.NET Core利用 AsyncLocal 实现共享变量的代码详解
作者：何言何语
在Web 应用程序中，我们经常会遇到这样的场景，如用户信息，租户信息本次的请求过程中都是固定的，我们希望是这种信息在本次请求内，一次赋值，到处使用。本文就来探讨一下，如何在.NET Core 下去利用AsyncLocal 实现全局共享变量
简介
我们如果需要整个程序共享一个变量，我们仅需将该变量放在某个静态类的静态变量上即可（不满足我们的需求，静态变量上，整个程序都是固定值）。我们在Web 应用程序中，每个Web 请求服务器都为其分配了一个独立线程，如何实现用户，租户等信息隔离在这些独立线程中。这就是今天要说的线程本地存储。针对线程本地存储 .NET 给我们提供了两个类 ThreadLocal 和 AsyncLocal。我们可以通过查看以下例子清晰的看到两者的区别：
[TestClass]
public class TastLocal {
private static ThreadLocal<string> threadLocal = new ThreadLocal<string>();
private static AsyncLocal<string> asyncLocal = new AsyncLocal<string>();
[TestMethod]
public void Test() {
threadLocal.Value = "threadLocal";
asyncLocal.Value = "asyncLocal";
var threadId = Thread.CurrentThread.ManagedThreadId;
Task.Factory.StartNew(() => {
var threadId = Thread.CurrentThread.ManagedThreadId;
Debug.WriteLine($"StartNew:threadId:{ threadId}; threadLocal:{threadLocal.Value}");
Debug.WriteLine($"StartNew:threadId:{ threadId}; asyncLocal:{asyncLocal.Value}");
});
CurrThread();
}
public void CurrThread() {
var threadId = Thread.CurrentThread.ManagedThreadId;
Debug.WriteLine($"CurrThread:threadId:{threadId};threadLocal:{threadLocal.Value}");
Debug.WriteLine($"CurrThread:threadId:{threadId};asyncLocal:{asyncLocal.Value}");
}
}
输出结果：
CurrThread:threadId:4;threadLocal:threadLocal
StartNew:threadId:11; threadLocal:
CurrThread:threadId:4;asyncLocal:asyncLocal
StartNew:threadId:11; asyncLocal:asyncLocal
从上面结果中可以看出 ThreadLocal 和 AsyncLocal 都能实现基于线程的本地存储。但是当线程切换后，只有 AsyncLocal 还能够保留原来的值。在Web 开发中，我们会有很多异步场景，在这些场景下，可能会出现线程的切换。所以我们使用AsyncLocal 去实现在Web 应用程序下的共享变量。
AsyncLocal 解读
源码查看：
public sealed class AsyncLocal<T> : IAsyncLocal
{
private readonly Action<AsyncLocalValueChangedArgs<T>>? m_valueChangedHandler;
//
// 无参构造函数
//
public AsyncLocal()
{
}
//
// 构造一个带有委托的AsyncLocal<T>，该委托在当前值更改时被调用
// 在任何线程上
//
public AsyncLocal(Action<AsyncLocalValueChangedArgs<T>>? valueChangedHandler)
{
m_valueChangedHandler = valueChangedHandler;
}
[MaybeNull]
public T Value
{
get
{
object? obj = ExecutionContext.GetLocalValue(this);
return (obj == null) ? default : (T)obj;
}
set => ExecutionContext.SetLocalValue(this, value, m_valueChangedHandler != null);
}
void IAsyncLocal.OnValueChanged(object? previousValueObj, object? currentValueObj, bool contextChanged)
{
Debug.Assert(m_valueChangedHandler != null);
T previousValue = previousValueObj == null ? default! : (T)previousValueObj;
T currentValue = currentValueObj == null ? default! : (T)currentValueObj;
m_valueChangedHandler(new AsyncLocalValueChangedArgs<T>(previousValue, currentValue, contextChanged));
}
}
//
// 接口，允许ExecutionContext中的非泛型代码调用泛型AsyncLocal<T>类型
//
internal interface IAsyncLocal
{
void OnValueChanged(object? previousValue, object? currentValue, bool contextChanged);
}
public readonly struct AsyncLocalValueChangedArgs<T>
{
public T? PreviousValue { get; }
public T? CurrentValue { get; }
//
// If the value changed because we changed to a different ExecutionContext, this is true. If it changed
// because someone set the Value property, this is false.
//
public bool ThreadContextChanged { get; }
internal AsyncLocalValueChangedArgs(T? previousValue, T? currentValue, bool contextChanged)
{
PreviousValue = previousValue!;
CurrentValue = currentValue!;
ThreadContextChanged = contextChanged;
}
}
//
// Interface used to store an IAsyncLocal => object mapping in ExecutionContext.
