在.net core web 启动过程(1)中介绍了IHostStartup的执行过程，该文章主要介绍IStartup的执行过程。
最常用的配置Startup方式，通过调用webHostBuilder扩展方法UseStartup<T> 来指定。
var host = new HostBuilder()
.ConfigureWebHost(webHostBuilder =>
{
webHostBuilder
.UseConfiguration(config)
.UseKestrel()
.UseStartup<StartupBlockingOnStart>();
})
.Build();
这里看一下WebHostBuilder.UseStartup<T>扩展方法的具体实现
/// <summary>
/// Specify the startup type to be used by the web host.
/// </summary>
/// <param name="hostBuilder">The <see cref="IWebHostBuilder"/> to configure.</param>
/// <typeparam name ="TStartup">The type containing the startup methods for the application.</typeparam>
/// <returns>The <see cref="IWebHostBuilder"/>.</returns>
public static IWebHostBuilder UseStartup<[DynamicallyAccessedMembers(StartupLinkerOptions.Accessibility)] TStartup>(this IWebHostBuilder hostBuilder) where TStartup : class
{
return hostBuilder.UseStartup(typeof(TStartup));
}
internal sealed class GenericWebHostBuilder : WebHostBuilderBase, ISupportsStartup
{
private object? _startupObject;
private readonly object _startupKey = new object();
private AggregateException? _hostingStartupErrors;
private HostingStartupWebHostBuilder? _hostingStartupWebHostBuilder;
public IWebHostBuilder UseStartup([DynamicallyAccessedMembers(StartupLinkerOptions.Accessibility)] Type startupType)
{
//这里得到指定的startup类型所在的Assembly名称
var startupAssemblyName = startupType.Assembly.GetName().Name;
//记录在设置中，多次执行会覆盖上一次的
UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName);
//UseStartup 允许多次调用，但是只会允许最后一次，这里和IHostStartup的机制不同，IHostStartup允许多个实现
// UseStartup can be called multiple times. Only run the last one.
//_startupObject是object类型，他是GenericWebHostBuilder对象中的一个私有变量，多次调用UseStartup，_startObject会被覆盖为最后一次的设置对象。
_startupObject = startupType;
//重点关注，UseStartup这里会把具体操作给 转化成一个Action给暂存到ConfigureSercices。稍后会在HostBuilder.Build()中执行，
_builder.ConfigureServices((context, services) =>
{
// Run this delegate if the startup type matches
//多次调用UseStartup，暂存多个Action，但是在执行的时候，这里会有一个验证，仅最后一次设置的才能通过，
if (object.ReferenceEquals(_startupObject, startupType))
{
UseStartup(startupType, context, services);
}
});
return this;
}
}
看完上述代码后，需要提注意的是，虽然UseStartup具体操作被暂存了起来，但是 startupAssemblyName 和_startupObject是被立即记录起来的。
配置Startup的其他方式,通过传入startupFactory来实现动态改变startup的目的
/// <summary>
/// Specify a factory that creates the startup instance to be used by the web host.
/// </summary>
/// <param name="hostBuilder">The <see cref="IWebHostBuilder"/> to configure.</param>
/// <param name="startupFactory">A delegate that specifies a factory for the startup class.</param>
/// <returns>The <see cref="IWebHostBuilder"/>.</returns>
/// <remarks>When in a trimmed app, all public methods of <typeparamref name="TStartup"/> are preserved. This should match the Startup type directly (and not a base type).</remarks>
public static IWebHostBuilder UseStartup<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] TStartup>(this IWebHostBuilder hostBuilder, Func<WebHostBuilderContext, TStartup> startupFactory) where TStartup : class
{
//通过传入一个Func<WebHostBuilderContext,TStartup>的startupFactory来实现动态改变Startup对象
}
// Note: This method isn't 100% compatible with trimming. It is possible for the factory to return a derived type from TStartup.
