// Represents the root of an <see cref="IConfiguration"/> hierarchy. => 配置根路径
/// </summary>
public interface IConfigurationRoot : IConfiguration
{
/// <summary>
/// Force the configuration values to be reloaded from the underlying <see cref="IConfigurationProvider"/>s. => 从配置源重新加载配置
/// </summary>
void Reload();
/// <summary>
/// The <see cref="IConfigurationProvider"/>s for this configuration. => 依赖的配置源集合
/// </summary>
IEnumerable<IConfigurationProvider> Providers { get; }
}
IConfigurationRoot（继承IConfiguration）维护着一个IConfigurationProvider集合列表，也就是我们的配置源。IConfiguration实例的创建并非通过new()方式，而是由IConfigurationBuilder来构建，实现了按需加载配置源，是建造者模式的充分体现。IConfigurationBuilder上的所有操作如：
HostBuilder.ConfigureAppConfiguration((context, builder) =>
{
builder.AddCommandLine(args); // 命令行配置源
builder.AddEnvironmentVariables(); // 环境配置源
builder.AddJsonFile("demo.json"); // json文件配置源
builder.AddInMemoryCollection(); // 内存配置源
// ...
})
皆是为IConfigurationRoot.Providers做准备，最后通过Build()方法生成ConfigurationRoot实例注册到服务容器
public class HostBuilder : IHostBuilder
{
private HostBuilderContext _hostBuilderContext;
// 配置构建 委托
private List<Action<HostBuilderContext, IConfigurationBuilder>> _configureAppConfigActions = new List<Action<HostBuilderContext, IConfigurationBuilder>>();
private IConfiguration _appConfiguration;
private void BuildAppConfiguration()
{
IConfigurationBuilder configBuilder = new ConfigurationBuilder();
foreach (Action<HostBuilderContext, IConfigurationBuilder> buildAction in _configureAppConfigActions)
{
buildAction(_hostBuilderContext, configBuilder);
}
_appConfiguration = configBuilder.Build(); // 调用Build()创建IConfiguration 实例 ConfigurationRoot
_hostBuilderContext.Configuration = _appConfiguration;
}
private void CreateServiceProvider()
{
var services = new ServiceCollection();
// register configuration as factory to make it dispose with the service provider
services.AddSingleton(_ => _appConfiguration); // 注册 IConfiguration - 单例
}
}
2.IConfiguration/IConfigurationSection读取配置与配置储存本质
程序中我们会通过如下方式获取配置值（当然还有绑定IOptions）
IConfiguration["key"]
IConfiguration.GetSection("key").Value
...
而IConfiguration注册的实例是ConfigurationRoot，代码如下，其索引器实现竟是倒序遍历配置源，获取配置值。原来当我们通过IConfiguration获取配置时，其实就是倒序遍历IConfigurationBuilder加载进来的配置源。
public class ConfigurationRoot : IConfigurationRoot, IDisposable
{
private readonly IList<IConfigurationProvider> _providers;
public IEnumerable<IConfigurationProvider> Providers => _providers;
public string this[string key]
{
get
{
// 倒序遍历配置源，获取到配置 就返回，这也是配置覆盖的根本原因，后添加的相同配置会覆盖前面的
for (int i = _providers.Count - 1; i >= 0; i--)
{
IConfigurationProvider provider = _providers[i];
if (provider.TryGet(key, out string value))
{
return value;
}
}
return null;
}
}
}
那么配置数据是以什么形式存储的呢？在Microsoft.Extensions.Configuration项目中，提供了一个IConfigurationProvider默认实现存储抽象类ConfigurationProvider，部分代码如下
/// <summary>
/// Base helper class for implementing an <see cref="IConfigurationProvider"/>
/// </summary>
public abstract class ConfigurationProvider : IConfigurationProvider
{
protected ConfigurationProvider()
{
Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
}
/// <summary>
/// The configuration key value pairs for this provider.
