前言
笔者的这篇文章和上篇文章思路一样，不注重依赖注入的使用方法，更加注重源码的实现，我尽量的表达清楚内容，让读者能够真正的学到东西。如果有不太清楚依赖注入是什么或怎么在.Net项目中使用的话，请点击这里，这是微软的官方文档，把用法介绍的很清晰了，相信你会有很大收获。那么废话不多说，咱们进入正题（可能篇幅有点长，耐心读完你会有收获的??）。
DependencyInjection类之间的关系
下图中只列举重要的类和接口(实际的类和接口有很多)，里面的方法和属性也只列出重要的，这里只是让你有个大概的印象，看不懂没关系，继续往下读。
源码解析
- 首先我们要知道，.Net程序在启动的时候会构建一个Host(Host.Build().Run()),在其Build方法中来构建容器(只构建根容器，下面会解释)，具体在哪构建容器，源码如下图：
public class HostBuilder : IHostBuilder
{
private List<Action<IConfigurationBuilder>> _configureHostConfigActions = new List<Action<IConfigurationBuilder>>();
public IHostBuilder ConfigureAppConfiguration(Action<HostBuilderContext, IConfigurationBuilder> configureDelegate)
{
_configureAppConfigActions.Add(configureDelegate ?? throw new ArgumentNullException(nameof(configureDelegate)));
return this;
}
//即我们在main函数中看到的Build方法
public IHost Build()
{
//只能执行一次这个方法
if (_hostBuilt)
{
throw new InvalidOperationException(SR.BuildCalled);
}
_hostBuilt = true;
using var diagnosticListener = new DiagnosticListener("Microsoft.Extensions.Hosting");
const string hostBuildingEventName = "HostBuilding";
const string hostBuiltEventName = "HostBuilt";
if (diagnosticListener.IsEnabled() && diagnosticListener.IsEnabled(hostBuildingEventName))
{
Write(diagnosticListener, hostBuildingEventName, this);
}
//执行Host配置(应用程序执行路径，增加_dotnet环境变量，获取命令行参数,加载预配置)
BuildHostConfiguration();
//设置主机环境变量
CreateHostingEnvironment();
//设置上下文
CreateHostBuilderContext();
//构建程序配置(加载appsetting.json)
BuildAppConfiguration();
//构造容器，加载服务
CreateServiceProvider();
var host = _appServices.GetRequiredService<IHost>();
if (diagnosticListener.IsEnabled() && diagnosticListener.IsEnabled(hostBuiltEventName))
{
Write(diagnosticListener, hostBuiltEventName, host);
}
return host;
}
private void CreateServiceProvider()
{
//构建服务集合
var services = new ServiceCollection();
services.AddSingleton<IHostingEnvironment>(_hostingEnvironment);
services.AddSingleton<IHostEnvironment>(_hostingEnvironment);
services.AddSingleton(_hostBuilderContext);
services.AddSingleton(_ => _appConfiguration);
services.AddSingleton<IApplicationLifetime>(s => (IApplicationLifetime)s.GetService<IHostApplicationLifetime>());
services.AddSingleton<IHostApplicationLifetime, ApplicationLifetime>();
AddLifetime(services);
services.AddSingleton<IHost>(_ =>
{
return new Internal.Host(_appServices,
_hostingEnvironment,
_defaultProvider,
_appServices.GetRequiredService<IHostApplicationLifetime>(),
_appServices.GetRequiredService<ILogger<Internal.Host>>(),
_appServices.GetRequiredService<IHostLifetime>(),
_appServices.GetRequiredService<IOptions<HostOptions>>());
});
services.AddOptions().Configure<HostOptions>(options => { options.Initialize(_hostConfiguration); });
services.AddLogging();
//加载StartUp里面的ConfigService方法
foreach (Action<HostBuilderContext, IServiceCollection> configureServicesAction in _configureServicesActions)
{
configureServicesAction(_hostBuilderContext, services);
}
//构建容器
object containerBuilder = _serviceProviderFactory.CreateBuilder(services);
//配置容器适配器，可以构建自己的容器
foreach (IConfigureContainerAdapter containerAction in _configureContainerActions)
{
containerAction.ConfigureContainer(_hostBuilderContext, containerBuilder);
}
//生成容器服务
