Java 8并行流和ThreadLocal

Question

我试图找出如何在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);
            }
        }
    }

哪个输出

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

有没有办法让我将main()方法中的ThreadLocal克隆到为每个并行执行生成的线程中？

这样我的结果就是:

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

正如路易斯在评论中所说的那样,你的例子很可能被简化为捕获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);
    });
}

从您的示例中可以看出,完整的用例是什么.

如果您确切知道将从主线程启动哪些线程,您可以考虑使用InheritableThreadLocal

此类扩展ThreadLocal为提供从父线程到子线程的值的继承:当创建子线程时,子进程接收父对象具有值的所有可继承线程局部变量的初始值.

在你的情况下,宣布val作为InheritableThreadLocal,因为Thread对创建的实例parallel()中ForkJoinPool#commonPool()懒洋洋地创建,他们都从值继承set的main方法(和线程).

如果您在原始线程中设置值之前以某种方式使用commonPool(或parallel调用终端操作的任何池),则情况不会如此InhertiableThreadLocal.