java - Java 中的 TCP 服务器和客户端

Question

我尝试使用 Java 构建 TCP 服务器和客户端。它们可以连接，运行良好，但我有一个错误。

这是服务器端:

package com.company;
import java.io.*;
import java.net.*;

public class Main {

    public static void main (String[] args) throws IOException {

        System.out.println("The server is ready");
        ServerSocket serverSocket = new ServerSocket (1234);
        Socket clientSocket = serverSocket.accept ();

        BufferedReader in = new BufferedReader (new InputStreamReader (clientSocket.getInputStream ()));
        PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
        String message, modifiedMessage;
        message = in.readLine ();
        System.out.print("The received message from client: " + message);
        modifiedMessage = message.toUpperCase();
        out.print(modifiedMessage);
        System.out.println ("\nModified message which is sent to client: " + modifiedMessage);
    }

}

服务器必须从客户端接收消息，然后将其转换为大写字符串。

客户端是:

package com.company;
import java.io.*;
import java.net.*;

public class Main {

    public static void main(String[] args) throws IOException {

        Socket socket = new Socket("127.0.0.1", 1234);
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
        System.out.println("Enter a lowercase sentence: ");
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
       BufferedReader in = new BufferedReader((new InputStreamReader(socket.getInputStream())));

        String messageSent = reader.readLine();
        System.out.println("The message sent is: " + messageSent);
        out.println(messageSent);


        String messageReceived = in.readLine();
        System.out.println("The modified message is: " + messageReceived);
    }
}

我希望客户端能够打印小写句子和接收到的(修改后的)大写句子。问题是，当我输入一个简单的单词时，你好，我的客户端只会打印原始字符串，而不是修改后的字符串。

服务器的输出是:

The received message from client: hello 
The modified message sent to the client is: HELLO

但是客户端的输出是:

The message sent is: hello 
The modified message is: null

我知道服务器能够将字符串转换为大写版本并连接到我的客户端。为什么我的客户端不打印收到的消息？它真的没有收到吗？

Answer 1

您需要刷新消息。 PrintWriter 在 println 中调用 flush，而 print 不会。

在服务器端你需要更改为:out.println(modifiedMessage);代替out.print(modifiedMessage);