_appServices = _serviceProviderFactory.CreateServiceProvider(containerBuilder);
if (_appServices == null)
{
throw new InvalidOperationException(SR.NullIServiceProvider);
}
//在根容器中先生成IConfiguration
_ = _appServices.GetService<IConfiguration>();
}
}
从上面可以看出，_serviceProviderFactory实际上就是DefaultServiceProviderFactory对象实例，它通过调用自己的CreateServiceProvider方法，来构建默认容器，而默认容器是作为根容器存在，全局只有一个。
- _serviceProviderFactory.CreateBuilder(services)方法只是简单的转换(把IServiceCollection转化成Object)，为什么他要返回object呢？因为在.net中默认实现是个IServiceCollection，那么如果要是需要自定义容器的话，则需要通过自己的ProviderFactory返回自己的Builder，来生成容器，在这里就不细讲了，以后其他文章会详细说下怎么自定义一个IOC，这里只讲默认实现，接下来我们具体讲它是怎么生成容器服务的，请看CreateServiceProvider方法实现：
//.net提供的默认的ProviderFactory
public class DefaultServiceProviderFactory : IServiceProviderFactory<IServiceCollection>
{
private readonly ServiceProviderOptions _options;
public DefaultServiceProviderFactory() : this(ServiceProviderOptions.Default)
{
}
public DefaultServiceProviderFactory(ServiceProviderOptions options)
{
if (options == null)
{
throw new ArgumentNullException(nameof(options));
}
_options = options;
}
public IServiceCollection CreateBuilder(IServiceCollection services)
{
return services;
}
//通过调用扩展方法,来实现ServiceProvider
public IServiceProvider CreateServiceProvider(IServiceCollection containerBuilder)
{
return containerBuilder.BuildServiceProvider(_options);
}
}
public static class ServiceCollectionContainerBuilderExtensions
{
public static ServiceProvider BuildServiceProvider(this IServiceCollection services, ServiceProviderOptions options)
{
if (services == null)
{
throw new ArgumentNullException(nameof(services));
}
if (options == null)
{
throw new ArgumentNullException(nameof(options));
}
return new ServiceProvider(services, options);
}
}
public sealed class ServiceProvider : IServiceProvider, IDisposable, IAsyncDisposable
{
//验证Scoped服务是否有在Singleton服务中存在，默认是关闭的，因为可能会把Scoped服务提升为Singleton服务，那么这个本身就是不正常的调用，建议修改调用方法。
private readonly CallSiteValidator _callSiteValidator;
//用来获取服务
private readonly Func<Type, Func<ServiceProviderEngineScope, object>> _createServiceAccessor;
//服务提供的引擎
internal ServiceProviderEngine _engine;
//是否已被释放
private bool _disposed;
//对每种type做了一个缓存，来提高效率
private ConcurrentDictionary<Type, Func<ServiceProviderEngineScope, object>> _realizedServices;
//服务描述工厂，保存着服务描述的集合，描述服务的类型
internal CallSiteFactory CallSiteFactory { get; }
//根容器
internal ServiceProviderEngineScope Root { get; }
internal static bool VerifyOpenGenericServiceTrimmability { get; } =
AppContext.TryGetSwitch("Microsoft.Extensions.DependencyInjection.VerifyOpenGenericServiceTrimmability", out bool verifyOpenGenerics) ? verifyOpenGenerics : false;
internal ServiceProvider(ICollection<ServiceDescriptor> serviceDescriptors, ServiceProviderOptions options)
{
//根容器就是本身，并且IsRootScope是true标识
Root = new ServiceProviderEngineScope(this, isRootScope: true);
//获取用那种引擎来获取服务
_engine = GetEngine();
//获取服务时调用的函数
_createServiceAccessor = CreateServiceAccessor;
_realizedServices = new ConcurrentDictionary<Type, Func<ServiceProviderEngineScope, object>>();
//主要是生成服务的ServiceCallSite这个很重要，用来描述服务类型
CallSiteFactory = new CallSiteFactory(serviceDescriptors);
//额外的一些内部服务描述
CallSiteFactory.Add(typeof(IServiceProvider), new ServiceProviderCallSite());
