Question

I am sending a bufferedImage over a socket and I am using the example found in this post:

Sender

   BufferedImage image = ....;
   ImageIO.write(image, "PNG", socket.getOutputStream());

Receiver

   BufferedImage image = ImageIO.read(socket.getInputStream());

It works – IF, and ONLY IF, I close the sender’s outputStream after this line:

 ImageIO.write(image, "PNG", socket.getOutputStream());

Is there anything I can do apart from closing the outputStream?

Also, is there anything else I can do to avoid using ImageIO altogether? It seems to take ages to do anything.
Also note that reading or writing to the hard disk in anyway should be avoided at all costs due to performance issues. I need to make this transfer as fast as possible, (I’m experimenting and trying to create a client similar to VNC and saving each screenshot to the hard disk would greatly slow down everything)..

@Jon Skeet

Edit 3:

Sender: (Note that I am sending a JPG image not a PNG).

                    int filesize;
                    OutputStream out = c.getClientSocket().getOutputStream();

                    ByteArrayOutputStream bScrn = new ByteArrayOutputStream();
                    ImageIO.write(screenshot, "JPG", bScrn);
                    byte[] imgByte = bScrn.toByteArray();
                    bScrn.flush();
                    bScrn.close();

                    filesize = bScrn.size();
                    out.write(new String("#FS " + filesize).getBytes()); //Send filesize
                    out.write(new String("#<IM> \n").getBytes());        //Notify start of image

                    out.write(imgByte); //Write file
                    System.out.println("Finished");                                 

Reciever: (where input is the socket input stream)

Attempt #1:

String str = input.toString();
imageBytes = str.getBytes();

InputStream in = new ByteArrayInputStream(imageBytes);
BufferedImage image = ImageIO.read(in);
in.close();

System.out.println("width=" + image.getWidth());

(failed: Nullpointer exception on getWidth() line)
I understand this error to mean “corrupt image” because it couldn’t initialize it. correct?

Attempt #2:

byte[] imageBytes = new byte[filesize];
for (int j = 0; i < filesize; i++)
{
    imageBytes[j] = (byte) input.read();
}

InputStream in = new ByteArrayInputStream(imageBytes);
BufferedImage image = ImageIO.read(in);
in.close();

System.out.println("width=" + image.getWidth());

(failed: Nullpointer exception on getWidth() line)

Attempt #3:

if (filesize > 0)
{

    int writtenBytes = 0;
    int bufferSize = client.getReceiveBufferSize();

    imageBytes = new byte[filesize];     //Create a byte array as large as the image
    byte[] buffer = new byte[bufferSize];//Create buffer


    do {
        writtenBytes += input.read(buffer); //Fill up buffer
        System.out.println(writtenBytes + "/" + filesize); //Show progress

        //Copy buffer to the byte array which will contain the full image
        System.arraycopy(buffer, 0, imageBytes, writtenBytes, client.getReceiveBufferSize());
        writtenBytes+=bufferSize;

    } while ((writtenBytes + bufferSize) < filesize);

    // Read the remaining bytes
    System.arraycopy(buffer, 0, imageBytes, writtenBytes-1, filesize-writtenBytes);
    writtenBytes += filesize-writtenBytes;
    System.out.println("Finished reading! Total read: " + writtenBytes + "/" + filesize);

}

InputStream in = new ByteArrayInputStream(imageBytes);
BufferedImage image = ImageIO.read(in);
in.close();

(failed: Reciever gives: Null pointer exception)

Attempt 4:

   int readBytes = 0;
   imageBytes = new byte[filesize];     //Create a byte array as large as the image



    while (readBytes < filesize)
    {
        readBytes += input.read(imageBytes);
    }

    InputStream in = new ByteArrayInputStream(imageBytes);
    BufferedImage image = ImageIO.read(in);
    in.close();

    System.out.println("width=" + image.getWidth());

(failed: sender gives: java.net.SocketException: Connection reset by peer: socket write error)

Attempt #5:

Using Jon skeet’s code snippet, the image arrives, but only partially. I saved it to a file (1.jpg) to see what was going on, and it actually sends 80% of the image, while the rest of the file is filled with blank spaces. This results in a partially corrupt image. Here is the code I tried: (note that captureImg() is not at fault, saving the file directly works)

Sender:

    Socket s = new Socket("127.0.0.1", 1290);
    OutputStream out = s.getOutputStream();

    ByteArrayOutputStream bScrn = new ByteArrayOutputStream();
    ImageIO.write(captureImg(), "JPG", bScrn);
    byte imgBytes[] = bScrn.toByteArray();
    bScrn.close();

    out.write((Integer.toString(imgBytes.length)).getBytes());
    out.write(imgBytes,0,imgBytes.length);

Reciever:

    InputStream in = clientSocket.getInputStream();
    long startTime = System.currentTimeMillis();

    byte[] b = new byte[30];
    int len = in.read(b);

    int filesize = Integer.parseInt(new String(b).substring(0, len));

    if (filesize > 0)
    {
        byte[] imgBytes = readExactly(in, filesize);
        FileOutputStream f = new FileOutputStream("C:\\Users\\Dan\\Desktop\\Pic\\1.jpg");
        f.write(imgBytes);
        f.close();

        System.out.println("done");

The sender still gives a Connection reset by peer: socket write error.
Click here for full sized image

Answer 1

One option would be to write the image to a ByteArrayOutputStream so you can determine the length, then write that length to the output stream first.

Then on the receiving end, you can read the length, then read that many bytes into a byte array, then create a ByteArrayInputStream to wrap the array and pass that to ImageIO.read().

I’m not entirely surprised that it doesn’t work until the output socket is closed normally – after all, a file which contains a valid PNG file and then something else isn’t actually a valid PNG file in itself, is it? So the reader needs to read to the end of the stream before it can complete – and the "end" of a network stream only comes when the connection is closed.

EDIT: Here’s a method to read the given number of bytes into a new byte array. It’s handy to have as a separate "utility" method.

public static byte[] readExactly(InputStream input, int size) throws IOException
{
    byte[] data = new byte[size];
    int index = 0;
    while (index < size)
    {
        int bytesRead = input.read(data, index, size - index);
        if (bytesRead < 0)
        {
            throw new IOException("Insufficient data in stream");
        }
        index += bytesRead;
    }
    return data;
}