传递 Threadlocal 给子线程

使用 InheritableThreadLocal

InheritableThreadLocal 类是 ThreadLocal 的子类，专门用于在子线程中继承父线程的 ThreadLocal 变量。它能够自动将父线程的 ThreadLocal 变量值传递给子线程。

public class InheritableThreadLocalExample {
    private static final InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();

    public static void main(String[] args) throws InterruptedException {
        threadLocal.set("Parent Thread Local Value");

        Thread childThread = new Thread(() -> {
            System.out.println("Child Thread: " + threadLocal.get());
        });

        childThread.start();
        childThread.join();

        System.out.println("Parent Thread: " + threadLocal.get());
    }
}

手动传递 ThreadLocal 值

另一种方法是手动将父线程的 ThreadLocal 值传递给子线程。这可以通过在创建子线程时将 ThreadLocal 值作为参数传递给子线程的构造函数或任务。

public class ManualThreadLocalExample {
    private static final ThreadLocal<String> threadLocal = new ThreadLocal<>();

    public static void main(String[] args) throws InterruptedException {
        threadLocal.set("Parent Thread Local Value");

        String parentValue = threadLocal.get();
        Thread childThread = new Thread(() -> {
            threadLocal.set(parentValue);
            System.out.println("Child Thread: " + threadLocal.get());
        });

        childThread.start();
        childThread.join();

        System.out.println("Parent Thread: " + threadLocal.get());
    }
}

使用自定义 ThreadFactory

你还可以创建一个自定义的 ThreadFactory，在创建新线程时自动传递 ThreadLocal 值。

import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;

public class CustomThreadFactoryExample {
    private static final ThreadLocal<String> threadLocal = new ThreadLocal<>();

    public static void main(String[] args) {
        threadLocal.set("Parent Thread Local Value");

        ThreadFactory factory = new CustomThreadFactory(threadLocal.get());
        Executors.newSingleThreadExecutor(factory).execute(() -> {
            System.out.println("Child Thread: " + threadLocal.get());
        });
    }

    static class CustomThreadFactory implements ThreadFactory {
        private final String parentValue;

        CustomThreadFactory(String parentValue) {
            this.parentValue = parentValue;
        }

        @Override
        public Thread newThread(Runnable r) {
            return new Thread(() -> {
                threadLocal.set(parentValue);
                r.run();
            });
        }
    }
}

使用 TransmittableThreadLocal

TransmittableThreadLocal 是阿里巴巴开源的一个工具类，它扩展了 Java 标准库中的 InheritableThreadLocal，使得父线程的 ThreadLocal 变量不仅能够传递给子线程，还能在使用线程池等复杂场景下进行传递。

import com.alibaba.ttl.TransmittableThreadLocal;

public class TransmittableThreadLocalExample {
    private static final TransmittableThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();

    public static void main(String[] args) {
        // 设置父线程的 ThreadLocal 变量
        threadLocal.set("Parent Thread Local Value");

        // 创建子线程
        Thread childThread = new Thread(() -> {
            // 获取子线程中的 ThreadLocal 变量
            System.out.println("Child Thread: " + threadLocal.get());
        });

        // 启动子线程
        childThread.start();

        // 等待子线程结束
        try {
            childThread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // 获取父线程中的 ThreadLocal 变量
        System.out.println("Parent Thread: " + threadLocal.get());
    }
}

在这个示例中，子线程能够正确获取到父线程设置的 ThreadLocal 变量值，即 “Parent Thread Local Value”。

在线程池中使用 TtlRunnable

对于线程池中的任务，你可以使用 TtlRunnable 来包装任务，从而确保 TransmittableThreadLocal 变量能够正确传递。

import com.alibaba.ttl.TransmittableThreadLocal;
import com.alibaba.ttl.TtlRunnable;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class TransmittableThreadLocalThreadPoolExample {
    private static final TransmittableThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();

    public static void main(String[] args) {
        threadLocal.set("Parent Thread Local Value");

        ExecutorService executorService = Executors.newFixedThreadPool(2);

        Runnable task = () -> {
            System.out.println("Task Thread: " + threadLocal.get());
        };

        // 使用 TtlRunnable 包装任务
        executorService.submit(TtlRunnable.get(task));

        // 关闭线程池
        executorService.shutdown();
    }
}