Java 8个并行流和ThreadLocal

Question

我想知道如何在 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);
            }
        }
    }

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

有没有办法让我从 main() 方法中将 ThreadLocal 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

，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 from parent thread to child thread: when a child thread is created, the child receives initial values for all inheritable thread-local variables for which the parent has values.

在您的情况下，将 val 声明为 InheritableThreadLocal，因为 ForkJoinPool#commonPool() 中为 parallel() 创建的 Thread 实例是延迟创建的，它们都将从 main 方法（和线程）中的值 set 继承。

如果您在设置原始线程。

Java 8个并行流和ThreadLocal

Java 8 parallel stream and ThreadLocal

java

threadpool

java-8

forkjoinpool