Question

I am sending byte arrays over a socket. The sent data starts off with 4 bytes indicating the length of the following byte array.

// get the amount of data being sent
byte[] lengthOfReplyAsArray = new byte[4];
forceRead(inputStream, lengthOfReplyAsArray);
int lengthOfReply = byteArrayToInt(lengthOfReplyAsArray);

// read the data into a byte array
byte[] reply = new byte[lengthOfReply];
forceRead(inputStream, reply);

The method used to read data from an InputStream:

private byte[] forceRead(InputStream inputStream, byte[] result)
        throws IOException {

    int bytesRead = 0;
    int total = result.length;
    int remaining = total;

    while (remaining > 0)
        remaining -= inputStream.read(result, bytesRead, remaining);

    return result;

}

The method used to convert a byte array to an integer:

private int byteArrayToInt(byte[] byteArray) {
    int result = 0;

    for (int i = 0; (i<byteArray.length) && (i<8); i++) {
        result |= (byteArray[3-i] & 0xff) << (i << 3);
    }
    return result;                  
}

The problem is, that the data is not read in the order it arrives. The first 4 bytes are being read just fine. The rest is mixed up. I made a TCP dump to ensure the data correctly arrives at the client. It seems as if the data is split up into 4 TCP packets. The InputStream returns the first 4 bytes of the first packet, then the entire data of the fourth packet, the last part (starting from “length of last packet”) of the second packet and the entire data of the third packet. In this order.

Does anyone have a clue what might cause this issue?

Answer 1

Your logic for reading the byte array is not quite right:

From to the docs:

Reads up to len bytes of data from the input stream into an array of
bytes. An attempt is made to read as many as len bytes, but a smaller
number may be read. The number of bytes actually read is returned as
an integer.

and

The first byte read is stored into element b[off], the next one into
b[off+1], and so on. The number of bytes read is, at most, equal to
len. Let k be the number of bytes actually read; these bytes will be
stored in elements b[off] through b[off+k-1], leaving elements
b[off+k] through b[off+len-1] unaffected.

However, as your bytesRead variable stays at 0 for the whole loop, any data from the inputstream is always written to the beginning of your buffer, overwriting the data already in there.

What will work better (checking for -1 will also ensure that you don’t subtract -1 from remaining if the stream runs out of data prematurely which would result in remaining increase, which would mean the loop would run unnecessarily until a buffer overrun would make remaining negative):

while ((bytesRead = inputStream.read(result, total - remaining, remaining)) != -1
     && remaining > 0) {
    remaining -= bytesRead;