java - CompletableFuture 如何知道任务是独立的？

Question

假设我们有以下虚拟代码:

CompletableFuture<BigInteger> cf1 = CompletableFuture.supplyAsync(() -> BigInteger.valueOf(2L));
CompletableFuture<BigInteger> cf2 = CompletableFuture.supplyAsync(() -> BigInteger.valueOf(3L));
cf1.thenCombine(cf2, (x, y) -> x.add(y)).thenAccept(System.out::println);

在这种情况下，JVM 是否知道 cf1 和 cf2 承载独立线程？如果线程相互依赖(例如，使用一个数据库连接)，将会发生什么变化？

更一般地说，CompletableFuture 如何同步线程？

Answer 1

CompletableFuture 与任何线程都没有关系。它只是异步检索结果的持有者，并使用对该结果进行操作的方法。

静态 supplyAsync和 runAsync 方法只是辅助方法。 supplyAsync 的 javadoc 状态

Returns a new CompletableFuture that is asynchronously completed by a task running in the ForkJoinPool.commonPool() with the value obtained by calling the given Supplier.

这或多或少相当于

Supplier<R> sup = ...;
CompletableFuture<R> future = new CompletableFuture<R>();
ForkJoinPool.commonPool().submit(() -> {
     try {
        R result = sup.get();
        future.complete(result);
    } catch (Throwable e) {
        future.completeExceptionally(e);
    }
});
return future;

返回CompletableFuture，甚至允许您在任务提交到池之前完成它。

More general, how does CompletableFuture synchronize threads?

它不知道，因为它不知道哪些线程正在对其进行操作。 javadoc 中进一步暗示了这一点。

Since (unlike FutureTask) this class has no direct control over the computation that causes it to be completed, cancellation is treated as just another form of exceptional completion. Method cancel has the same effect as completeExceptionally(new CancellationException()). Method isCompletedExceptionally() can be used to determine if a CompletableFuture completed in any exceptional fashion.

CompletableFuture 对象不控制处理。