阿里巴巴TransmittableThreadLocal使用指南

前言

ThreadLocal在上下文的数据传输上非常的方便和简洁。工业实践中，比较常用的有三个，ThreadLocal、InheritableThreadLocal、TransmittableThreadLocal，那么他们三个之间有什么区别呢？

常见的三种ThreadLocal比较

	ThreadLocal	InheritableThreadLocal	TransmittableThreadLocal
来源	jdk	jdk	阿里开源
单线程数据传输	支持	支持	支持
new线程数据传输	不支持	支持	支持
线程池数据传输	不支持	部分支持【简单场景】	支持

针对线程池的数据传输，InheritableThreadLocal仅仅能在一些简单场景下做到，下面就用一个案例来说明

先看下线程工厂的定义

package com.tml.mouseDemo.core.threadLocalDemo;import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicInteger;public class SimpleThreadFactory implements ThreadFactory {private static AtomicInteger atomicInteger = new AtomicInteger(1);@Overridepublic Thread newThread(Runnable r) {Thread t = new Thread(r);t.setName("tml-"+atomicInteger.getAndIncrement());return t;}
}

 InheritableThreadLocal部分支持的案例

package com.tml.mouseDemo.core.threadLocalDemo;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class ThreadLocalDemo1 {private static ThreadLocal<String> threadLocal = new InheritableThreadLocal<>();private static ExecutorService service = Executors.newFixedThreadPool(2, new SimpleThreadFactory());public static void main(String[] args) throws InterruptedException {threadLocal.set("hello main");for (int i = 0; i < 2; i++) {service.execute(() -> {String s = threadLocal.get();ThreadUtils.printLog("get data " + s);});}//修改threadLocal中的值threadLocal.set("hello world");Thread.sleep(2000);for (int i = 0; i < 2; i++) {service.execute(() -> {String s = threadLocal.get();ThreadUtils.printLog("get data " + s);});}ThreadUtils.printLog("get data " + threadLocal.get());service.shutdown();}
}

运行结果如下 

2025-01-10 19:41:50 | INFO  | tml-1 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 19:41:50 | INFO  | tml-2 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 19:41:52 | INFO  | tml-2 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 19:41:52 | INFO  | main | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world
2025-01-10 19:41:52 | INFO  | tml-1 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main

从运行结果来看，前面的两次循环提交的任务，在子线程中确实是能正常的获取主线程设置的变量，即hello main，但是紧接着，我修改了主线程上绑定的变量为hello world，然后继续循环两次提交两个任务，这个时候子线程中获取的线程变量依然是hello main，这明显是与是期望不一致的。

从这个层面来讲，InheritableThreadLocal确实在线程池的层面支持不够友好，可以说仅支持部分简单场景。

根本原因就死线程池的池化机制，从上面的运行日志上也可以看出，提交了4个任务执行的线程依然是两个。线程池中的线程是复用的，InheritableThreadLocal是在创建子线程的时候，会将主线程上绑定的数据拷贝过来，如果我不创建新的线程，但是主线程上绑定的数据改变了呢？那我依然还是读取到之前拷贝的数据。

这个就是InheritableThreadLocal的短板。针对这个问题，阿里开源的TransmittableThreadLocal就能顺利丝滑的解决这个问题。

TransmittableThreadLocal实践

maven依赖

<dependency><groupId>com.alibaba</groupId><artifactId>transmittable-thread-local</artifactId><version>2.12.4</version>
</dependency>

装饰Runnable任务

直接看代码

package com.tml.mouseDemo.core.threadLocalDemo;import com.alibaba.ttl.TransmittableThreadLocal;
import com.alibaba.ttl.TtlRunnable;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class ThreadLocalDemo1 {private static ThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();private static ExecutorService service = Executors.newFixedThreadPool(2, new SimpleThreadFactory());public static void main(String[] args) throws InterruptedException {threadLocal.set("hello main");for (int i = 0; i < 2; i++) {service.execute(TtlRunnable.get(() -> {String s = threadLocal.get();ThreadUtils.printLog("get data " + s);}));}//修改threadLocal中的值threadLocal.set("hello world");Thread.sleep(2000);for (int i = 0; i < 2; i++) {service.execute(TtlRunnable.get(() -> {String s = threadLocal.get();ThreadUtils.printLog("get data " + s);}));}ThreadUtils.printLog("get data " + threadLocal.get());service.shutdown();}
}

