TCP：如何从Android客户端中的Vb.net服务器获取响应？

Question

This is not a possible duplicate. No answer on this site adequately answers or solves my issue.

I am trying to connect to a VB.NET server via TCP socket and get response in Android application. The response is always null as string or -1 as bytes.

I have to connect and get a response for multiple platforms but for the moment I just want to focus on the Android app. Maybe if I figure it out, it will be easier to move forward to other platforms.

I do not have access to edit any code in the VB.NET live server. There system is pretty old and has been only sending responses to other Windows clients up until now.

Here is my Android client. It is inside a background task which is called from the mainActivity. The below command string should return coordinates in the form of a string from the server. Nothing is returned.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintStream;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;

public class Client {


    public static void sendMessage() throws IOException, InterruptedException {

        Socket socket = null;
        String host = "";
        int port = ;
        PrintStream stream = null;
        String command="";

        try {
            Socket s = new Socket(host,port);
            System.out.println("Socket created");

            //outgoing stream redirect to socket
            OutputStream out = s.getOutputStream();

            PrintWriter output = new PrintWriter(out);
            output.println(command);
            output.flush();
            System.out.println("command sent");
            BufferedReader input = new BufferedReader(new InputStreamReader(s.getInputStream()));

            //read line(s)
            System.out.println("Getting response:");
            String st = input.readLine();
            System.out.println("Response : " + st);
            //Close connection
            s.close();
        }

        catch (UnknownHostException e) {
            System.out.println("Don't know about host : " + host);
            e.printStackTrace();
            System.exit(1);
        }

        catch (IOException e) {
            System.out.println("Couldn't get I/O for the connection to : " + host);
            e.printStackTrace();
            System.exit(1);
        }

    }
}

A developer also sent me a test client in VB which connects, sends and recieves without problem

Here is a class of a VB:NET Dummy server project the developer has sent me to see how the live server is setup code-wise. I can see it gets the string as unicode but I am not confident in VB to know where my Java code is going wrong.

When I open the project and start the server on localhost I cant connect to it from the java client anyway. Then I have written another client in PHP, same problem, connection established but no response. I downloaded a socket tester software but it also can connect but does not get a response.

Option Explicit On
Imports System
Imports System.Net
Imports System.Net.Sockets
Imports System.Text
Imports Microsoft.VisualBasic

Imports System.Net.Dns
Imports System.Text.UnicodeEncoding
Imports System.Threading
Imports System.Data.SqlClient



Public Enum glenConnectionType
    ConstantConnection = 1
    ConnectOnDemand = 2
    AsyncConnection = 3
End Enum

Public Class clsDynaListner
    Public tcpServer As Socket
    Public tcpClient As Socket
    Public tcpPort As Integer
    Public tcpBilnr As Integer ' was shared SHOULD PROB BE INITIALISED TO 0
    Public ThreadClient As Thread
    Public LastKontakt As Date = Nothing
    Public ConActive As Boolean = False
    Private tcpClientEndPoint As System.Net.IPEndPoint
    Private bCommandLength(15), bReplyLength(15) As Byte
    Private iCommandLength, iReplyLength As Integer
    Private sReplyLength As String
    Private sCommand, sReply As String
    Private theCommandBytes() As Byte
    Private theReplyBytes() As Byte
    Private Const AsyncMaxBytes As Integer = 600000 '1024
    Public Shared AsyncData As String = Nothing

    Public Sub New(ByVal currentTCPPort As Integer, ByVal theConnectionType As glenConnectionType)
        tcpPort = currentTCPPort
        tcpClientEndPoint = New System.Net.IPEndPoint(System.Net.IPAddress.Any, tcpPort)

        'Select Case theConnectionType
        '    Case glenConnectionType.ConstantConnection
        '        ThreadClient = New Threading.Thread(AddressOf ListenForConstantConnection)
        '    Case glenConnectionType.ConnectOnDemand

        ThreadClient = New Threading.Thread(AddressOf ListenForConnectOnDemand)

        '    Case glenConnectionType.AsyncConnection
        'ThreadClient = New Threading.Thread(AddressOf ListenForAsyncConnection)
        'End Select

        ThreadClient.Start()
    End Sub


Private Sub ListenForConnectOnDemand()
        While (True)
            Try
                tcpServer = New Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)
                tcpServer.SendBufferSize = TCP_BUFFER_SIZE
                tcpServer.ReceiveBufferSize = TCP_BUFFER_SIZE
                tcpServer.Blocking = True
                tcpServer.Bind(tcpClientEndPoint)
                tcpServer.Listen(0)
                tcpClient = tcpServer.Accept

                tcpClient.SendBufferSize = TCP_BUFFER_SIZE
                tcpClient.ReceiveBufferSize = TCP_BUFFER_SIZE