/// </summary>
protected IDictionary<string, string> Data { get; set; }
public virtual bool TryGet(string key, out string value)
=> Data.TryGetValue(key, out value);
/// <summary>
/// 虚方法，供具体配置源重写，加载配置到 Data中
/// </summary>
public virtual void Load() { }
}
从上可知，所有加载到程序中的配置源，其本质还是存储在Provider里面一个类型为IDictionary<string, string> Data属性中。由此推论： 当通过IConfiguration获取配置时，就是通过各个Provider的Data读取！
要实现自定义的配置源，只需实现IConfigurationProvider，IConfigurationSource两个接口即可，这里通过一个QueryString格式的简易配置来演示。虫洞隧道
1.queryString.config数据格式如下
server=localhost&port=3306&datasource=demo&user=root&password=123456&charset=utf8mb4
2.实现IConfigurationSource接口（QueryStringConfiguationSource）
public class QueryStringConfiguationSource : IConfigurationSource
{
public QueryStringConfiguationSource(string path)
{
Path = path;
}
/// <summary>
/// QueryString文件相对路径
/// </summary>
public string Path { get; }
public IConfigurationProvider Build(IConfigurationBuilder builder)
{
return new QueryStringConfigurationProvider(this);
}
}
3.实现IConfigurationProvider接口（QueryStringConfiguationProvider）
public class QueryStringConfigurationProvider : ConfigurationProvider
{
public QueryStringConfigurationProvider(QueryStringConfiguationSource source)
{
Source = source;
}
public QueryStringConfiguationSource Source { get; }
/// <summary>
/// 重写Load方法，将自定义的配置解析到 Data 中
/// </summary>
public override void Load()
{
// server=localhost&port=3306&datasource=demo&user=root&password=123456&charset=utf8mb4 例子格式
string queryString = File.ReadAllText(Path.Combine(AppContext.BaseDirectory, Source.Path));
string[] arrays = queryString.Split(new[] { "&" }, StringSplitOptions.RemoveEmptyEntries); // & 号分隔
foreach (var item in arrays)
{
string[] temps = item.Split(new[] { "=" }, StringSplitOptions.RemoveEmptyEntries); // = 号分隔
if (temps.Length != 2) continue;
Data.Add(temps[0], temps[1]);
}
}
}
4.IConfigurationBuilder配置源构建
public static class QueryStringConfigurationExtensions
{
/// <summary>
/// 默认文件名称 queryString.config
/// </summary>
/// <param name="builder"></param>
/// <returns></returns>
public static IConfigurationBuilder AddQueryStringFile(this IConfigurationBuilder builder)
=> AddQueryStringFile(builder, "queryString.config");
public static IConfigurationBuilder AddQueryStringFile(this IConfigurationBuilder builder, string path)
=> builder.Add(new QueryStringConfiguationSource(path));
}
5.Program加载配置源
public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args)
.ConfigureAppConfiguration(builder =>
{
// 加载QueryString配置源
builder.AddQueryStringFile();
//builder.AddQueryStringFile("queryString.config");
})
.ConfigureWebHostDefaults(webBuilder =>
{
webBuilder.UseStartup<Startup>();
});
至此，自定义QueryString配置源实现完成，便可通过IConfiguration接口获取值，结果如下
IConfiguration["server"] => localhost
IConfiguration["datasource"] => demo
IConfiguration["charset"] => utf8mb4
...
