如何调整这3个线程的控制流？

Question

// Task 1 -- the main thread        SimilarityResponse w2vResponse = questionClassifier.compute(questionInfo);        // Task 2        String sku = questionInfo.getSku();        String question = questionInfo.getQuestion();        Callable<ResponseList> dssmTask = () -> this.dssmCompute(sku, question);        Future<ResponseList> dssmService = executorService.submit(dssmTask);        ResponseList dssmResponse;        try {            LOGGER.info("start dssm ... {} ", question);            dssmResponse = dssmService.get(Parameters.getParserTimeLimit(), TimeUnit.MILLISECONDS);            LOGGER.info("dssmResponse ... {} ", dssmResponse);        } catch (ExecutionException | InterruptedException e) {            LOGGER.warn("ExecutionException | InterruptedException");            e.printStackTrace();        } catch (TimeoutException te) {            dssmService.cancel(true);            LOGGER.warn("DSSM time out for {} {}", sku, question);        }        // Task 3        Callable<ResponseList> stsTask = () -> this.stsCompute(sku, question);        Future<ResponseList> stsService = executorService.submit(stsTask);        ResponseList stsResponse;        try {            LOGGER.info("start sts ... {} ", question);            stsResponse = stsService.get(Parameters.getParserTimeLimit(), TimeUnit.MILLISECONDS);            LOGGER.info("stsResponse ... {} ", stsResponse);        } catch (ExecutionException | InterruptedException e) {            LOGGER.warn("ExecutionException | InterruptedException");            e.printStackTrace();        } catch (TimeoutException te) {            stsService.cancel(true);            LOGGER.warn("STS time out for {} {}", sku, question);        }        // Last step == do something for above        SimilarityResponse ensemble = new SimilarityResponse();        return ensemble;在执行最后一步之前，如何确保任务 1-3 已经完成？当前代码似乎是先完成Task 1，然后直接返回。

Answer 1

您应该使用 CountDownLatch。在您的主线程中创建它的实例并将此实例传递给您的任务（Callables）。然后当任务完成时调用latch.countDown()。在代码的最后一步调用 latch.await() 以等待每个任务完成。它看起来像这样（如果你的 callables 是作为 lambdas 创建的）：

final CountDownLatch latch = new CountDownLatch(3);

for(int x = 0; x < 3; x++) {

    service.submit( () -> {

        // do something

        latch.countDown();

    });

}

// in the end wait for tasks to finish

latch.await();