Java Sockets - Data duplicated on last write

Question

I have the following code that I am using to send a file to a client:

private void sendFile(Socket client) throws Exception {
        byte[] data = new byte[4096];
        DataOutputStream dos = new DataOutputStream(client.getOutputStream());
        FileInputStream fis = new FileInputStream("test.txt");

        while (fis.read(data, 0, data.length) != -1) {
            dos.write(data);
        }

        fis.close();
        dos.close();
}

The problem that I'm finding is that the file sizes are different after transfer. Upon further investigation, I discovered that the files were being duplicated on the last dos.write(data).

Example:

Original File:

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Cras dictum diam neque, eu dictum sem efficitur ut.
Ut eu hendrerit risus.
In dapibus vel lectus at egestas.

Transferred File:

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Cras dictum diam neque, eu dictum sem efficitur ut.
Ut eu hendrerit risus.
In dapibus vel lectus at egestas.
Ut eu hendrerit risus.
In dapib

I'm at wit's end here and I've already looked at hundreds of examples trying to fix this. I've tried dos.flush(), changing the read to dos.read(data), and changing the while loop condition. I'm expecting to transfer very large files so I don't want to load a file all at once.

EDIT:

I have been using both telnet and netcat from the command line for testing the download.

You need to check return values from both read and write. The value from read tells you how much was read and can safely be written. The value from write tells you how much was written and might prompt a retry. — Alexander Torstling
– Alexander Torstling, Commented Nov 30, 2016 at 6:27

Scary Wombat · Accepted Answer · 2016-11-30 06:23:54Z

1

see this method

https://docs.oracle.com/javase/7/docs/api/java/io/DataOutputStream.html#write(byte[],%20int,%20int)

and

https://docs.oracle.com/javase/7/docs/api/java/io/FileInputStream.html#read(byte[],%20int,%20int)

len - the maximum number of bytes read.

and returns

the total number of bytes read into the buffer

You only want to write the amount of bytes that were read previously

so

  int br = -1;
  while ((br = fis.read(data, 0, data.length)) != -1) {
        dos.write(data, 0, br);
  }

edited Nov 30, 2016 at 6:23

answered Nov 30, 2016 at 6:14

Scary Wombat

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11 Comments

pvg Over a year ago

your link should be to InputStream read. The root cause is that the poster assumed that call to read guarantees reading as many bytes as are passed in the third argument. Of course, it doesn't.

Scary Wombat Over a year ago

I think that OP assumed that the array would be cleared

pvg Over a year ago

which is also a problem with the question - there isn't any 'duplicate data'. you're just writing stuff you didn't actually read, over and over. and the reason you're doing that is you never checked how much you actually read.

pvg Over a year ago

no. you should be keeping track of what you read and writing just that.

Scary Wombat Over a year ago

With my solution it is not necessary, but it is also another possible solution

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