您必须加入线程以确保其计算完成

Question

我有一个实用程序方法（用于单元测试，碰巧）在另一个线程中执行 Runnable。它启动线程运行，但不等待 Thread 完成，而是依赖 Future。该方法的调用者应 get() Future。但这足以确保安全发布 Runnable 所做的计算吗？

方法如下：

private static Future<Void> runInOtherThread(final CountDownLatch ready, final Runnable operation) {
    final CompletableFuture<Void> future = new CompletableFuture<Void>();
    final Thread thread = new Thread(() -> {
        try {
            ready.await();
            operation.run();
        } catch (Throwable e) {
            future.completeExceptionally(e);
            return;
        }
        future.complete(null);
    });
    thread.start();
    return future;
}

在返回的 Future 上调用 Future.get() 后，方法的调用者是否可以安全地假设 Runnable 已完成执行，并且其结果已安全发布？

Answer 1

如果我们看看 Shipilev 的 Safe Publication，获得安全发布的一种简单方法就是工作：

Exchange the reference via a volatile field (JLS 17.4.5), or as the consequence of this rule, via the AtomicX classes

由于 CompletableFuture 使用 volatile 字段来写入和读取值，因此安全发布不需要额外的内存屏障。这在 CompletableFuture class overview comment:

中有解释

 * A CompletableFuture may have dependent completion actions,
 * collected in a linked stack. It atomically completes by CASing
 * a result field, and then pops off and runs those actions. This
 * applies across normal vs exceptional outcomes, sync vs async
 * actions, binary triggers, and various forms of completions.
 *
 * Non-nullness of volatile field "result" indicates done.  It may
 * be set directly if known to be thread-confined, else via CAS.
 * An AltResult is used to box null as a result, as well as to
 * hold exceptions.

它还处理已发布对象的安全初始化，根据稍后的相同概述评论：

 * Completion fields need not be declared as final or volatile
 * because they are only visible to other threads upon safe
 * publication.

Answer 2

不，你不需要 join()。对未来调用 get() 就足够了。

CompletableFuture 接口是 Future 的子类型。 Future 的 javadoc 表示：

Memory consistency effects: Actions taken by the asynchronous computation happen-before actions following the corresponding Future.get() in another thread.

happen-before 关系足以确保安全发布 get().

返回的值

此外，在 CompletableFuture 完成、异常完成或取消之前，get() 调用不会完成。

您必须加入线程以确保其计算完成

Must you join on a Thread to ensure its computation is complete

java

multithreading

future