                ' Find out how big the command is going to be
                tcpClient.Receive(bCommandLength)
                iCommandLength = CType(Unicode.GetString(bCommandLength), Integer)

                ' Bring that command to daddy
                Array.Resize(theCommandBytes, iCommandLength + 1)
                tcpClient.Receive(theCommandBytes)
                sCommand = Unicode.GetString(theCommandBytes)
                gInMessage = sCommand

                ' Get the reply
                sReply = "Response:"
                gOutMessage = sReply

                ' Inform the controller of the length of the reply transmission
                iReplyLength = (sReply.Length * 2) - 1
                sReplyLength = iReplyLength.ToString.PadLeft(8, "0")
                bReplyLength = Unicode.GetBytes(sReplyLength)
                tcpClient.Send(bReplyLength)

                ' Send the reply data
                Array.Resize(theReplyBytes, iReplyLength + 1)
                theReplyBytes = Unicode.GetBytes(sReply)
                tcpClient.Send(theReplyBytes)

                Array.Clear(theCommandBytes, 0, theCommandBytes.Length)
                Array.Clear(theReplyBytes, 0, theReplyBytes.Length)

                tcpClient.Close()
                tcpServer.Close()
                tcpClient = Nothing
                tcpServer = Nothing

            Catch ex1 As Exception
                Try
                    tcpClient.Close()
                    tcpServer.Close()
                    tcpClient = Nothing
                    tcpServer = Nothing
                    ' ErrMessage = "LisForContr :" & tcpPort.ToString & ex1.Message
                Catch
                End Try
            End Try
        End While

    End Sub

    Protected Overrides Sub Finalize()
        Try
            tcpServer.Close()
            ThreadClient.Abort()
        Catch
        End Try

        MyBase.Finalize()
    End Sub

End Class

I have been working with this for a while. The apps I have built are complete for PHP Web App, Android Native, and iPhone Native. The problem is only getting the response from the VB server.

Would like some help to push me in the right direction.

Also I enquired with the developers if the response has a line-break. It does not and it does seem they are willing to mess with the code as it served there purpose for many many years. So I have to find away around that.

If you need me to provide more info just ask.

Answer 1

To send to a VB.NET server from Java you must send the string as unicoded bytes. Send the length at the bytes to expect first and then send the primary data and flush. You have to know exactly what the server is expecting in order to format and encode the data accordingly in preparation to send it.

I was able to successfully fix my issue after debugging VB client and server. This client will process byte arrays, stream advanced sql commands and get the response. The update will be getting xml table data. Anyone who wants to input how to make this client better. You are welcome.

Updated my java client as follows.

/**
 * MR-TCP Java & VB.NET DataExchange Client 0.95 - Maze Runner 2015
 */

import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;

public class Client {

    public static String sendCommand(String commandString, Boolean IsID) throws IOException, InterruptedException {

        String host;
    int port;
    String command;
    byte[] commandBytes;
    String commandLength;
    byte[] cmdLengthBytes;
    Socket s;
    DataOutputStream stream;
    String dataString = "";

    host = ""; //
    port = 0; // 

    command = commandString;
    commandBytes = command.getBytes("UTF-16LE"); // returns byte[18]

    if(IsID){
        commandLength = "00000021";
        cmdLengthBytes = commandLength.getBytes("UTF-16LE"); 
    } else {
        String iCommandLength = command; // Get the command length
        cmdLengthBytes = iCommandLength.getBytes("UTF-16LE");
        int commandNewLength = cmdLengthBytes.length-1;
        String newLength = "0000" + String.valueOf(commandNewLength);
        cmdLengthBytes = newLength.getBytes("UTF-16LE");

    }

    try {
        s = new Socket(host, port); // Connect to server

        //Send the command to the server
        stream = new DataOutputStream(s.getOutputStream()); // get ready to send

        stream.write(cmdLengthBytes); // tell the server how many bytes to expect

        System.out.println("Command length sent");

        stream.write(commandBytes); // Send the command to papa...

        stream.flush(); // guaranteed sending

        System.out.println("Command sent");

        //Receive the command from the server.
        DataInputStream is = new DataInputStream(s.getInputStream()); // get ready to receive

        ByteArrayOutputStream buffer = new ByteArrayOutputStream(); // prepare to get array

        int nRead;
        byte[] data = new byte[1024];

        while ((nRead = is.read(data, 0, data.length)) != -1) { // read the array byte by byte
            buffer.write(data, 0, nRead);
        }

        byte[] dataBytes =  buffer.toByteArray(); // get complete array

        dataString = buffer.toString("UTF-16LE").substring(8); // get rid of the array length and convert to string


        stream.close(); // close the dataStream

        s.close(); // close the connection

        System.out.println("Disconnected");

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

    System.out.println("Function Complete");

    System.out.println("Server response:" + dataString);


    return dataString;

    }
}