Question

I’m trying to send an image upload in a Qt server trough the socket and visualize it in a client created using Java. Until now I have only transferred strings to communicate on both sides, and tried different examples for sending images but with no results.

The code I used to transfer the image in qt is:

QImage image;
image.load("../punton.png");
qDebug()<<"Image loaded";
QByteArray ban; // Construct a QByteArray object
QBuffer buffer(&ban);   // Construct a QBuffer object using the QbyteArray
image.save(&buffer, "PNG"); // Save the QImage data into the QBuffer
socket->write(ban);        

In the other end the code to read in Java is:

BufferedInputStream in = new BufferedInputStream(socket.getInputStream(),1);
File f = new File("C:\\Users\\CLOUDMOTO\\Desktop\\JAVA\\image.png");
System.out.println("Receiving...");
FileOutputStream fout = new FileOutputStream(f);
byte[] by = new byte[1];
for(int len; (len = in.read(by)) > 0;){
fout.write(by, 0, len);
System.out.println("Done!");
}

The process in Java gets stuck until I close the Qt server and after that the file generated is corrupt.

I’ll appreciate any help because it’s neccessary for me to do this and I’m new to programming with both languages.

Also I’ve used the following commands that and the receiving process now ends and show a message, but the file is corrupt.

socket->write(ban+"-1"); 
socket->close(); in qt. 

And in java:

System.out.println(by); 
String received = new String(by, 0, by.length, "ISO8859_1");
System.out.println(received);
System.out.println("Done!");

Answer 1

You cannot transport file over socket in such simple way. You are not giving the receiver any clue, what number of bytes is coming. Read javadoc for InputStream.read() carefully. Your receiver is in endless loop because it is waiting for next byte until the stream is closed. So you have partially fixed that by calling socket->close() at the sender side. Ideally, you need to write the length of ban into the socket before the buffer, read that length at receiver side and then receive only that amount of bytes. Also flush and close the receiver stream before trying to read the received file.

I have absolutely no idea what you wanted to achieve with socket->write(ban+"-1"). Your logged output starts with %PNG which is correct. I can see there "-1" at the end, which means that you added characters to the image binary file, hence you corrupted it. Why so?

And no, 1×1 PNG does not have size of 1 byte. It does not have even 4 bytes (red,green,blue,alpha). PNG needs some things like header and control checksum. Have a look at the size of the file on filesystem. This is your required by size.