java - 通过套接字发送压缩的 JPG 图像

Question

我正在尝试使用 Java 创建一个程序，它可以截取用户屏幕的屏幕截图，压缩图像，然后通过套接字将其发送到服务器。 由于某种原因，图像最终被保存损坏(无法读取)。你能帮我找出问题所在吗？

CLIENT:(屏幕截图作为 BufferedImage 输入，然后返回的字节数组返回到第二个函数，该函数将其发送到服务器)

    public static byte[] compressImage(BufferedImage image) throws IOException {
    System.out.println("starting compression");

    ByteArrayOutputStream os = new ByteArrayOutputStream(37628);

    float quality = 0.16f;

    // create a BufferedImage as the result of decoding the supplied InputStream

    // get all image writers for JPG format
    Iterator<ImageWriter> writers = ImageIO.getImageWritersByFormatName("jpg");
    //Iterator iter = ImageIO.getImageWritersByFormatName("jpeg");

    if (!writers.hasNext())
        throw new IllegalStateException("No writers found");

    ImageWriter writer = (ImageWriter) writers.next();
    ImageOutputStream ios = ImageIO.createImageOutputStream(os);
    writer.setOutput(ios);

    ImageWriteParam param = writer.getDefaultWriteParam();

    // compress to a given quality
    param.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
    param.setCompressionQuality(quality);

    // appends a complete image stream containing a single image and
    //associated stream and image metadata and thumbnails to the output
    writer.write(null, new IIOImage(image, null, null), param);
    os.flush();

    return os.toByteArray();
}

    public void uploadShot(byte[] imgData, String nickname) {

    try {
        /* Try to connect to the server on localhost, port 5555 */

        Socket sk = new Socket("localhost", 23232);
        OutputStream output = sk.getOutputStream();

        /* Send filename to server */

        OutputStreamWriter outputStream = new OutputStreamWriter(sk.getOutputStream());
        outputStream.write(nickname + "\n");
        outputStream.flush();

        /* Get response from server */

        BufferedReader inReader = new BufferedReader(new InputStreamReader(sk.getInputStream()));

        String serverStatus = inReader.readLine(); // Read the first line

        /* If server is ready, send the file */

        if (serverStatus.equals("READY")){
            int len = imgData.length;
            int start = 0;

            if (len < 0)
                throw new IllegalArgumentException("Negative length not allowed");
            if (start < 0 || start >= imgData.length)
                throw new IndexOutOfBoundsException("Out of bounds: " + start);
            // Other checks if needed.

            // May be better to save the streams in the support class;
            // just like the socket variable.
            OutputStream out = sk.getOutputStream(); 
            DataOutputStream dos = new DataOutputStream(out);

            dos.writeInt(len);
            if (len > 0) {
                dos.write(imgData, start, len);
            }
            dos.close();
            output.close();
            sk.close();

            System.out.println("Transfer complete.");
        }
    } catch (Exception ex){
        /* Catch any errors */
        System.out.println(ex.getMessage());
    }
}

服务器:(接收到的图像保存到带有时间戳的文件夹中)

public static void main(String args[]) throws Exception{
    System.out.println("Server running...");

    /* Listen on port 5555 */

    ServerSocket server = new ServerSocket(23232);

    /* Accept the sk */

    Socket sk = server.accept();

    System.out.println("Server accepted client");
    InputStream input = sk.getInputStream();
    BufferedReader inReader = new BufferedReader(new InputStreamReader(sk.getInputStream()));
    BufferedWriter outReader = new BufferedWriter(new OutputStreamWriter(sk.getOutputStream()));

    /* Read the filename */
    String nickname = inReader.readLine();

    if ( !nickname.equals("") ){

        /* Reply back to client with READY status */

        outReader.write("READY\n");
        outReader.flush();
    }


    String current = "/home/kasgel/screenshots";

    DateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy__HH:mm:ss");
    Date timestamp = new Date();
    File filename = new File(current + "/" + nickname + "-" + dateFormat.format(timestamp) + ".jpg");
    if (!filename.exists()) {
        filename.createNewFile();
    }
    FileOutputStream wr = new FileOutputStream(filename);

    byte[] buffer = new byte[sk.getReceiveBufferSize()];

    int bytesReceived = 0;

    while((bytesReceived = input.read(buffer))>0) {
        wr.write(buffer,0,bytesReceived);
    }
    wr.close();
}

打开保存的屏幕截图时收到的错误消息如下: display.im6:不是 JPEG 文件:以 0x00 0x03 `MyNick-30-03-2015__19:27:58.jpg' @ error/jpeg.c/JPEGErrorHandler/316 开头。

Answer 1

当您写入图像时，首先写入一个 32 位有符号整数，其中包含图像的字节长度:

        dos.writeInt(len);
        if (len > 0) {
            dos.write(imgData, start, len);
        }

但是当你读回图像时，你不会先读长度；而是先读长度。您正在读取所有数据(包括长度)，就好像它们是图像的一部分一样。

但是你还有第二个问题，它本身也会导致这个问题。当您创建 BufferedReader 并对其调用 readLine 时，它​​将读取换行符以外的内容 - 它将读取直到缓冲区已满。如果您继续从中读取，这不是问题，但在读取该行后，您将继续从底层 InputStream 中读取，这通常会在换行符之后消耗更多字节。

解决方案是:仅使用一种抽象来读/写数据。在这种情况下，最简单的绝对是使用 DataOutputStream 和 DataInputStream。使用writeUTF写入文件名并使用readUTF读回。使用writeInt写入文件的长度并使用readInt读取它。使用 write 写入数据并使用 read 读取数据 - 并确保仅读取从 readInt 调用接收到的字节数。最重要的是，继续使用相同的 DataOutputStream 和 DataInputStream 实例；不要在同一底层 InputStream 上构造缓冲读取器和输入流