子线程如何向Java中的主线程（方法）发送连续消息？

Question

我有一个HashMap的人想按如下方式相互交流：

{zidane=[rooney, rooney, rooney, scholes, rooney], rooney=[scholes, messi, scholes], scholes=[ronaldo], ronaldo=[rooney, messi, scholes], messi=[zidane]}

[这里，密钥中的每个人都有各自的线程，他们将在其中向列表中的每个人发送消息并接收响应。

public class Master {
    public Map callsMap = new HashMap<String, List>();

    public static void main(String[] args) throws InterruptedException, ExecutionException {
        Master m = new Master();
        m.readFile();
        ExecutorService executorService = Executors.newFixedThreadPool(6);
        List<Future<String>> returnFutures = new ArrayList<>();
        Iterator<Map.Entry<String, List>> it = m.callsMap.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry<String, List> pair = it.next();
            String caller = pair.getKey();
            List receiverList = pair.getValue();
            returnFutures.add(executorService.submit(new SubTasks(caller, receiverList) ));
        }

        while(executorService.awaitTermination(1, TimeUnit.SECONDS));
        for(Future<String> returnFuture: returnFutures){
            System.out.println(returnFuture.get());
        }
        executorService.shutdown();
    }
}

Master类将为HashMap中的每个键迭代创建一个线程。请注意，readFile方法只会从文本文件中读取并产生我上面提到的输入（到callsMap变量中）。对于这种情况，它并不是特别重要，因此在此省略了它。

我具有SubTasks类，该类实现Callable接口并为每次交互创建消息。此类不会创建任何其他线程，而只会在接收到的调用方和接收方列表上进行操作，如下所示：

import java.util.List;
import java.util.concurrent.Callable;

public  class SubTasks implements Callable<String> {
    private String caller;
    private List receiverList;
    private volatile String returnMessage;
    private volatile String callerMessage;
    private volatile String str = "";

    SubTasks(String s, List l){
        caller = s;
        receiverList = l;
    }

    @Override
    public String call (){
        for (int i = 0; i < receiverList.size(); i++) {
            callerMessage = receiverList.get(i)+" received intro message from "+caller +" ["+System.currentTimeMillis()+"]";
            //System.out.println(callerMessage);
            returnMessage = caller+" received reply message from "+receiverList.get(i) +" ["+System.currentTimeMillis()+"]";
            //System.out.println(returnMessage);
            str = str + callerMessage +"\n"+ returnMessage +"\n";
        }
        return null;
    }
}

如果直接在子线程中打印，由于消息以随机顺序打印，所以我得到了预期的结果：

rooney received intro message from zidane [1592165414388]
ronaldo received intro message from scholes [1592165414388]
scholes received intro message from rooney [1592165414388]
rooney received intro message from ronaldo [1592165414389]
zidane received intro message from messi [1592165414389]
scholes received reply message from ronaldo [1592165414434]
zidane received reply message from rooney [1592165414434]
...
...

但是我希望这些消息在main方法而不是子线程中打印。我试图通过连接returnMessage和callerMessage来返回消息，但是由于我在每个线程的末尾都返回了消息，因此它们似乎是按顺序打印的（每个对的回叫）如下：

rooney received intro message from zidane [1592165584566]
zidane received reply message from rooney [1592165584614]
rooney received intro message from zidane [1592165584632]
zidane received reply message from rooney [1592165584632]
rooney received intro message from zidane [1592165584632]
zidane received reply message from rooney [1592165584632]
...
...

所以，我的问题是如何在创建主线程后立即在主方法中打印每个callerMessage和returnMessage而不是子线程？

Answer 1

这似乎是生产者－消费者问题的经典示例，其中您的SubTasks是生产者，而Master是单个消费者。可以使用底层Java并发构造以多种不同方式解决该问题，但最简单的解决方法可能是使用某种线程安全的中介队列。您可以先查看Producer/Consumer threads using a Queue。