启动2个线程。dataListUpdateThread将数字2加到List中。processFlowThread将相同List中的值相加，并将相加后的列表输出到控制台。代码如下：

import akka.NotUsed;
import akka.actor.ActorSystem;
import akka.stream.javadsl.Sink;
import akka.stream.javadsl.Source;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.ExecutionException;

import static java.lang.Thread.sleep;

public class SourceExample {

    private final static ActorSystem system = ActorSystem.create("SourceExample");

    private static void delayOneSecond() {
        try {
            sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private static void printValue(CompletableFuture<Integer> integerCompletableFuture) {
        try {
            System.out.println("Sum is " + integerCompletableFuture.get().intValue());
        } catch (ExecutionException | InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {

        final List dataList = new ArrayList<Integer>();
        final Thread dataListUpdateThread = new Thread(() -> {
            while (true) {
                dataList.add(2);
                System.out.println(dataList);
                delayOneSecond();
            }
        });
        dataListUpdateThread.start();

        final Thread processFlowThread = new Thread(() -> {
            while (true) {
                final Source<Integer, NotUsed> source = Source.from(dataList);

                final Sink<Integer, CompletionStage<Integer>> sink =
                        Sink.fold(0, (agg, next) -> agg + next);

                final CompletionStage<Integer> sum = source.runWith(sink, system);

                printValue(sum.toCompletableFuture());

                delayOneSecond();
            }
        });

        processFlowThread.start();
    }
}

我试着创建了一个最简单的例子来回答这个问题。dataListUpdateThread可以从REST服务或Kafka主题填充List，而不仅仅是将值2添加到List。如果不使用Java线程，该如何实现这个场景？换句话说，如何将dataList共享到Akka Stream进行处理？