如何使用Java使TCP连接保持活动状态？

Question

我想做的是：

1）创建一个侦听本地计算机中特定端口号的TCP套接字。（伺服器）

2）创建另一个将与我创建的第一个TCP套接字连接的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。

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()));
        }
    }

}

因此，此TCP套接字侦听端口6789。请注意，它从客户端发送回接收到的字符串，将其大写并返回。然后服务器发送currentTimeMillis，等待1000毫秒，然后再次发送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);
    }

}  

但是，当我测试代码时，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)

如何在客户端保持活动连接？我也希望能够从客户端发送更多文本。

Answer 1

最简单的事情是在客户端代码中放入一个循环。像这样：

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

该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。

下面是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