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

官方文档

源码地址

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}");
        }
    }
}

到此这篇关于.NET Core利用 AsyncLocal 实现共享变量的代码详解的文章就介绍到这了,更多相关.NET Core共享变量内容请搜索阿兔在线工具以前的文章或继续浏览下面的相关文章希望大家以后多多支持阿兔在线工具!

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