// RequiresUnreferencedCode isn't on this method because the majority of people won't do that.
public IWebHostBuilder UseStartup<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] TStartup>(Func<WebHostBuilderContext, TStartup> startupFactory)
{
//这里和useStartup(Type t)一样，都是记录startup所在的程序集
var startupAssemblyName = startupFactory.GetMethodInfo().DeclaringType!.Assembly.GetName().Name;
UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName);
//覆盖上次设置类型
// Clear the startup type
_startupObject = startupFactory;
_builder.ConfigureServices(ConfigureStartup);
[UnconditionalSuppressMessage("Trimmer", "IL2072", Justification = "Startup type created by factory can't be determined statically.")]
void ConfigureStartup(HostBuilderContext context, IServiceCollection services)
{
// UseStartup can be called multiple times. Only run the last one.
if (object.ReferenceEquals(_startupObject, startupFactory))
{
var webHostBuilderContext = GetWebHostBuilderContext(context);
var instance = startupFactory(webHostBuilderContext) ?? throw new InvalidOperationException("The specified factory returned null startup instance.");
UseStartup(instance.GetType(), context, services, instance);
}
}
return this;
}
对比两种配置方式，发现内部逻辑基本一致，最终都会暂存一个Action到HostBuilder.ConfigureServices，该action 都会有一个拦截验证，只允许最后一次配置startup的方式允许通过。 两种方式暂存的action 里面的操作都最终指向
private void UseStartup([DynamicallyAccessedMembers(StartupLinkerOptions.Accessibility)] Type startupType, HostBuilderContext context, IServiceCollection services, object? instance = null)
那我们来看看，他们暂存的action 到具体执行的时候都做了什么事。
private void UseStartup([DynamicallyAccessedMembers(StartupLinkerOptions.Accessibility)] Type startupType, HostBuilderContext context, IServiceCollection services, object? instance = null)
{
var webHostBuilderContext = GetWebHostBuilderContext(context);
var webHostOptions = (WebHostOptions)context.Properties[typeof(WebHostOptions)];
ExceptionDispatchInfo? startupError = null;
ConfigureBuilder? configureBuilder = null;
try
{
//我们UseStartup<T>和StartupFactory 他们指定的Startup具体类是不能继承IStartup接口，因为IStartup接口的ConfigureServices方法需要返回ServiceProvider
//指定的Startup具体类的ConfigureServices是无需返回值的
// We cannot support methods that return IServiceProvider as that is terminal and we need ConfigureServices to compose
if (typeof(IStartup).IsAssignableFrom(startupType))
{
throw new NotSupportedException($"{typeof(IStartup)} isn't supported");
}
if (StartupLoader.HasConfigureServicesIServiceProviderDelegate(startupType, context.HostingEnvironment.EnvironmentName))
{
throw new NotSupportedException($"ConfigureServices returning an {typeof(IServiceProvider)} isn't supported.");
}
//如果使用的UseStartup<T>(Type type)这里需要创建对象，
//如果使用的startupFactory instance是不等null的，具体可以看一下startupFactory那里的具体实现，暂存的action执行过程中会通过startupFactory创建指定的对象
instance ??= ActivatorUtilities.CreateInstance(new HostServiceProvider(webHostBuilderContext), startupType);
context.Properties[_startupKey] = instance;
// Startup.ConfigureServices
var configureServicesBuilder = StartupLoader.FindConfigureServicesDelegate(startupType, context.HostingEnvironment.EnvironmentName);
var configureServices = configureServicesBuilder.Build(instance);
//这里执行执行Startup.ConfigureServices方法
configureServices(services);
// REVIEW: We're doing this in the callback so that we have access to the hosting environment
// Startup.ConfigureContainer
var configureContainerBuilder = StartupLoader.FindConfigureContainerDelegate(startupType, context.HostingEnvironment.EnvironmentName);
if (configureContainerBuilder.MethodInfo != null)
{
// Store the builder in the property bag
_builder.Properties[typeof(ConfigureContainerBuilder)] = configureContainerBuilder;
InvokeContainer(this, configureContainerBuilder);
}
// Resolve Configure after calling ConfigureServices and ConfigureContainer
configureBuilder = StartupLoader.FindConfigureDelegate(startupType, context.HostingEnvironment.EnvironmentName);
}
catch (Exception ex) when (webHostOptions.CaptureStartupErrors)
{
startupError = ExceptionDispatchInfo.Capture(ex);
}
// Startup.Configure
services.Configure<GenericWebHostServiceOptions>(options =>
{
options.ConfigureApplication = app =>
{
// Throw if there was any errors initializing startup
startupError?.Throw();
//这里执行 Startup.Configure
// Execute Startup.Configure
if (instance != null && configureBuilder != null)
{
configureBuilder.Build(instance)(app);
}
};
});
[UnconditionalSuppressMessage("AOT", "IL3050:RequiresDynamicCode",
Justification = "There is a runtime check for ValueType startup container. It's unlikely anyone will use a ValueType here.")]