// Implementations are specialized based on the number of elements in the immutable
// map in order to minimize memory consumption and look-up times.
//
internal interface IAsyncLocalValueMap
{
bool TryGetValue(IAsyncLocal key, out object? value);
IAsyncLocalValueMap Set(IAsyncLocal key, object? value, bool treatNullValueAsNonexistent);
}我们知道在.NET 里面，每个线程都关联着执行上下文。我们可以通 Thread.CurrentThread.ExecutionContext 属性进行访问 或者通过 ExecutionContext.Capture() 获取。
从上面我们可以看出 AsyncLocal 的 Value 存取是通过 ExecutionContext.GetLocalValue 和GetLocalValue.SetLocalValue 进行操作的，我们可以继续从 ExecutionContext 里面取出部分代码查看（源码地址），为了更深入地理解 AsyncLocal 我们可以查看一下源码，看看内部实现原理。
internal static readonly ExecutionContext Default = new ExecutionContext();
private static volatile ExecutionContext? s_defaultFlowSuppressed;
private readonly IAsyncLocalValueMap? m_localValues;
private readonly IAsyncLocal[]? m_localChangeNotifications;
private readonly bool m_isFlowSuppressed;
private readonly bool m_isDefault;
private ExecutionContext()
{
m_isDefault = true;
}
private ExecutionContext(
IAsyncLocalValueMap localValues,
IAsyncLocal[]? localChangeNotifications,
bool isFlowSuppressed)
{
m_localValues = localValues;
m_localChangeNotifications = localChangeNotifications;
m_isFlowSuppressed = isFlowSuppressed;
}
public void GetObjectData(SerializationInfo info, StreamingContext context)
{
throw new PlatformNotSupportedException();
}
public static ExecutionContext? Capture()
{
ExecutionContext? executionContext = Thread.CurrentThread._executionContext;
if (executionContext == null)
{
executionContext = Default;
}
else if (executionContext.m_isFlowSuppressed)
{
executionContext = null;
}
return executionContext;
}
internal static object? GetLocalValue(IAsyncLocal local)
{
ExecutionContext? current = Thread.CurrentThread._executionContext;
if (current == null)
{
return null;
}
Debug.Assert(!current.IsDefault);
Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context");
current.m_localValues.TryGetValue(local, out object? value);
return value;
}
internal static void SetLocalValue(IAsyncLocal local, object? newValue, bool needChangeNotifications)
{
ExecutionContext? current = Thread.CurrentThread._executionContext;
object? previousValue = null;
bool hadPreviousValue = false;
if (current != null)
{
Debug.Assert(!current.IsDefault);
Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context");
hadPreviousValue = current.m_localValues.TryGetValue(local, out previousValue);
}
if (previousValue == newValue)
{
return;
}
// Regarding 'treatNullValueAsNonexistent: !needChangeNotifications' below:
// - When change notifications are not necessary for this IAsyncLocal, there is no observable difference between
// storing a null value and removing the IAsyncLocal from 'm_localValues'
// - When change notifications are necessary for this IAsyncLocal, the IAsyncLocal's absence in 'm_localValues'
// indicates that this is the first value change for the IAsyncLocal and it needs to be registered for change
// notifications. So in this case, a null value must be stored in 'm_localValues' to indicate that the IAsyncLocal
// is already registered for change notifications.
IAsyncLocal[]? newChangeNotifications = null;
IAsyncLocalValueMap newValues;
bool isFlowSuppressed = false;
if (current != null)
{
Debug.Assert(!current.IsDefault);
Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context");
isFlowSuppressed = current.m_isFlowSuppressed;
newValues = current.m_localValues.Set(local, newValue, treatNullValueAsNonexistent: !needChangeNotifications);
newChangeNotifications = current.m_localChangeNotifications;
}
else
{
// First AsyncLocal
newValues = AsyncLocalValueMap.Create(local, newValue, treatNullValueAsNonexistent: !needChangeNotifications);
}
//
// Either copy the change notification array, or create a new one, depending on whether we need to add a new item.
