取消计算

和完成异常类似，我们可以调用cancel(boolean mayInterruptIfRunning)取消计算。对于CompletableFuture类，布尔参数并没有被使用，这是因为它并没有使用中断去取消操作，相反，cancel等价于completeExceptionally(new CancellationException())。

static void cancelExample() {CompletableFuture cf = CompletableFuture.completedFuture("message").thenApplyAsync(String::toUpperCase,CompletableFuture.delayedExecutor(1, TimeUnit.SECONDS));CompletableFuture cf2 = cf.exceptionally(throwable -> "canceled message");assertTrue("Was not canceled", cf.cancel(true));assertTrue("Was not completed exceptionally", cf.isCompletedExceptionally());assertEquals("canceled message", cf2.join());
}