CallSiteFactory.Add(typeof(IServiceScopeFactory), new ConstantCallSite(typeof(IServiceScopeFactory), Root));
CallSiteFactory.Add(typeof(IServiceProviderIsService), new ConstantCallSite(typeof(IServiceProviderIsService), CallSiteFactory));
if (options.ValidateScopes)
{
_callSiteValidator = new CallSiteValidator();
}
//在build阶段就验证服务注入是否正确
if (options.ValidateOnBuild)
{
List<Exception> exceptions = null;
foreach (ServiceDescriptor serviceDescriptor in serviceDescriptors)
{
try
{
ValidateService(serviceDescriptor);
}
catch (Exception e)
{
exceptions = exceptions ?? new List<Exception>();
exceptions.Add(e);
}
}
if (exceptions != null)
{
throw new AggregateException("Some services are not able to be constructed", exceptions.ToArray());
}
}
}
public object GetService(Type serviceType) => GetService(serviceType, Root);
private void OnCreate(ServiceCallSite callSite)
{
_callSiteValidator?.ValidateCallSite(callSite);
}
private void OnResolve(Type serviceType, IServiceScope scope)
{
_callSiteValidator?.ValidateResolution(serviceType, scope, Root);
}
internal object GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
{
if (_disposed)
{
ThrowHelper.ThrowObjectDisposedException();
}
Func<ServiceProviderEngineScope, object> realizedService = _realizedServices.GetOrAdd(serviceType, _createServiceAccessor);
OnResolve(serviceType, serviceProviderEngineScope);
DependencyInjectionEventSource.Log.ServiceResolved(this, serviceType);
var result = realizedService.Invoke(serviceProviderEngineScope);
System.Diagnostics.Debug.Assert(result is null || CallSiteFactory.IsService(serviceType));
return result;
}
private Func<ServiceProviderEngineScope, object> CreateServiceAccessor(Type serviceType)
{
ServiceCallSite callSite = CallSiteFactory.GetCallSite(serviceType, new CallSiteChain());
if (callSite != null)
{
DependencyInjectionEventSource.Log.CallSiteBuilt(this, serviceType, callSite);
OnCreate(callSite);
// Optimize singleton case
if (callSite.Cache.Location == CallSiteResultCacheLocation.Root)
{
object value = CallSiteRuntimeResolver.Instance.Resolve(callSite, Root);
return scope => value;
}
return _engine.RealizeService(callSite);
}
return _ => null;
}
internal IServiceScope CreateScope()
{
if (_disposed)
{
ThrowHelper.ThrowObjectDisposedException();
}
return new ServiceProviderEngineScope(this, isRootScope: false);
}
private ServiceProviderEngine GetEngine()
{
ServiceProviderEngine engine;
#if NETFRAMEWORK || NETSTANDARD2_0
engine = new DynamicServiceProviderEngine(this);
#else
if (RuntimeFeature.IsDynamicCodeCompiled)
{
engine = new DynamicServiceProviderEngine(this);
}
else
{
engine = RuntimeServiceProviderEngine.Instance;
}
#endif
return engine;
}
}
从上面的代码中可以看出，在构建ServiceProvider时，主要是构造默认的根容器和采用哪种引擎来获取服务，并且把服务的类型描述给构建好并缓存起来。我们再来看看GetService方法，里面的调用服务分为Root和非Root，也就是说，需要区分哪些是属于根容器的，哪些不是属于根容器的。在.Net中，默认情况下添加的添加的Singleton类型属于Root,Scoped类型属于Scope，Transient类型属于Dispose，具体请看下面代码：
internal struct ResultCache
{
public ResultCache(ServiceLifetime lifetime, Type type, int slot)
{
switch (lifetime)
{
case ServiceLifetime.Singleton:
Location = CallSiteResultCacheLocation.Root;
break;
case ServiceLifetime.Scoped:
Location = CallSiteResultCacheLocation.Scope;
