Java erlang 套接字通信

Question

我在通过线程读取和写入套接字时遇到问题。服务器使用 erlang，客户端使用 Java。我的做法是这样的：

PrintWriter printer = new PrintWriter(socket.getOutputStream(), true);
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
TransmitterTwo trans = new TransmitterTwo(socket);
trans.start(); // Gets the message from socket

TransmitterTwo 类：

public class TransmitterTwo extends Thread {
Socket socket;
String message;

TransmitterTwo(Socket socket) {
  this.socket = socket;
}

public String getMessageFromSocket() {
  return message;
}

public void run() {
  try {
    String response = null;
    BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
  while ((response = reader.readLine()) != null) {
    System.out.println("Server response: "+ response);
    this.message = response;
  }

  socket.close();
} catch(IOException e) {
  e.printStackTrace();
}
}
}

问题出在主客户端类中。这个想法是：从控制台接收输入，将其发送到套接字，服务器处理逻辑并通过套接字发送响应。然后在这个客户端上，我检查响应并做我需要做的任何事情。这是一个注册流程，我注册，我收到“register_ok”，然后我登录……等等。我遇到问题的循环部分是这样的：

while(true) {
  String readerInput = reader.readLine(); // Read from console
  printer.println(readerInput.trim()); // Sends it to the socket

  while(trans.message == null);
  socketMessage = trans.message;

这是正确的方法吗？问题是'socketmessage'打印了之前收到的消息，就像..落后了一步，显然这是与线程相关的，但我无法弄清楚问题......帮助？谢谢

Answer 1

您当前的方法不是最理想的，因为您正在浪费主线程旋转等待该变量进行更新。由于内存可见性在 java 中的工作方式，它可能看起来永远不会被更新（即使它实际上是），或者当您访问该变量时您可能会得到陈旧的值。更健壮的方法是使用 java 中的一些内置集合在线程之间传递消息：

public static void main(String[] args) {
    // This queue will be the link between the threads where 
    // they can pass messages to each other
    BlockingQueue<String> messages = new LinkedBlockingQueue<>();

    PrintWriter printer = new PrintWriter(socket.getOutputStream(), true);
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    TransmitterTwo trans = new TransmitterTwo(socket, queue);
    trans.start(); // Gets the message from socket

    ...

    while(true) {
          String readerInput = reader.readLine(); // Read from console
          printer.println(readerInput.trim()); // Sends it to the socket

          // Wait for the other thread to push a message in to the queue.
          String recv = messages.take();
    }
}

public class TransmitterTwo extends Thread {
    final Socket socket;
    final BlockingQueue<String> queue;

    TransmitterTwo(Socket socket, BlockingQueue<String> queue) {
        this.socket = socket;
        this.queue = queue;
    }

    public void run() {
        try {
            String response = null;
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            while ((response = reader.readLine()) != null) {
                System.out.println("Server response: " + response);
                // Add the response from the server to the queue
                queue.add(response);
            }

            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

这种方法永远不会得到陈旧的值，并且主线程上的 take() 操作将阻塞，直到服务器有一些响应。