Java 8 并行流和 ThreadLocal

Question

我想弄清楚如何在 Java 8 并行流中复制 ThreadLocal 值。

所以如果我们考虑这个:

    public class ThreadLocalTest {

        public static void main(String[] args)  {
            ThreadContext.set("MAIN");
            System.out.printf("Main Thread: %s\n", ThreadContext.get());

            IntStream.range(0,8).boxed().parallel().forEach(n -> {
                System.out.printf("Parallel Consumer - %d: %s\n", n, ThreadContext.get());
            });
        }

        private static class ThreadContext {
            private static ThreadLocal<String> val = ThreadLocal.withInitial(() -> "empty");

            public ThreadContext() {
            }

            public static String get() {
                return val.get();
            }

            public static void set(String x) {
                ThreadContext.val.set(x);
            }
        }
    }

哪些输出

Main Thread: MAIN
Parallel Consumer - 5: MAIN
Parallel Consumer - 4: MAIN
Parallel Consumer - 7: empty
Parallel Consumer - 3: empty
Parallel Consumer - 1: empty
Parallel Consumer - 6: empty
Parallel Consumer - 2: empty
Parallel Consumer - 0: MAIN

有没有办法将 ThreadLocal 从 main() 方法克隆到为每次并行执行生成的线程中？

这样我的结果是:

Main Thread: MAIN
Parallel Consumer - 5: MAIN
Parallel Consumer - 4: MAIN
Parallel Consumer - 7: MAIN
Parallel Consumer - 3: MAIN
Parallel Consumer - 1: MAIN
Parallel Consumer - 6: MAIN
Parallel Consumer - 2: MAIN
Parallel Consumer - 0: MAIN

而不是第一个？

Answer 1

As Louis has stated in the comments ，您的示例可以很好地简化为捕获 lambda 表达式中局部变量的值

public static void main(String[] args)  {
    String value = "MAIN";
    System.out.printf("Main Thread: %s\n", value);

    IntStream.range(0,8).boxed().parallel().forEach(n -> {
        System.out.printf("Parallel Consumer - %d: %s\n", n, value);
    });
}

从您的示例中看不出完整的用例是什么。

如果您确切知道哪些线程将从您的主线程启动，您可以考虑使用 InheritableThreadLocal

This class extends ThreadLocal to provide inheritance of values fromparent thread to child thread: when a child thread is created, thechild receives initial values for all inheritable thread-localvariables for which the parent has values.

在你的例子中，将 val 声明为 InheritableThreadLocal，因为 Thread 实例是为 parallel() 创建的ForkJoinPool#commonPool() 是延迟创建的，它们都继承自 main 方法(和线程)中的值 set。

如果您在设置 InhertiableThreadLocal 之前以某种方式使用 commonPool(或调用 parallel 终端操作的任何池)，情况就不会如此 原始线程中的值。