//
if (needChangeNotifications)
{
if (hadPreviousValue)
{
Debug.Assert(newChangeNotifications != null);
Debug.Assert(Array.IndexOf(newChangeNotifications, local) >= 0);
}
else if (newChangeNotifications == null)
{
newChangeNotifications = new IAsyncLocal[1] { local };
}
else
{
int newNotificationIndex = newChangeNotifications.Length;
Array.Resize(ref newChangeNotifications, newNotificationIndex + 1);
newChangeNotifications[newNotificationIndex] = local;
}
}
Thread.CurrentThread._executionContext =
(!isFlowSuppressed && AsyncLocalValueMap.IsEmpty(newValues)) ?
null : // No values, return to Default context
new ExecutionContext(newValues, newChangeNotifications, isFlowSuppressed);
if (needChangeNotifications)
{
local.OnValueChanged(previousValue, newValue, contextChanged: false);
}
}
从上面可以看出，ExecutionContext.GetLocalValue 和GetLocalValue.SetLocalValue 都是通过对 m_localValues 字段进行操作的。
m_localValues 的类型是 IAsyncLocalValueMap ,IAsyncLocalValueMap 的实现 和 AsyncLocal.cs 在一起，感兴趣的可以进一步查看 IAsyncLocalValueMap 是如何创建，如何查找的。
可以看到，里面最重要的就是ExecutionContext 的流动，线程发生变化时ExecutionContext 会在前一个线程中被默认捕获，流向下一个线程，它所保存的数据也就随之流动。在所有会发生线程切换的地方，基础类库(BCL) 都为我们封装好了对执行上下文的捕获 (如开始的例子，可以看到 AsyncLocal 的数据不会随着线程的切换而丢失)，这也是为什么 AsyncLocal 能实现 线程切换后，还能正常获取数据，不丢失。
总结
AsyncLocal 本身不保存数据，数据保存在 ExecutionContext 实例。
ExecutionContext 的实例会随着线程切换流向下一线程（也可以禁止流动和恢复流动），保证了线程切换时，数据能正常访问。
1.在.NET Core 中的使用示例先创建一个上下文对象
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
namespace NetAsyncLocalExamples.Context
{
/// <summary>
/// 请求上下文 租户ID
/// </summary>
public class RequestContext
{
/// <summary>
/// 获取请求上下文
/// </summary>
public static RequestContext Current => _asyncLocal.Value;
private readonly static AsyncLocal<RequestContext> _asyncLocal = new AsyncLocal<RequestContext>();
/// <summary>
/// 将请求上下文设置到线程全局区域
/// </summary>
/// <param name="userContext"></param>
public static IDisposable SetContext(RequestContext userContext)
{
_asyncLocal.Value = userContext;
return new RequestContextDisposable();
}
/// <summary>
/// 清除上下文
/// </summary>
public static void ClearContext()
{
_asyncLocal.Value = null;
}
/// <summary>
/// 租户ID
/// </summary>
public string TenantId { get; set; }
}
}
namespace NetAsyncLocalExamples.Context
{
/// <summary>
/// 用于释放对象
/// </summary>
internal class RequestContextDisposable : IDisposable
{
internal RequestContextDisposable() { }
public void Dispose()
{
RequestContext.ClearContext();
}
}
}
2.创建请求上下文中间件
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using NetAsyncLocalExamples.Context;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
namespace NetAsyncLocalExamples.Middlewares
{
/// <summary>
/// 请求上下文
/// </summary>
public class RequestContextMiddleware : IMiddleware
{
protected readonly IServiceProvider ServiceProvider;
private readonly ILogger<RequestContextMiddleware> Logger;
public RequestContextMiddleware(IServiceProvider serviceProvider, ILogger<RequestContextMiddleware> logger)
{
ServiceProvider = serviceProvider;
Logger = logger;
}
public virtual async Task InvokeAsync(HttpContext context, RequestDelegate next)
{
var requestContext = new RequestContext();
using (RequestContext.SetContext(requestContext))
{
requestContext.TenantId = $"租户ID:{DateTime.Now.ToString("yyyyMMddHHmmsss")}";
await next(context);
}
}
}
}
3.注册中间件
public void ConfigureServices(IServiceCollection services)
{
services.AddTransient<RequestContextMiddleware>();
services.AddRazorPages();
}
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
app.UseExceptionHandler("/Error");
// The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthorization();
//增加上下文
app.UseMiddleware<RequestContextMiddleware>();
app.UseEndpoints(endpoints =>
{
endpoints.MapRazorPages();
});
}
一次赋值，到处使用
namespace NetAsyncLocalExamples.Pages
{
public class IndexModel : PageModel
{
private readonly ILogger<IndexModel> _logger;
public IndexModel(ILogger<IndexModel> logger)
{
_logger = logger;
_logger.LogInformation($"测试获取全局变量1：{RequestContext.Current.TenantId}");
}
public void OnGet()
{
_logger.LogInformation($"测试获取全局变量2：{RequestContext.Current.TenantId}");
}
}
}
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