服务端：

public class Test1 {
    public static void main(String[] args) {
        ServerRead serverRead = new ServerRead();
        serverRead.start();
        while (true){
            ServerSocket serverSocket = null;
            Socket socket = null;
            DataInputStream in = null;
            DataOutputStream out = null;
            try {
                serverSocket = new ServerSocket(6666);
                socket = serverSocket.accept();
                //阻塞
                in = new DataInputStream(socket.getInputStream());
                System.out.println(in.readUTF());
                out = new DataOutputStream(socket.getOutputStream());
                //阻塞
                Scanner scanner = new Scanner(System.in);
                String s = scanner.nextLine();
                out.writeUTF("server:"+s);
            } catch (IOException e) {
                e.printStackTrace();
            }finally {
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    in.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    serverSocket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

public class Test{
    public static void main(String args[]){
        while (true){
            Socket socket = null;
            DataInputStream in = null;
            DataOutputStream out = null;
            try {
                socket = new Socket("127.0.0.1",6666);
                out = new DataOutputStream(socket.getOutputStream());
                Scanner scanner = new Scanner(System.in);
                //阻塞
                String s = scanner.nextLine();
                out.writeUTF("client:"+s);
                //阻塞
                in = new DataInputStream(socket.getInputStream());
                System.out.println(in.readUTF());
            } catch (IOException e) {
                e.printStackTrace();
            }finally {
                try {
                    in.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

问题：当任意一端发送两个消息以上，第一个问题是第二条（包括第二条）以后的消息不会立即发送到另外一端；第二个问题是从第三条开始的消息消失了。由于每个程序都是单线程的，而有的代码会造成线程阻塞，所以导致后面的代码执行不了。
解决方法：使用多线程
客户端

public class Test{
    public static void main(String args[]){
        while (true){
            Socket socket = null;
            DataInputStream in = null;
            DataOutputStream out = null;
            try {
                socket = new Socket("127.0.0.1",6666);
                out = new DataOutputStream(socket.getOutputStream());
                Scanner scanner = new Scanner(System.in);
                //阻塞
                in = new DataInputStream(socket.getInputStream());
                ClientRead clientRead = new ClientRead(in);
                clientRead.start();
//                System.out.println(in.readUTF());
                //阻塞
                while (scanner.hasNext()){
                    String s = scanner.nextLine();
                    out.writeUTF("client:"+s);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }finally {
                try {
                    in.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

class ClientRead extends Thread{
    DataInputStream in;
    public ClientRead(DataInputStream in){
            this.in = in;
        }
        public void run(){
            while (true){
                try {
                    System.out.printf("%30s\n",in.readUTF());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

服务器端

public class Test1 {
    public static void main(String[] args) {
            ServerSocket serverSocket = null;
            Socket socket = null;
            DataInputStream in = null;
            DataOutputStream out = null;
            try {
                serverSocket = new ServerSocket(6666);
                socket = serverSocket.accept();
                //阻塞
                in = new DataInputStream(socket.getInputStream());
                ServerRead serverRead = new ServerRead(in);
                serverRead.start();
//                System.out.println(in.readUTF());
                out = new DataOutputStream(socket.getOutputStream());
                //阻塞
                Scanner scanner = new Scanner(System.in);
                while (scanner.hasNext()){
                    String s = scanner.nextLine();
                    out.writeUTF("server:"+s);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }finally {
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    in.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    serverSocket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
    }
}

class ServerRead extends Thread{
    DataInputStream in = null;
    public ServerRead(DataInputStream in){
        this.in = in;
    }
    public void run(){
        while (true){
            try {
                System.out.printf("%30s\n",in.readUTF());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}