JAVA多线程Socket通信时遇到的问题?

测试了你的代码，存在的问题如下：
1 你的ServerSocketMult类的finally{socket.close();}导致你的Mul线程刚开始处理Socket时，这个socket就已经关闭了。所以这个操作多余而且还是问题的根源，因为你已经咋Mul类处理完成后写了socket.close代码了。
2 ClientSocketMult这个类启动指定N个线程，所以对线程数的操作threadcount--;是在多个线程中的，需要对threadcount变量进行同步操作。
3 所有的异常分支，你都是空语句，所以淹没了异常导致你无法排错。其实收不到响应数据就是因为1中你关闭了Socket导致的，只要打印了异常就很容易定位的。
修正ServerSocketMult代码：

 public class ServerSocketMult {
    static final int PORT = 8080;

    public static void main(String[] args) throws IOException {
        ServerSocket s = new ServerSocket(PORT);
        System.out.println("Server Startes");
        try {
            while (true) {
                Socket socket = s.accept();
                System.out.println("Connection success!");
                new Mult(socket);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            s.close();
        }
    }
}

再修正Mult

 class Mult extends Thread {
    private Socket socket;
    private BufferedReader in;
    private PrintWriter out;

    public Mult(Socket s) throws IOException {
        socket = s;
        in = new BufferedReader(new InputStreamReader(s.getInputStream()));
        out = new PrintWriter(new OutputStreamWriter(s.getOutputStream()), true);
        start();
    }

    public void run() {
        System.out.println("print received ...");
        try {
            String str = in.readLine();
            StringBuffer buffer = new StringBuffer();
            //读取Socket的请求数据，并回应
            while (str!=null&&!str.equals("END")) {
                buffer.append(str);
                str = in.readLine();
            }
            String value = buffer.toString();
            out.println(value);
            System.out.println("Receiving and echoing:" + value);
            System.out.println("closing...");
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                socket.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

修正ClientSocketMultThread

 class ClientSocketMultThread extends Thread {
    private Socket socket;
    private BufferedReader in;
    private PrintWriter out;
    private static int counter = 0;
    private int id = counter++;
    private static int threadcount = 0;

    public synchronized static int threadCount() {
        return threadcount;
    }

    public synchronized static void addThreadCount(){
        threadcount++;
    }

    public synchronized static void descThreadCount(){
        threadcount--;
    }

    public ClientSocketMultThread(InetAddress addr) {
        System.out.println("Making client" + id);
        threadcount++;
        try {
            socket = new Socket(addr, ServerSocketMult.PORT);
        } catch (IOException e) {
        }
        try {
            in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())), true);
            start();
        } catch (IOException e) {
            try {
                socket.close();
            } catch (IOException e2) {
            }
        }

    }

    public void run() {
        try {
            for (int i = 0; i < 25; i++) {
                out.println("Client" + id + i);
            }
            out.println("END");

            String str = in.readLine();
            System.out.println("getResponse:"+str);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                socket.close();
            } catch (IOException e) {
            }

            descThreadCount();
        }
    }
}

测试Ok，就能收到正确的请求及回复信息。