.NET Core官方已默认提供了：环境变量、命令行参数，Json、Ini等配置源，不过适用场景却应有不同。不妨可分为两类：一类是宿主配置源，一类是应用配置源
1.宿主配置源
宿主配置源：供IHost宿主启动时使用的配置源。环境变量、命令行参数就可归为这类，以IHostEnvironment为例
/// <summary>
/// 提供运行环境相关信息
/// </summary>
public interface IHostEnvironment
{
string EnvironmentName { get; set; }
string ApplicationName { get; set; }
string ContentRootPath { get; set; }
}
IHostEnvironment接口提供了当前应用运行环境相关信息，可以通过IsEnvironment()方法判断当前运行环境是Development还是Production、Staging。
public static bool IsEnvironment(this IHostEnvironment hostEnvironment, string environmentName)
{
if (hostEnvironment == null)
{
throw new ArgumentNullException(nameof(hostEnvironment));
}
return string.Equals(hostEnvironment.EnvironmentName, environmentName, StringComparison.OrdinalIgnoreCase);
}
hostEnvironment.EnvironmentName是什么？这就得益于它注册到服务容器时所赋的值：HostBuilder
public class HostBuilder:IHostBuilder
{
private void CreateHostingEnvironment()
{
_hostingEnvironment = new HostingEnvironment()
{
ApplicationName = _hostConfiguration[HostDefaults.ApplicationKey], // _hostConfiguration 类型是 IConfiguration
EnvironmentName = _hostConfiguration[HostDefaults.EnvironmentKey] ?? Environments.Production, // 当未配置环境时，默认Production环境，在使用vs开发启动时，lanuchSetting.json 配置了 环境变量："ASPNETCORE_ENVIRONMENT": "Development"
ContentRootPath = ResolveContentRootPath(_hostConfiguration[HostDefaults.ContentRootKey], AppContext.BaseDirectory),
};
if (string.IsNullOrEmpty(_hostingEnvironment.ApplicationName))
{
// Note GetEntryAssembly returns null for the net4x console test runner.
_hostingEnvironment.ApplicationName = Assembly.GetEntryAssembly()?.GetName().Name;
}
}
}
由此可见，IHostEnvironment所提供的信息根由仍是从IConfiguration读取，而这些配置正是来自环境变量、命令行参数配置源。
2.应用配置源
应用配置源：供应用业务逻辑使用的配置源。Json、Ini、Xml以及自定义的QueryString等就可归为类。
对于文件配置源，.NET Core默认提供了两个抽象类：FileConfigurationSource 和 FileConfigurationProvider
public abstract class FileConfigurationProvider : ConfigurationProvider, IDisposable
{
private readonly IDisposable _changeTokenRegistration;
public FileConfigurationProvider(FileConfigurationSource source)
{
if (source == null)
{
throw new ArgumentNullException(nameof(source));
}
Source = source;
if (Source.ReloadOnChange && Source.FileProvider != null)
{
_changeTokenRegistration = ChangeToken.OnChange( // 文件改变，重新加载配置
() => Source.FileProvider.Watch(Source.Path),
() =>
{
Thread.Sleep(Source.ReloadDelay);
Load(reload: true);
});
}
}
/// <summary>
/// The source settings for this provider.
/// </summary>
public FileConfigurationSource Source { get; }
private void Load(bool reload)
{
IFileInfo file = Source.FileProvider?.GetFileInfo(Source.Path);
if (file == null || !file.Exists)
{
if (Source.Optional || reload) // Always optional on reload
{
Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase); // Data 被重新创建新的实例赋值了
}
else
{
var error = new StringBuilder($"The configuration file '{Source.Path}' was not found and is not optional.");
if (!string.IsNullOrEmpty(file?.PhysicalPath))
{
error.Append($" The physical path is '{file.PhysicalPath}'.");
}
HandleException(ExceptionDispatchInfo.Capture(new FileNotFoundException(error.ToString())));
}
}
else
{
// Always create new Data on reload to drop old keys
if (reload)
{
Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase); // Data 被重新创建新的实例赋值了
}
static Stream OpenRead(IFileInfo fileInfo)
{
if (fileInfo.PhysicalPath != null)
{
// The default physical file info assumes asynchronous IO which results in unnecessary overhead
// especally since the configuration system is synchronous. This uses the same settings
// and disables async IO.
return new FileStream(
fileInfo.PhysicalPath,
FileMode.Open,
FileAccess.Read,
FileShare.ReadWrite,
bufferSize: 1,
FileOptions.SequentialScan);
}
return fileInfo.CreateReadStream();
}
using Stream stream = OpenRead(file);
try
{
Load(stream);
}
catch (Exception e)
{
HandleException(ExceptionDispatchInfo.Capture(e));
}
}
}
public override void Load()
{
Load(reload: false);
}
public abstract void Load(Stream stream);
}
所有基于文件配置源（如果要监控配置文件更新，如：appsetting.json）都应实现这个两个抽象类，尽管不懂ChangeToken是个什么东东，只需明白Provider.Data 在文件变更时被重新赋值也未尝不可。
