Java reading a file and sending it via DatagramSocket

Question

Using Java, I am trying to send some file data over a DatagramSocket. I need to read a file in 1000-byte chunks and send them over as packets. My code:

1. reads a file into a byte array wrapped in a byte buffer
2. places the data in a packet and sends it
3. has the receiver open the packet and re-write the contents to a new file.

I am having a problem with writing the byte array back to a file. Please see my code below.

Client/Sender:

byte[] data = new byte[1000];
ByteBuffer b = ByteBuffer.wrap(data);
DatagramPacket pkt;
File file = new File(sourceFile);
FileInputStream fis = new FileInputStream(file);
BufferedInputStream bis = new BufferedInputStream(fis);
CRC32 crc = new CRC32();

while(true){
    b.clear();
    b.putLong(0); // I need to put the checksum at the beginning for easy retrieval
    bytesRead = bis.read(data);
    if(bytesRead==-1) { break; }
    crc.reset();
    crc.update(data, 8, data.length-8);
    long chksum = crc.getValue();
    b.rewind();
    b.putLong(chksum);
    pkt = new DatagramPacket(data, 1000, addr); // addr is valid, works fine
    sk.send(pkt);
}

bis.close();
fis.close();

Server/Receiver:

DatagramSocket sk = new DatagramSocket(port);

File destfile = new File("hello.txt");
FileOutputStream fos = new FileOutputStream(destfile);
BufferedOutputStream bos = new BufferedOutputStream(fos);
PrintStream ps = new PrintStream(fos);

byte[] data = new byte[1000];
DatagramPacket pkt = new DatagramPacket(data, data.length);
ByteBuffer b = ByteBuffer.wrap(data);
CRC32 crc = new CRC32();

while(true) {
    pkt.setLength(data.length);
    sk.receive(pkt);
    b.rewind();

    // compare checksum, print error if checksum is different
    // if checksum is the same:
    bos.write(data);  // Where the problem seems to be occurring.

    // send acknowledgement packet. 
}
bos.close();
fos.close();

Here, I am mainly having issues with writing the file back. With a small text file that says Hello World! , I get a strange output that says vˇ]rld! . Also, the input file is only 12 bytes but the file that the receiver creates is 1KB.

I think my issue is dealing with a byte buffer - I've written a program that copies files using file streams and buffered streams, which worked well. But I'm confused with how streams work in this sort of situation, and I would really appreciate any help. Thanks!

Answer 1

In the sender's data[] you overwrite the text, which was read from the file by the crc! You have to read the text in a position after the long. When correcting this in the Sender, it works:

//int bytesRead = bis.read(data); --old code
int bytesRead=bis.read(data,8,data.length-8);

Furthermore you send 1000 bytes, so will receive 1000 bytes, which will go into the destfile.

BTW: you do not check the crc in the server.... so why send it ?