精通Java并发编程N+5：TransmittableThreadLocal解决线程池中ThreadLocal无法在父子线程之间传递的问题

在上一篇博文（https://blog.csdn.net/Saintmm/article/details/124461915）中介绍了InheritableThreadLocal如何实现父子线程之间传递ThreadLocal信息；最后提到InheritableThreadLocal的一个问题：在使用线程池时InheritableThreadLocal会完全失效。

因为线程池中的线程是会被复用的，所以在使用线程池进行多线程编程时，操作线程本地变量时需要特别注意；下面我们使用TransmittableThreadLocal 解决线程本地变量在线程池之间的传递问题。

1> 复现案例：
创建一个核心线程数为2，最大线程数为2的线程池，在主线程中每次对线程本地变量赋值之后，提交一个线程任务到线程池中运行，如下：

public class InheritableThreadLocalTest1 {

    public final static InheritableThreadLocal inheritableThreadLocal = new InheritableThreadLocal<>();
    public static final int CORE_THREAD_SIZE = 2;

    public static ThreadPoolExecutor executorService = new ThreadPoolExecutor(CORE_THREAD_SIZE,
            CORE_THREAD_SIZE,
            10,
            TimeUnit.SECONDS,
            new LinkedBlockingQueue<>());

    public static void main(String[] args) throws InterruptedException {

        inheritableThreadLocal.set("main thread-1");
        executorService.submit(() -> {
            System.out.println("1 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get());
        });

        inheritableThreadLocal.set("main thread-2");
        executorService.submit(() -> {
            System.out.println("2 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get());
        });

        inheritableThreadLocal.set("main thread-3");
        executorService.submit(() -> {
            System.out.println("3 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get());
        });

        inheritableThreadLocal.set("main thread-4");
        executorService.submit(() -> {
            System.out.println("4 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get());
        });

    }
}

2> 运行结果如下：

3> 结果分析：从运行结果结合线程池的运行机制可以看出：

1. 第一任务提交到线程池时，线程池中没有核心线程，创建一个线程，此时可以获取到父线程（main线程）中最新的InheritableThreadLocal值；
2. 第二个任务提交到线程池时，线程池中核心线程数为1，1 < 2，所以再创建一个线程，此时也可以获取到父线程（main线程）中最新的InheritableThreadLocal值；
3. 第三个任务提交到线程池时，线程池中核心线程数为2，线程进入到阻塞队列，等待空闲线程来调度任务。假如此时coreThread1刚执行完任务1，来执行任务3，则此时的InheritableThreadLocal是历史版本的（main thread-1）。
4. 第四个任务提交到线程池时，线程池中核心线程数为2，线程进入到阻塞队列，等待空闲线程来调度任务。假如此时coreThread2刚执行完任务2，来执行任务4，则此时的InheritableThreadLocal是历史版本的（main thread-2）。

因此，我们可以确定，只有当核心线程数不足时才会正确的传递的InheritableThreadLocal，而如果线程池中的线程在循环使用了，则不会加载最新的InheritableThreadLocal，而是使用历史上的某个版本数据。

线程池部分内容请见博文：源码分析ThreadPoolExecutor线程池实现原理。

引入阿里提供的TransmittableThreadLocal，github地址为：https://github.com/alibaba/transmittable-thread-local。

    com.alibaba
    transmittable-thread-local
    2.12.6

1> 修改线程本地变量类型为TransmittableThreadLocal

public final static TransmittableThreadLocal inheritableThreadLocal = new TransmittableThreadLocal<>();

其次，官方给定了三种方式保证TransmittableThreadLocal的正确传递：

2.1> 修饰Runnable和Callable
2.2> TtlExecutors 修饰线程池
2.3> 使用Java Agent来修饰JDK线程池实现类

public class TransmittableThreadLocalTest {

    public final static TransmittableThreadLocal inheritableThreadLocal = new TransmittableThreadLocal<>();
    public static final int CORE_THREAD_SIZE = 2;

    public static ThreadPoolExecutor executorService = new ThreadPoolExecutor(CORE_THREAD_SIZE,
            CORE_THREAD_SIZE,
            10,
            TimeUnit.SECONDS,
            new LinkedBlockingQueue<>());

    public static void main(String[] args) throws InterruptedException {

        inheritableThreadLocal.set("main thread-1");
        executorService.submit(TtlRunnable.get(
                        () -> System.out.println("1 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get())
                )
        );

        inheritableThreadLocal.set("main thread-2");
        executorService.submit(TtlRunnable.get(
                        () -> System.out.println("2 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get())
                )
        );

        inheritableThreadLocal.set("main thread-3");
        executorService.submit(TtlRunnable.get(
                        () -> System.out.println("3 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get())
                )
        );

        inheritableThreadLocal.set("main thread-4");
        executorService.submit(TtlRunnable.get(
                        () -> System.out.println("4 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get())
                )
        );

    }
}

运行结果正常，如下所示：

public class TransmittableThreadLocalTest {

    public final static TransmittableThreadLocal inheritableThreadLocal = new TransmittableThreadLocal<>();
    public static final int CORE_THREAD_SIZE = 2;

    public static ExecutorService executorService =
            TtlExecutors.getTtlExecutorService(new ThreadPoolExecutor(CORE_THREAD_SIZE,
                    CORE_THREAD_SIZE,
                    10,
                    TimeUnit.SECONDS,
                    new LinkedBlockingQueue<>())
            );

    public static void main(String[] args) throws InterruptedException {

        inheritableThreadLocal.set("main thread-1");
        executorService.submit(
                () -> System.out.println("1 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get())
        );

        inheritableThreadLocal.set("main thread-2");
        executorService.submit(
                () -> System.out.println("2 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get())
        );

        inheritableThreadLocal.set("main thread-3");
        executorService.submit(
                () -> System.out.println("3 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get())
        );

        inheritableThreadLocal.set("main thread-4");
        executorService.submit(
                () -> System.out.println("4 obtain inheritableThreadLocal in threadPool: " + inheritableThreadLocal.get())
        );

    }
}

运行结果和上述一致；

该种方式我们在后续分析TransmittableThreadLocal的实现原理之后，再做演示、推论。

可以先参考：github <–> 23-使用java-agent来修饰jdk线程池实现类。