运行结果如下：

 2025-01-10 19:57:03 | INFO  | tml-2 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 19:57:03 | INFO  | tml-1 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 19:57:05 | INFO  | tml-1 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world
2025-01-10 19:57:05 | INFO  | main | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world
2025-01-10 19:57:05 | INFO  | tml-2 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world

与第一个案例的差异点在于，使用了

public static TtlRunnable get(@Nullable Runnable runnable) {return get(runnable, false, false);
}

 来增强了了Runnable任务，执行的结果也是符合预期。

装饰线程池

直接看代码

package com.tml.mouseDemo.core.threadLocalDemo;import com.alibaba.ttl.TransmittableThreadLocal;
import com.alibaba.ttl.TtlRunnable;
import com.alibaba.ttl.threadpool.TtlExecutors;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class ThreadLocalDemo1 {private static ThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();private static ExecutorService service = Executors.newFixedThreadPool(2, new SimpleThreadFactory());private static ExecutorService ttlExecutorService = TtlExecutors.getTtlExecutorService(service);public static void main(String[] args) throws InterruptedException {threadLocal.set("hello main");for (int i = 0; i < 2; i++) {ttlExecutorService.execute(() -> {String s = threadLocal.get();ThreadUtils.printLog("get data " + s);});}//修改threadLocal中的值threadLocal.set("hello world");Thread.sleep(2000);for (int i = 0; i < 2; i++) {ttlExecutorService.execute(() -> {String s = threadLocal.get();ThreadUtils.printLog("get data " + s);});}ThreadUtils.printLog("get data " + threadLocal.get());service.shutdown();}
}

 运行结果如下

2025-01-10 20:05:05 | INFO  | tml-2 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 20:05:05 | INFO  | tml-1 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 20:05:07 | INFO  | tml-2 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world
2025-01-10 20:05:07 | INFO  | main | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world
2025-01-10 20:05:07 | INFO  | tml-1 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world

与上一个案例的差异点在于，这里没有包装Runnable任务，而是包装了线程池，使用了

public static ExecutorService getTtlExecutorService(@Nullable ExecutorService executorService) {if (TtlAgent.isTtlAgentLoaded() || executorService == null || executorService instanceof TtlEnhanced) {return executorService;}return new ExecutorServiceTtlWrapper(executorService, true);
}

包装了ExecutorService，执行结果也是符合预期

使用java Agent无侵入增强线程池

直接看代码

package com.tml.mouseDemo.core.threadLocalDemo;import com.alibaba.ttl.TransmittableThreadLocal;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class ThreadLocalDemo1 {private static ThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();private static ExecutorService service = Executors.newFixedThreadPool(2, new SimpleThreadFactory());public static void main(String[] args) throws InterruptedException {threadLocal.set("hello main");for (int i = 0; i < 2; i++) {service.execute(() -> {String s = threadLocal.get();ThreadUtils.printLog("get data " + s);});}//修改threadLocal中的值threadLocal.set("hello world");Thread.sleep(2000);for (int i = 0; i < 2; i++) {service.execute(() -> {String s = threadLocal.get();ThreadUtils.printLog("get data " + s);});}ThreadUtils.printLog("get data " + threadLocal.get());service.shutdown();}
}

 项目运行的时候，需要添加额外的jvm启动参数，如下

运行结果如下，也是符合预期

 2025-01-10 20:11:59 | INFO  | tml-2 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 20:11:59 | INFO  | tml-1 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello main
2025-01-10 20:12:01 | INFO  | tml-2 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world
2025-01-10 20:12:01 | INFO  | main | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world
2025-01-10 20:12:01 | INFO  | tml-1 | com.tml.mouseDemo.core.threadLocalDemo.ThreadUtils | get data hello world

总结

阿里巴巴的TransmittableThreadLocal是继承自InheritableThreadLocal，对他的功能进行了增强，增强的点也主要是在线程池的支持上。

通过上面的三个案例，可以看到TransmittableThreadLocal是非常灵活的，大家可以根据自己的需要，选择对应的方式来实现。

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