Question

I’m trying to send an image file from a server to a client via a socket. The socket was previously used to send some strings from the server to the client (with buffered input/output streams).

The trouble is the image file can’t be received properly, with “Premature end of JPEG file” error.

The server first sends the file size to the client, the client then creates a byte[] of that size, and starts to receive the file.

Here are the codes:

Server:

        DataOutputStream dos = new DataOutputStream(socket.getOutputStream());
        //Send file size
        dos.writeInt((int) file.length());

        BufferedInputStream bis = new BufferedInputStream(new FileInputStream(file));
        byte[] fileBytes = new byte[bis.available()];
        bis.read(fileBytes);
        bis.close();

        BufferedOutputStream bos = new BufferedOutputStream(socket.getOutputStream());
        bos.write(fileBytes);
        bos.flush();

Client:

        DataInputStream dis = new DataInputStream(socket.getInputStream());
        //Receive file size
        int size = dis.readInt();

        BufferedInputStream bis = new BufferedInputStream(socket.getInputStream());
        byte[] fileBytes = new byte[size];

        bis.read(fileBytes, 0, fileBytes.length);

More interestingly, if I let server sleep for about 2 seconds between sending the file size and writing the byte[], then the image is received properly. I wonder if there’s some kind of race condition between the server and the client

Answer 1

The error is most likely here:

byte[] fileBytes = new byte[bis.available()];

The method available does not return the size of the file. It might return only the size of the input buffer, which is smaller than the size of the file. See the API documentation of the method in BufferedInputStream.

Also, read in the line below is not guaranteed to read the whole file in one go. It returns the number of bytes that were actually read, which can be less than what you asked for. And in the client code, you are using read in the same way, without actually checking if it read all the data.