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POJava Socket synchronization behavior
 

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    <p>I tried to solve the problem in many ways but without success and I have also looked for information in this forum but with same results, so here we go.</p> <p>I am actually doing a server daemon that accepts client requests and then it (the server) transfers all the files contained in a specific folder. I'm going to post the code of the sendFileData (on the server) and the receiveFileData (on the client).</p> <p>The server uses:</p> <pre><code>public static void sendFileData(File file, Socket socket) throws FileNotFoundException, IOException, SocketException { byte[] auxiliar = new byte[8192]; byte[] mybytearray = new byte[(int) file.length()]; int longitud = mybytearray.length; BufferedInputStream bis = new BufferedInputStream(new FileInputStream(file)); bis.read(mybytearray, 0, longitud); DataOutputStream os = new DataOutputStream(socket.getOutputStream()); int paquetes = longitud / 8187; int resto = longitud % 8187; int i = 0; while(i&lt;paquetes){//The length goes on the first 4 bytes and the 5th tells if there are more packets to send (8192 bytes or less). byte[] bytes = ByteBuffer.allocate(4).putInt(8187).array(); auxiliar[0] = bytes[0]; auxiliar[1] = bytes[1]; auxiliar[2] = bytes[2]; auxiliar[3] = bytes[3]; auxiliar[4] = 1; for(int j = 5; j &lt; 8192; j++){ auxiliar[j] = mybytearray[i*8187+(j-5)]; } os.write(auxiliar, 0, 8192); i+=1; } if(resto &gt; 0){ byte[] bytes = ByteBuffer.allocate(4).putInt(resto).array(); auxiliar[0] = bytes[0]; auxiliar[1] = bytes[1]; auxiliar[2] = bytes[2]; auxiliar[3] = bytes[3]; auxiliar[4] = 0; for(int j = 5; j &lt; resto+5; j++){ auxiliar[j] = mybytearray[i*8187+(j-5)]; } os.write(auxiliar, 0, resto+5); } os.flush(); } </code></pre> <p>And in the client side:</p> <pre><code>public static void receiveFileData(String nombreFichero, Socket s) throws IOException{ File monitored = new File(nombreFichero); if(monitored.exists() == false){ monitored.createNewFile(); } byte[] mybytearray; DataInputStream is = new DataInputStream(s.getInputStream()); FileOutputStream fos = new FileOutputStream(monitored); BufferedOutputStream bos = new BufferedOutputStream(fos); int bytesRead = 0; int hasNext = 1; do { bytesRead = is.readInt();//Leo longitud try { Thread.sleep(1);// HERE!!!! } catch (InterruptedException e) { } // System.out.println("Bytes read "+bytesRead ); if(bytesRead &lt;= 8187 &amp;&amp; bytesRead &gt; 0){ // System.out.println("Bytes leídos "+bytesRead ); hasNext = is.readByte();//Leo si hay más datos por enviar mybytearray = new byte[bytesRead]; is.read(mybytearray); if(monitored.exists()){ synchronized(monitored){ bos.write(mybytearray, 0, mybytearray.length); } } mybytearray = null; }else{ System.out.println("Fuera de rango "+bytesRead); } }while(hasNext == 1); bos.close(); mybytearray = null; System.out.println("Fichero recibido: "+monitored.getAbsolutePath()); } </code></pre> <p>In the receiveFileData code, if I do not put a Thread.sleep(1) or a System.out.println() or whatever who takes time to execute, I am not receiving the data in the correct way on the client, because readInt() returns a very high number randomly negative or positive (which implies Heap out of memory and other exceptions).</p> <p>Sure it's something about synchronization but I think the transfering schema between the two methods is correct (maybe the client is too slow and server too fast).</p> <p>What is happening?? Because I do not want to put a Thread.sleep, this is not good programming here I think.</p> <p>Thank you so much!</p>
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    1. COHow many different threads are potentially calling receiveFileData() ? Same question for sendFileData()... Also, if you are just doing a File -> File transfer... why even mess with anything other than a byte[] ? You'll need a single int in front of the data load to tell the other side how many bytes to expect, but other than that, just send raw bytes!
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    2. COThe server has only one thread who is listening for new connections (accept()). When a client attempts to connect to server, the server creates a new thread that will manage separately the data flows between client and server and after that he continues to listen. The client side has only one thread. I think I tried to send the complete buffer with all the bytes of the file, but I think this crashed (I have to check because I am not at home now). Thank you for your reply
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    3. COAnd this code transfers an entire file, unaltered, from one place to another?
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