static void InvokeContainer(GenericWebHostBuilder genericWebHostBuilder, ConfigureContainerBuilder configureContainerBuilder)
{
var containerType = configureContainerBuilder.GetContainerType();
// Configure container uses MakeGenericType with the container type. MakeGenericType + struct container type requires IsDynamicCodeSupported.
if (containerType.IsValueType && !RuntimeFeature.IsDynamicCodeSupported)
{
throw new InvalidOperationException("A ValueType TContainerBuilder isn't supported with AOT.");
}
var actionType = typeof(Action<,>).MakeGenericType(typeof(HostBuilderContext), containerType);
// Get the private ConfigureContainer method on this type then close over the container type
var configureCallback = typeof(GenericWebHostBuilder).GetMethod(nameof(ConfigureContainerImpl), BindingFlags.NonPublic | BindingFlags.Instance)!
.MakeGenericMethod(containerType)
.CreateDelegate(actionType, genericWebHostBuilder);
// _builder.ConfigureContainer<T>(ConfigureContainer);
typeof(IHostBuilder).GetMethod(nameof(IHostBuilder.ConfigureContainer))!
.MakeGenericMethod(containerType)
.InvokeWithoutWrappingExceptions(genericWebHostBuilder._builder, new object[] { configureCallback });
}
}
根据代码中标注红色的部分可以看出Startup类中方法的执行顺序
1. void ConfigureServices(IServiceCollection services) ;
2. void ConfigureContainer(ContainerBuilder builder);
3. void Configure(IApplicationBuilder app, IWebHostEnvironment env);
看到 Startup.Configure的参数和IHostStartup.Configure区别了吗
1. IHostStartup.Configure(IWebHostBuilder builder) 用于配置WebHostBuilder ,
2. Starup.Configure(IApplicationBuilder app, IWebHostEnvironment env)用于配置ApplicationBuilder ，配置HTTP 请求管道的中间件。
IHostStartup允许多个实现，执行顺序比较靠前，Starup允许多次UseStartup ，但是只允许执行最后一个Startup的方法
讲完上面，下面给出一个常见.net core web StartUp类
public class Startup
{
// ConfigureServices
public void ConfigureServices(IServiceCollection services)
{
services.AddControllers();
}
//ConfigureContainer
public void ConfigureContainer(ContainerBuilder builder)
{
}
// Configure 是用于配置 HTTP 请求管道的地方
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
app.UseExceptionHandler("/Home/Error");
app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthorization();
app.UseEndpoints(endpoints =>
{
endpoints.MapControllers();
});
}
}
写到这里总感觉对于启动过程总是缺少了什么，比如说IServiceConllection是如何创建的，是什么时候创建的 。Startup.ConfigureContainer是做什么。他们直接是如何在上层对象中调用的。
想讲明白启动过程还需要了解一下HostBuilder中的方法。