Asp.net Core启动流程讲解（四）
Asp.net Core内 DI（DependencyInjection）贯穿了项目的始终，要学习Asp.net Core就无法越过DI。
下面讲解一下DI在Asp.Net Core内的流程
asp.net core 3.0以下
Asp.Net core 3.0以下有两种自定义替换DI容器的方式
替换 IServiceProviderFactory
1、IServiceProviderFactory
查看 WebHostBuilder.Build
public IWebHost Build()
{
var hostingServices = BuildCommonServices(out var hostingStartupErrors);
var applicationServices = hostingServices.Clone();
var hostingServiceProvider = GetProviderFromFactory(hostingServices);
AddApplicationServices(applicationServices, hostingServiceProvider);
var host = new WebHost(
applicationServices,
hostingServiceProvider,
_options,
_config,
hostingStartupErrors);
try
{
host.Initialize();
//省略不重要的代码段
return host;
}
catch
{
host.Dispose();
throw;
}
IServiceProvider GetProviderFromFactory(IServiceCollection collection)
{
var provider = collection.BuildServiceProvider();
var factory = provider.GetService<IServiceProviderFactory<IServiceCollection>>();
if (factory != null && !(factory is DefaultServiceProviderFactory))
{
using (provider)
{
return factory.CreateServiceProvider(factory.CreateBuilder(collection));
}
}
return provider;
}
}
再跟进 BuildCommonServices 函数
private IServiceCollection BuildCommonServices(out AggregateException hostingStartupErrors)
{
var services = new ServiceCollection();
//省略不重要的代码段
services.AddTransient<IServiceProviderFactory<IServiceCollection>, DefaultServiceProviderFactory>();
_configureServices?.Invoke(_context, services);
return services;
}
思路很简，注册DI，然后返回 IServiceProvider 给 Host 容器，找到 IServiceProviderFactory
DefaultServiceProviderFactory，则转换DI，最后返回新的 IServiceProvider。
public interface IServiceProviderFactory<TContainerBuilder>
{
TContainerBuilder CreateBuilder(IServiceCollection services);
IServiceProvider CreateServiceProvider(TContainerBuilder
containerBuilder);
}
WebHostBuilder.ConfigureServices 内DI注册 默认了实现的IServiceProviderFactory
IServiceProviderFactory 的流程是 IServiceProviderFactory.CreateBuilder->IServiceProviderFactory.CreateServiceProvider
先把DI容器 IServiceCollection 转换成自定义容器，再通过自定义容器转换成 IServiceProvider，最后依赖于根节点的IServiceProvider 即可
注册 IServiceProviderFactory
WebHostBuilder.UseDefaultServiceProvider 转到源码
public static IWebHostBuilder UseDefaultServiceProvider(this IWebHostBuilder hostBuilder, Action<WebHostBuilderContext, ServiceProviderOptions> configure)
{
return hostBuilder.ConfigureServices((context, services) =>
{
var options = new ServiceProviderOptions();
configure(context, options);
//替换默认的IServiceProviderFactory<IServiceCollection>
services.Replace(ServiceDescriptor.Singleton<IServiceProviderFactory<IServiceCollection>>(new DefaultServiceProviderFactory(options)));
});
}
异曲同工之妙，不再赘述。
2、IStartup.Configure
还是打开熟悉的 WebHostBuilder.BuildCommonServices
private IServiceCollection BuildCommonServices(out AggregateException hostingStartupErrors)
{
var services = new ServiceCollection();
//忽略无关代码
if (!string.IsNullOrEmpty(_options.StartupAssembly))
{
try
{
var startupType = StartupLoader.FindStartupType(_options.StartupAssembly, _hostingEnvironment.EnvironmentName);
if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo()))
{
services.AddSingleton(typeof(IStartup), startupType);
}
else
{
services.AddSingleton(typeof(IStartup), sp =>
{
var hostingEnvironment = sp.GetRequiredService<IHostEnvironment>();
var methods = StartupLoader.LoadMethods(sp, startupType, hostingEnvironment.EnvironmentName);
return new ConventionBasedStartup(methods);
});
}
}
catch (Exception ex)
{
var capture = ExceptionDispatchInfo.Capture(ex);
services.AddSingleton<IStartup>(_ =>
{
capture.Throw();
return null;
});
}
}
_configureServices?.Invoke(_context, services);
return services;
}
注册 IStartup 到 DI
再查看 WebHostBuilder.Build
public IWebHost Build()
{
var hostingServices = BuildCommonServices(out var hostingStartupErrors);
var applicationServices = hostingServices.Clone();
var hostingServiceProvider = GetProviderFromFactory(hostingServices);
var host = new WebHost(
applicationServices,
hostingServiceProvider,
_options,
_config,
hostingStartupErrors);
try
{
host.Initialize(); //关建代码行
//省略不重要的代码段
return host;
}
catch
{
host.Dispose();
throw;
}
}
查看 WebHost.Initialize 代码
public void Initialize()
{
try
{
EnsureApplicationServices();
}
catch (Exception ex)
{
// EnsureApplicationServices may have failed due to a missing or throwing Startup class.
