TCP套接字上的多个图像文件

Question

I am trying to send multiple images on a TCP socket.

The following code is running fine without a problem: the client and server exchange a greeting. Then the client is sending a single image file which the server receives.

Client side:

...        
try      {
    Socket clientSocket = new Socket(serverName, port);
    DataInputStream in=new DataInputStream(clientSocket.getInputStream());
    System.out.println("Heard from server: "+in.readUTF());

    DataOutputStream out = new DataOutputStream(clientSocket.getOutputStream());
    out.writeUTF("Hello from " + clientSocket.getLocalSocketAddress());

    BufferedImage bimg;

//    for (int i=0; i<3; i++)  {
    bimg = ImageIO.read(new File("flowers.jpg"));  
    ImageIO.write(bimg,"jpg",clientSocket.getOutputStream());   
    out.flush(); 
    System.out.println("Image sent");
//    }
    clientSocket.close();
  } catch(Exception e) { ... }

Server side:

...
Socket socket=null;
DataInputStream din=null;
DataOutputStream dout=null;
...
socket = serverSocket.accept();  
din=new DataInputStream(socket.getInputStream());
dout=new DataOutputStream(socket.getOutputStream());

for (;;)  {  
    try  {

        dout.writeUTF("ImageReceiver here at your service: ");

        System.out.println("from the client: "+din.readUTF());
        BufferedImage img;

//for (int i=0; i<3; i++) {
        img=ImageIO.read(socket.getInputStream());   
        ImageIO.write(img, "png", new File(timeStamp+"img.png"));
        System.out.println("Image received...");
//} 
        socket.close();
        break;
   }
  catch(Exception e) {  .... }
 }

When I uncomment the 2*2 lines in the above and thus send&receive images in a loop, I am getting an exception that the image is null:

the server is receiving the 1st image fine without errors. However, after that, it is not seeing the input stream as an image. The call to ImageIO.read() at

img=ImageIO.read(socket.getInputStream());    

is returning null and

        ImageIO.write(img, "png", new File(timeStamp+"img.png"));

is throwing the exception.

The client process is running fine without any errors.

Is this a matter of managing the I/O buffers?

Where to look to fix it?

TIA.

//=======================================

I'm getting the same error when I'm sending 2 images: the first image is transmitted&received fine.

The client is sending the second image as the last thing before it closes. However, the server isn't seeing the second one as an image. After the first image is transmitted, there's nothing in the server-socket's input buffer but the second image.

Answer 1

ImageIO closes the input stream after reading the image.It needs to be created again and again for reading multiple images using ImageIO.createImageInputStream(). You can see it here- https://docs.oracle.com/javase/8/docs/api/javax/imageio/ImageIO.html#read-javax.imageio.stream.ImageInputStream-

Answer 2

You can try this code. This code sends data in bytes.

client (Sender):

try {
        int port = 8989; // Your Server port number
        Socket sender = new Socket("Server IP", port);
        int count,i=0;

        byte[] data = new byte[1024]; // Here provide size of your file
        FileInputStream fileStream = null;
        BufferedInputStream fileBuffer = null;
        OutputStream out = sender.getOutputStream();
        InputStream in = sender.getInputStream();

        while (i < 10) {

            fileStream = new FileInputStream("fileURL");
            fileBuffer = new BufferedInputStream(fileStream);

            // Sending file data in bytes
            while ((count = fileBuffer.read(data)) > 0) {
                System.out.println("Data Sent : " + count);
                out.write(data, 0, count);
                out.flush();
            }

            // Waiting for response
            while (in.available() <= 0) {
            }
            if (in.read() == 0) {
                System.out.println("File Sent Successfully");
                i++;
            } else {
                System.out.println("File Not Sent Successfully");
            }
        }
    } catch (Exception e) {
    }

Server (Receiver):

try {
        ServerSocket server = new ServerSocket(port);

        // Waiting for client
        Socket receiveSocket = server.accept();
        InputStream in = receiveSocket.getInputStream();
        OutputStream out = receiveSocket.getOutputStream();
        int count, cnt = 10, i = 0;

        // cnt will be total number of files
        while (i < cnt) {
            byte data[] = new byte[1024]; // Provide file size here
            FileOutputStream fileOut = new FileOutputStream("fileURL");
            BufferedOutputStream fileBuffer = new BufferedOutputStream(fileOut);

            // Waiting till client is sending the file
            while (in.available() <= 0) {
            }
            // Receiving byte data and storing it in the file
            while ((count = in.read(data)) > 0) {
                fileBuffer.write(data, 0, count);
                System.out.println("Data Received : " + count);
                fileBuffer.flush();
            }
            // Notify client that you received that file successfully
            out.write(0);

            // Close all streams
            in.close();
            out.close();
            fileBuffer.close();
            fileOut.close();
            i++;
        }
    } catch (Exception e) {
    }

Hope this helps you.