同一个java程序在mac和win下运行结果不同？

[sinat_38170563]

用java写的简单的聊天室，分为客户端和服务端，同样的代码，客户端输入"exit"退出后，在mac下服务端无限循环输出读到的数据null，而在win下则正常只输出一句客户端下线，这是为什么？

客户端代码：
public class ChatroomClient {
private Socket socket;
public ChatroomClient(){
}
public void start() throws IOException{
socket=new Socket("localhost",8088);
System.out.println("[客户端]：和服务器建立连接");
//客户端向服务器发送消息
//获得输出流
OutputStream os=socket.getOutputStream();
PrintWriter out=new PrintWriter(os);
Scanner sc=new Scanner(http://System.in);
while(true){
String data=sc.next();
if("exit".equals(data)){
out.println("exit");
break;
}
out.println(data);
out.flush();
}
}
public static void main(String[] args) throws IOException{
ChatroomClient client=new ChatroomClient();
client.start();
}
}

服务端代码：
public class ChatroomServer {
private ServerSocket server;
private SimpleDateFormat sdf;

public ChatroomServer() throws IOException {
try {
sdf = new SimpleDateFormat("HH:ss:MM");
server = new ServerSocket(8088);
System.out.println("[服务端]" + printTime() + "：服务器启动成功...");
} catch (IOException e) {
throw e;
}
}

public String printTime() {
return sdf.format(new Date());
}

public void start() throws IOException {
while (true) {
// 监听客户端的连接
try {
Socket socket = server.accept();
System.out.println("[服务端]：与客户端建立连接");
System.out.println("[服务端]：" + socket);
// 启动一个线程维护一个Socket对象
new ClientHandler(socket).start();
} catch (IOException e) {
throw e;
}
}
}

private class ClientHandler extends Thread {
private Socket socket;
public ClientHandler(Socket socket) {
this.socket = socket;
}

public void run() {
try {
//服务器接受客户端的消息 需要获得输入流
InputStream in = socket.getInputStream();
BufferedReader br = new BufferedReader(new InputStreamReader(in));
while (true) {
String datas = br.readLine();
if ("exit".equals(datas)) {
break;
}
System.out.println("[读取的数据]:" + datas + " [" + printTime() + "]");
}
} catch (IOException e) {
System.out.println("[服务端]：客户端下线" + "[" + printTime() + "]");
}
}
}

public static void main(String[] args) throws IOException {
ChatroomServer server = new ChatroomServer();
server.start();
}
}

运行结果：
客户端输入信息服务端可正常接收，但当客户端输入"exit"退出时，客户端退出，服务端则无限循环输出"[读取数据]：null"。Windows下则正常输出"[服务端]：客户端下线"。感觉是因为某些原因在mac的系统下服务端没能正确接收到"exit"导致while循环没能退出？请问如何解决？