java Socketchannel始终为空

Question

我正在尝试向SMTP服务器发送EHLO命令。连接成功，但我似乎无法从中读取任何数据：

 ByteBuffer byteBuffer = null;
    try {
        socketChannel = SocketChannel.open();
        socketChannel.connect(new InetSocketAddress("host", 25));
        socketChannel.configureBlocking(true);
        byteBuffer = ByteBuffer.allocateDirect(4 * 1024);

    } catch (Exception e) {
        e.printStackTrace();
    }

    try {
        byteBuffer.clear();
        socketChannel.write(byteBuffer.put(SMTP_EHLO.getBytes()));
        byteBuffer.flip();
        socketChannel.read(byteBuffer);
        byteBuffer.get(subStringBytes);
        String ss = new String(subStringBytes);
        System.out.println(byteBuffer);

    } catch (IOException e) {
        e.printStackTrace();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

print语句的输出总是\u000（null）

Answer 1

    socketChannel.write(byteBuffer.put(SMTP_EHLO.getBytes()));

您可以put将SMTP\u EHLO写入缓冲区，但必须flip()在写入缓冲区之前。否则，将不向套接字通道写入任何内容。从SocketChannelJavadoc：

An attempt is made to write up to r bytes to the channel, where r is the number of bytes remaining in the buffer, that is, src.remaining(), at the moment this method is invoked.

从Buffer#remaining()开始：

public final int remaining()

Returns the number of elements between the current position and the limit.

因此，在byteBuffer.put(...)之后，当前位置==限制