Java客户端/服务器不通过Socket / ServerSocket进行通信

Question

I have a simple socket / serversocket example that I'm trying to get running, but both the client and the server hang when their BufferedReader s try to read. Here is the code for each:

SERVER

package picturePerfect;
--imports--

public class PictureServer implements Runnable{
static ServerSocket serverSocket;

public static void main(String[] args) throws IOException {
    serverSocket = new ServerSocket(2342);
    Thread firstSessionThread = new Thread(new PictureServer());
    firstSessionThread.start();
}

@Override
public void run() {
    Socket socket = null;
    try {
        socket = serverSocket.accept();
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        String clientRequest = bufferedReader.readLine();
        System.out.println(clientRequest);
        PrintWriter printWriter = new PrintWriter(socket.getOutputStream(), true);
        printWriter.println("Sent from server!");
    } catch (IOException e) {
        e.printStackTrace();
    }       
}

}

CLIENT

package picturePerfect;
--imports--

public class PictureClient {
    public static void main(String[] args) throws UnknownHostException, IOException, InterruptedException {
        Socket socket = new Socket("localhost", 2342);
        PrintWriter printWriter = new PrintWriter(socket.getOutputStream(), true);
        printWriter.write("Sent from client!");
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        String response = bufferedReader.readLine();
        System.out.println(response);
        socket.close();
    }
}

This is the barest I could simplify my code to. I have a sample program that I've been following, which seems nearly exactly the same. This is the sample server and client (that does work):

SAMPLE SERVER

--imports--

public class Server implements Runnable{
    static ServerSocket ss;

    public static void main(String[] args) throws IOException {
        ss = new ServerSocket(3142);
        Thread thread = new Thread(new Server());
        thread.start();
    }

    @Override
    public void run() {
        while ( true ) {
            Socket s = null;

            try {
                s = ss.accept();
                BufferedReader br = new BufferedReader(new InputStreamReader(s.getInputStream()));
                String operands = br.readLine();
                System.out.println(operands + " was received");
                PrintWriter pw = new PrintWriter(s.getOutputStream(), true);
                pw.println(operands + " right back!");
            } catch ( IOException e ) {
                e.printStackTrace();
            }
        }
    }
}

SAMPLE CLIENT

--imports--

public class Server implements Runnable{
    static ServerSocket ss;


    public static void main(String[] args) throws IOException {
        ss = new ServerSocket(3142);
        Thread thread = new Thread(new Server());
        thread.start();
    }

    @Override
    public void run() {
        while ( true ) {
            Socket s = null;

            try {
                s = ss.accept();
                BufferedReader br = new BufferedReader(new InputStreamReader(s.getInputStream()));
                String operands = br.readLine();
                System.out.println(operands + " was received");
                PrintWriter pw = new PrintWriter(s.getOutputStream(), true);
                pw.println(operands + " right back!");
            } catch ( IOException e ) {
                e.printStackTrace();
            }
        }
    }
}

I've tried putting the while loop into my server and moving my client and server to the default package, but neither helped. I also tried using read() instead of readLine() , and ending the printWriter's lines with \\r\\n , but was just as unsuccessful there.

Why does my code hang on readLine() , especially when the sample code doesn't?

Answer 1

What readline() does is wait until it sees a new line character until it returns, hence readLine().

In your client, you do not write a new line. You use:

printWriter.write("Sent from client!");

Instead, write a newline character into the stream using println,

printWriter.println("Sent from client!");

Answer 2

Server expects to read line, so you need to add line separators after your massage. To do this instead of

printWriter.write("Sent from client!");

in client use

printWriter.println("Sent from client!");
//          ^^^^^^^

or

printWriter.write("Sent from client!"+System.lineSeparator());
printWriter.flush();

You will need to flush yourself because autoflush in PrintWriter works only for println , printf , or format , not for write method