Java DataOutputStream doesnt write all messages ( TCP Socket )

Question

in my Client-Server application I found a strange error. I got the following Methods :

sendLoginResponse();
sendPlayerList();
sendCurrentLevelState();

Each Methods sends a byte array to the Client-Side

If I only call 2 of them, everything works fine and the Client-Side gets all the sent byte arrays. But if I call all 3 of them only the first and second one arrive the Client, the order of the following methods doesnt matter. but the Server says all of them were send. To write to the Client iam using the write(byte[] b, int off, int len); method all The lenghts within the packages make sense too. Here comes the strange point:

if I add a Thread.sleep(1000); after the second Method, the third one does now arrive the Client after the sleep. I Have also tried to flush the DataOutputStream after every write call, but this didnt help.

EDIT:

So let's say I'd send 3 Login-Response

The Method's that gives me the byte[]:

public byte[] getLoginResponse(int playerID){
    byte[] msg = new byte[4];
    short shortMsgLength = 4;
    byte[] msgLength = shortToBytes(shortMsgLength);
    msg[0] = 2;
    msg[1] = msgLength[0];
    msg[2] = msgLength[1];
    msg[3] = (byte) playerID;
    return msg;
}

private byte[] shortToBytes(short value) {
    byte[] returnByteArray = new byte[2];
    returnByteArray[0] = (byte) (value & 0xff);
    returnByteArray[1] = (byte) ((value >>> 8) & 0xff);
    return returnByteArray;
}

And the Send Method:

private void sendLoginResponse() {
    try{
        byte[] msg = rfcObject.getLoginResponse(playerID);
        out.write(msg,0,msg.length);
    }catch(Exception e){
        System.err.println(e.getMessage());
        System.exit(0);
    }
}

So if I call the sendLoginResponse(); three times in a row, the client only recieves 2 byte-arrays, but the server says it has been sent 3 times. If i add a

Thread.sleep(1000); `after the second Method-Call, everything works fine..`

The Client that reads the message runs in a Thread:

public void run(){
    while(true){
        try {
            byte[] data = new byte[MAX_DATA_SIZE]; // MAX_DATA = 255
            byteCount = in.read(data);

        } catch (IOException ex) {
            handleExceptionError(ex);
        }
    }
}

thank you!

Answer 1

DataOutputStream and TCP don't lose data.

As almost invariable seen in questions of this nature, the problem is at the receiving end. You are probably assuming that `read()' fills the buffer, and ignoring the count that it returns.

Based on your protocol description in comments, you should be using DataInputStream.readFully() in this circumstance:

byte type = din,readByte();
int length = din.readShort();
byte[] data = new byte[length];
din.readFully(data);

Answer 2

if I call the sendLoginResponse(); three times in a row, the client only recieves 2 byte-arrays, but the server says it has been sent 3 times.

This is because TCP is a stream-oriented protocol. Meaning it doesn't know or care how your messages are delimited. There's no concept of individual messages in TCP, just a stream of bytes, with the guarantee that the order of bytes is preserved.

So when the sender calls three write , the three byte arrays are simply concatenated over the connection and arrives at the receiver in the same order, but the receiver doesn't necessarily need three read to get all the bytes, and even if it does take three read , the read doesn't necessarily gives you the same byte array passed to each corresponding write .

Your message already have the necessary information to get the individual message back from the byte stream:

// Client code for reading individual messages from a TCP connection

byte type = din.readByte();

// Read the message length, little-endian.
// We cannot use din.readShort because it's big-endian
int lenLo = din.read();
int lenHi = din.read();
short len = (short)(lenLo | (lenHi << 8));
byte [] body = new byte[len];
din.readFully(body);