if (_applicationServices == null)
{
_applicationServices = _applicationServiceCollection.BuildServiceProvider();
}
if (!_options.CaptureStartupErrors)
{
throw;
}
_applicationServicesException = ExceptionDispatchInfo.Capture(ex);
}
}
查看 WebHost.EnsureApplicationServices 代码
private void EnsureApplicationServices()
{
if (_applicationServices == null)
{
EnsureStartup();
_applicationServices = _startup.ConfigureServices(_applicationServiceCollection);
}
}
private void EnsureStartup()
{
if (_startup != null)
{
return;
}
_startup = _hostingServiceProvider.GetService<IStartup>();
if (_startup == null)
{
throw new InvalidOperationException($"No application configured. Please specify startup via IWebHostBuilder.UseStartup, IWebHostBuilder.Configure, injecting {nameof(IStartup)} or specifying the startup assembly via {nameof(WebHostDefaults.StartupAssemblyKey)} in the web host configuration.");
}
}
获取 IStartup 接口的实例，然后调用 IStartup.ConfigureServices 返回 IServiceProvider 实例
Asp.Net Core 3.0以上
Asp.Net Core 3.0以下的设计代码过于混乱，在Asp.Net Core 3.0开始，替换DI的流程变得简洁了
默认程序入口
public class Program
{
public static void Main(string[] args)
{
CreateHostBuilder(args).Build().Run();
}
public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args)
.ConfigureWebHostDefaults(webBuilder =>
{
webBuilder.UseStartup<Startup>();
});
}
查看 Host.CreateDefaultBuilder
public static IHostBuilder CreateDefaultBuilder(string[] args)
{
var builder = new HostBuilder();
//忽略无关代码
return builder;
}
转向 HostBuilder.Build
public IHost Build()
{
//忽略无关代码
CreateServiceProvider();
return _appServices.GetRequiredService<IHost>();
}
查看 HostBuilder.CreateServiceProvider
private void CreateServiceProvider()
{
var services = new ServiceCollection();
//忽略无关代码
var containerBuilder = _serviceProviderFactory.CreateBuilder(services);
foreach (var containerAction in _configureContainerActions)
{
containerAction.ConfigureContainer(_hostBuilderContext, containerBuilder);
}
_appServices = _serviceProviderFactory.CreateServiceProvider(containerBuilder);
if (_appServices == null)
{
throw new InvalidOperationException($"The IServiceProviderFactory returned a null IServiceProvider.");
}
// resolve configuration explicitly once to mark it as resolved within the
// service provider, ensuring it will be properly disposed with the provider
_ = _appServices.GetService<IConfiguration>();
}
这里有个 _serviceProviderFactory，转到定义
public class HostBuilder : IHostBuilder
{
//忽略无关代码
private IServiceFactoryAdapter _serviceProviderFactory = new ServiceFactoryAdapter<IServiceCollection>(new DefaultServiceProviderFactory());
}
对应传参，有两个函数不同重载的UseServiceProviderFactory
public IHostBuilder UseServiceProviderFactory<TContainerBuilder>(IServiceProviderFactory<TContainerBuilder> factory)
{
_serviceProviderFactory = new ServiceFactoryAdapter<TContainerBuilder>(factory ?? throw new ArgumentNullException(nameof(factory)));
return this;
}
public IHostBuilder UseServiceProviderFactory<TContainerBuilder>(Func<HostBuilderContext, IServiceProviderFactory<TContainerBuilder>> factory)
{
_serviceProviderFactory = new ServiceFactoryAdapter<TContainerBuilder>(() => _hostBuilderContext, factory ?? throw new ArgumentNullException(nameof(factory)));
return this;
}
后记
柠檬大佬说Asp.Net Core至少有三处可以替换DI的地方~ 还有一处
看了一下源码，大概是 IApplicationBuilderFactory 吧
public interface IApplicationBuilderFactory
{
IApplicationBuilder CreateBuilder(IFeatureCollection serverFeatures);
}
这里IApplicationBuilderFactory.CreateBuilder返回 IApplicationBuilder
IApplicationBuilder里的Ioc对象可以在这里设置
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