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POJava - sending packets via UDP
 

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  1. POJava - sending packets via UDP
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    <p>In my android app, I am sending messages via UDP. I followed <a href="https://stackoverflow.com/questions/3997459/send-and-receive-serialize-object-on-udp-in-java">this</a> and <a href="http://www.javaworld.com/javaworld/javatips/jw-javatip40.html?page=2" rel="nofollow noreferrer">this</a>. However, I am not able to send the initial packet from the client to the server. Please let me know if I am doing something wrong in my code below:</p> <pre><code>class serverCommunicationThread implements Runnable { public void run() { try { serverSocket = new DatagramSocket(STATICPORT); ServerReceiveMessageThread rcvMsgThread = new ServerReceiveMessageThread(serverSocket); new Thread(rcvMsgThread).start(); } catch (IOException e) { e.printStackTrace(); } } </code></pre> <p>This is the ServerReceiveMessageThread class:</p> <pre><code>class ServerReceiveMessageThread implements Runnable { DatagramSocket clientSocket; byte[] buffer = new byte[108]; DatagramPacket packet = new DatagramPacket(buffer, buffer.length); ServerReceiveMessageThread(DatagramSocket clientSocket) { this.clientSocket = clientSocket; } @Override public void run() { int counter = 0; MsgStruct recvMsgStruct = null; try { ByteArrayOutputStream baosServer = new ByteArrayOutputStream(); serializeServer = new ObjectOutputStream(baosServer); clientSocket.receive(packet); ByteArrayInputStream baosRecv = new ByteArrayInputStream(buffer); deserializeServer = new ObjectInputStream(baosRecv); while(true) { try { recvMsgStruct = (MsgStruct) deserializeServer.readObject(); } catch (ClassNotFoundException e1) { e1.printStackTrace(); } boolean retVal = serverSendMessage(clientSocket, recvMsgStruct, baosServer.toByteArray()); } catch (IOException e2) { e2.printStackTrace(); } } } </code></pre> <p>The serverSendMessage method:</p> <pre><code>public boolean serverSendMessage(DatagramSocket clientSocket, MsgStruct msgStruct, byte[] buf) { MsgStruct recvMsgStruct = new MsgStruct(); recvMsgStruct.pingPong = true; recvMsgStruct.msgId = msgStruct.msgId; recvMsgStruct.bufferMsg = msgStruct.bufferMsg; try { serializeServer.writeObject(recvMsgStruct); serializeServer.flush(); DatagramPacket packet = new DatagramPacket(buf, buf.length, InetAddress.getByName(mobileIP), STATICPORT); clientSocket.send(packet); } catch (IOException e) { e.printStackTrace(); } return true; } </code></pre> <p>The code for the client send/receive is similar to that of the server. I get a NullPointerException here. </p> <pre><code>serializeClient.writeObject(mobileSendMsgStruct); </code></pre> <hr> <pre><code>public boolean mobileSendMessage(int msgId) { MsgStruct mobileSendMsgStruct = new MsgStruct(); mobileSendMsgStruct.bufferMsg = new byte[100]; mobileSendMsgStruct.pingPong = false; mobileSendMsgStruct.msgId = msgId; rand.nextBytes(mobileSendMsgStruct.bufferMsg); try { sendTime = System.nanoTime(); serializeClient.writeObject(mobileSendMsgStruct); /*I get a NullPointerException here*/ serializeClient.flush(); byte[] sendBuf = baosSend.toByteArray(); DatagramPacket packet = new DatagramPacket(sendBuf, sendBuf.length, InetAddress.getByName(staticIP), STATICPORT); mobileSocket.send(packet); outPing.write(Long.toString(sendTime) + ", " + String.valueOf(mobileSendMsgStruct.msgId) + ", " + String.valueOf(mobileSendMsgStruct.bufferMsg) + " | "); } catch (IOException e1) { e1.printStackTrace(); return false; } return true; } </code></pre>
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    1. PTQuestion
    1. USCommunity
    1. USSarvavyapi
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    1. COWhere do you initialize serializeClient?
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      1. POJava - sending packets via UDP
      1. USAmir T
    2. COI was initializing it in a thread similar to ServerReceiveMessageThread. The problem was that the initialization did not get over while the client started sending data. I found out that problem and am able to send data from client. But now, the server is blocking at this line - recvMsgStruct = (MsgStruct) deserializeServer.readObject();
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      1. POJava - sending packets via UDP
      1. USSarvavyapi
    3. COSee what deserializeServer.available() returns. It should tell you how many btyes are available without blocking. I would expect that the server would wait for data, so perhaps your packets aren't getting across (in which case you might look into a packet capture (wireshark) to debug the network and return to java land once you're sure it's coming across.
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      1. POJava - sending packets via UDP
      1. USAmir T
 

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