break;
case ServiceLifetime.Transient:
Location = CallSiteResultCacheLocation.Dispose;
break;
default:
Location = CallSiteResultCacheLocation.None;
break;
}
Key = new ServiceCacheKey(type, slot);
}
public CallSiteResultCacheLocation Location { get; set; }
public ServiceCacheKey Key { get; set; }
}
- 有了基本的了解之后，我们再来看通过GetService方法获取对象实例，当类型是位于根容器时，会将根容器的实例做在缓存里面，而类型如果是Transient，那么则每次获取时，都重新创建，不做缓存处理，至于Scoped方式的类型，稍后再说，我们先看下面代码
internal sealed class CallSiteRuntimeResolver : CallSiteVisitor<RuntimeResolverContext, object>
{
//对于Transient的类型实例
protected override object VisitDisposeCache(ServiceCallSite transientCallSite, RuntimeResolverContext context)
{
//直接构造类型实例，并记录在dispose的集合中，等待容器被Dispose时，同时dispose掉
return context.Scope.CaptureDisposable(VisitCallSiteMain(transientCallSite, context));
}
//获取在根容器的对象实例
protected override object VisitRootCache(ServiceCallSite callSite, RuntimeResolverContext context)
{
//对于已经做过缓存的服务，直接返回
if (callSite.Value is object value)
{
return value;
}
var lockType = RuntimeResolverLock.Root;
ServiceProviderEngineScope serviceProviderEngine = context.Scope.RootProvider.Root;
//对当前服务类型上锁
lock (callSite)
{
//相当于一个双检锁，再查一遍
if (callSite.Value is object resolved)
{
return resolved;
}
resolved = VisitCallSiteMain(callSite, new RuntimeResolverContext
{
Scope = serviceProviderEngine,
AcquiredLocks = context.AcquiredLocks | lockType
});
serviceProviderEngine.CaptureDisposable(resolved);
//将获取到的实例做缓存，返回拿到的实例
callSite.Value = resolved;
return resolved;
}
}
}
- 而对于注入的Scope类型，我们知道针对每次请求下，针对每个类型的实例是一个，而在ServiceProviderEngineScope类中有一个CreateScope方法，而每次接收到请求时都会构建HttpContext实例，在构建的同时调用CreateScope方法，来构建新的容器作为本次请求的Scope容器，期间获取的Scope类型的实例，都是在里面获取，先做缓存再返回，保证这个作用域内的实例是一个，请看下面代码：
internal sealed class CallSiteRuntimeResolver : CallSiteVisitor<RuntimeResolverContext, object>
{
protected override object VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context)
{
//如果是根容器作用域，就在根容器中找，否则就在当前作用域容器中找
return context.Scope.IsRootScope ?
VisitRootCache(callSite, context) :
VisitCache(callSite, context, context.Scope, RuntimeResolverLock.Scope);
}
private object VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
{
bool lockTaken = false;
object sync = serviceProviderEngine.Sync;
Dictionary<ServiceCacheKey, object> resolvedServices = serviceProviderEngine.ResolvedServices;
if ((context.AcquiredLocks & lockType) == 0)
{
Monitor.Enter(sync, ref lockTaken);
}
try
{
//每次查找前，先判断这个实例是否已经创建过，如果在本容器的缓存集合中存在就直接返回
if (resolvedServices.TryGetValue(callSite.Cache.Key, out object resolved))
{
return resolved;
}
resolved = VisitCallSiteMain(callSite, new RuntimeResolverContext
{
Scope = serviceProviderEngine,
AcquiredLocks = context.AcquiredLocks | lockType
});
serviceProviderEngine.CaptureDisposable(resolved);
//将类型实例添加到缓存中
resolvedServices.Add(callSite.Cache.Key, resolved);
return resolved;
}
finally
{
if (lockTaken)
{
Monitor.Exit(sync);
}
}
}
}
总结
通过源码可以看出默认的依赖注入有以下特点：
- 在一个Host中只能存在一个根容器，而其他容器(每次请求创建)都是从根容器中衍生出来的。
- Scope的类型可能被提升到Singleton。
- 对于Singleton的注入类型，都是存放在根容器中，并作缓存。
- 对于Scoped的注入类型，大部分是存放在每次请求构建的容器中，并作缓存。
- 对于Transient的注入类型，则不做缓存，每次访问都构建出一个新的对象实例。
关于.Net中默认的依赖注入，上面的代码也只是挑出重点的部分分享给大家，具体想看更多细节，读者可以根据本篇博客直接看源码，因为篇幅问题，实在不能贴太多的代码，主要是把思路给大家说一下。