在java中使用非阻塞I/O发送消息(NIO API)

I'm writing a server/client program that clients send text message to server.I have used non-blocking I/O (NIO API) but messages on the server do not display correctly.this is my code on server:

private JTextArea displayArea;
private int numBytes;
private ByteBuffer buffer;
/*...
some code is here
...*/
displayArea = new JTextArea();
add(new JScrollPane(displayArea), BorderLayout.CENTER);
setSize(400, 500);
setVisible(true);
/*...
some code is here
...*/
buffer = ByteBuffer.allocate(20);
buffer.clear();
displayArea.append("reading data...");
do{
   numBytes = socketChannel.read(buffer);
}while(numBytes == -1);
displayArea.append("
Data read.");
buffer.flip();
int usedBytes = buffer.position();
byte[] bufferArray = buffer.array();
String message = new String(bufferArray, 0, usedBytes);
displayArea.append("
"+message);

这是一段客户端代码:

byte[] byteData = message.getBytes();
buffer.put(byteData);
socketChannel.write(buffer);
buffer.clear();

在客户端向服务器发送消息的运行时，会显示空格字符或一条消息.