java - 如何使用 Java 保持 TCP 连接？

Question

我想做的是:

1) 在我的本地机器上创建一个监听特定端口号的 TCP 套接字。 (服务器)

2) 创建另一个 TCP 套接字，它将连接到我创建的第一个 TCP 套接字。(客户端)

3) 通过客户端向服务器发送一些数据。

4) 客户端从服务器接收数据。

5) 不要关闭连接..稍等片刻..

6) 从服务器接收更多数据。

这是我的代码:

public class MyTCPServer {

    public static void main(String argv[]) throws Exception {
        String clientSentence;
        String capitalizedSentence;
        ServerSocket welcomeSocket = new ServerSocket(6789);
        while (true) {
            Socket connectionSocket = welcomeSocket.accept();
            BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
            DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
            clientSentence = inFromClient.readLine();
            System.out.println("Received: " + clientSentence);
            capitalizedSentence = clientSentence.toUpperCase() + '\n';
            outToClient.writeBytes(capitalizedSentence);
            outToClient.writeBytes(String.valueOf(System.currentTimeMillis()));
            Thread.sleep(1000);
            outToClient.writeBytes(String.valueOf(System.currentTimeMillis()));
        }
    }

}

所以这个 TCP 套接字监听端口 6789。请注意它从客户端发回接收到的字符串，将其大写并返回。然后Server发送currentTimeMillis，等待1000毫秒，再次发送timeMillis。

这是我的客户:

public class MyTCPClient {

    public static void main(String[] args) throws Exception {
        String sentence;
        String modifiedSentence;
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
        Socket clientSocket = new Socket("localhost", 6789);
        DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
        BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        sentence = inFromUser.readLine();
        outToServer.writeBytes(sentence + '\n');
        modifiedSentence = inFromServer.readLine();
        System.out.println("FROM SERVER: " + modifiedSentence);
    }

}  

然而，当我测试我的代码时，我得到:

Received: asdf
Exception in thread "main" java.net.SocketException: Connection reset by peer: socket write error
    at java.net.SocketOutputStream.socketWrite0(Native Method)
    at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:92)
    at java.net.SocketOutputStream.write(SocketOutputStream.java:115)
    at java.io.DataOutputStream.writeBytes(DataOutputStream.java:259)
    at MyTCPServer.main(MyTCPServer.java:28)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

如何在客户端保持连接？此外，我还希望能够从客户端发送更多文本。

Answer 1

最简单的方法是在客户端代码中放置一个循环。像这样:

   while ((modifiedSentence = inFromServer.readLine()) != null) {
       System.out.println("FROM SERVER: " + modifiedSentence);
       sentence = inFromUser.readLine();
       outToServer.writeBytes(sentence + '\n');
   }

readLine() 方法在等待来自服务器的消息时阻塞，因此客户端不会退出。

请参阅下面有关如何编写套接字服务器和客户端的简单教程:

http://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html