java - 阻塞模式下SocketChannel读取返回0

Question

我有简单的客户端和服务器，客户端基于NIO，而服务器是一个简单的老式程序。

我正在以阻塞的默认模式使用客户端。在程序中，我尝试从客户端写入，服务器读取它。然后服务器回复，客户端读取。

我能够毫无问题地写入服务器，但在客户端中从服务器读取数据被证明是有问题的。由于它处于阻塞模式，根据文档，我希望它永远不会返回 0。但事实并非如此，我总是看到 client_channel.read 的返回值为 0。

********************************服务器**************** ******************************

class MyBlockingServer extends Thread
{
    private int M_PortNumber;
    private ServerSocket M_ServerSocket;

    MyBlockingServer(int PortNumber)
    {
        M_PortNumber = PortNumber;
        try {
            M_ServerSocket = new ServerSocket(M_PortNumber);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public void run()
    {
        int my_number = 0;
        while(true)
        {
            try {
                Socket ClientServerTuple = M_ServerSocket.accept();
                //System.out.println("Server address is "+ ClientServerTuple.getLocalAddress() + "Server Port is " + ClientServerTuple.getLocalPort());
                //System.out.println("Client address is " + ClientServerTuple.getRemoteSocketAddress() + "Client address is" + ClientServerTuple.getPort());


                DataInputStream inputStream = new DataInputStream(ClientServerTuple.getInputStream());

                byte b[] = new byte[48];
                inputStream.read(b);


                System.out.println("[SERVER]" + new String(b));


                DataOutputStream outputStream = new DataOutputStream(ClientServerTuple.getOutputStream());


                byte c[] = new byte[100];
                String output= new String("Thanks for connection, you suck tata" + " "+ my_number);

                c = output.getBytes();
                outputStream.write(c);

                my_number++;
                System.out.println("write done");

                ClientServerTuple.close();

            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    void socket_close()
    {
        try {
            M_ServerSocket.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

public class JavaBlocking
{

    public static void main(String []args)
    {
        MyBlockingServer Server = new MyBlockingServer(8000);
        try {
            Server.start();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }
}

********************************客户端**************** ******************************

public class JavaChannels 
{

    public static void main(String []args)
    {
        SocketChannel client_channel = null;


        try {
            client_channel = SocketChannel.open();

        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("[Async Client] Socket channel open");

        try {
            client_channel.connect(new InetSocketAddress("127.0.0.1",8000));

        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("[Async Client] Socket channel connected");

        ByteBuffer my_buffer = ByteBuffer.allocate(248);



        try {
            my_buffer.put("seven77".getBytes("UTF-8"));
        } catch (UnsupportedEncodingException e2) {
            // TODO Auto-generated catch block
            e2.printStackTrace();
        }

        my_buffer.flip();

        try {
            int bytes_written = client_channel.write(my_buffer);

            while(my_buffer.hasRemaining())
            {
                bytes_written = client_channel.write(my_buffer);
            }

            System.out.println("[Async Client] Wrote "+ bytes_written +" bytes");
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("[Async Client] Socket channel write finished");

        my_buffer.clear();
        my_buffer.flip();


        try {
            int read_length = client_channel.read(my_buffer);
            System.out.println("Initial read is " + read_length + " bytes");
            while(read_length !=-1)
            {
                read_length = client_channel.read(my_buffer);
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                System.out.println("Reading the buffer." +"Read "+read_length +"bytes");
            }
        } catch (IOException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }

        System.out.println("[Async Client] server says" + new String(my_buffer.array()));

        try {
            client_channel.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }


}

我从客户端看到的输出如下

Initial read is 0 bytes
Reading the buffer.Read 0bytes
Reading the buffer.Read 0bytes
Reading the buffer.Read 0bytes

Answer 1

我认为这是错误的:

    System.out.println("[Async Client] Socket channel write finished");
    my_buffer.clear();
    my_buffer.flip();

clear 通过将位置设置为零和容量限制来准备缓冲区以供读取。

但是flip然后设置了该位置的限制；即零。这意味着当您尝试读入缓冲区时，有零字节的空间。

摆脱那个flip调用。

<小时/>

As it is in blocking mode, i expect that it never returns 0 according to the documentation.

哪个文档？ javadocs对于SocketChannel.read(ByteBuffer)说:

"It is guaranteed, however, that if a channel is in blocking mode and there is at least one byte remaining in the buffer then this method will block until at least one byte is read. "

在这种情况下，突出显示的条件为 false。