Why does Java read random amounts from a socket but not the whole message?

I am working on a project and have a question about Java sockets. The source file which can be found here.

After successfully transmitting开发者_StackOverflow中文版 the file size in plain text I need to transfer binary data. (DVD .Vob files)

I have a loop such as

                // Read this files size
                long fileSize = Integer.parseInt(in.readLine());

                // Read the block size they are going to use
                int blockSize = Integer.parseInt(in.readLine());
                byte[] buffer = new byte[blockSize];

                // Bytes "red"
                long bytesRead = 0;
                int read = 0;

                while(bytesRead < fileSize){
                System.out.println("received " + bytesRead + " bytes" + " of " + fileSize + " bytes in file " + fileName);
                read = socket.getInputStream().read(buffer);
                if(read < 0){
                    // Should never get here since we know how many bytes there are
                    System.out.println("DANGER WILL ROBINSON");
                    break;
                }
                binWriter.write(buffer,0,read);
                bytesRead += read;
            }

I read a random number of bytes close to 99%. I am using Socket, which is TCP based, so I shouldn't have to worry about lower layer transmission errors.

The received number changes but is always very near the end received 7258144 bytes of 7266304 bytes in file GLADIATOR/VIDEO_TS/VTS_07_1.VOB

The app then hangs there in a blocking read. I am confounded. The server is sending the correct file size and has a successful implementation in Ruby but I can't get the Java version to work.

Why would I read less bytes than are sent over a TCP socket?

The above is because of a bug many of you pointed out below.

BufferedReader ate 8Kb of my socket's input. The correct implementation can be found Here

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If your in is a BufferedReader then you've run into the common problem with buffering more than needed. The default buffer size of BufferedReader is 8192 characters which is approximately the difference between what you expected and what you got. So the data you are missing is inside BufferedReader's internal buffer, converted to characters (I wonder why it didn't break with some kind of conversion error).

The only workaround is to read the first lines byte-by-byte without using any buffered classes readers. Java doesn't provide an unbuffered InputStreamReader with readLine() capability as far as I know (with the exception of the deprecated DataInputStream.readLine(), as indicated in the comments below), so you have to do it yourself. I would do it by reading single bytes, putting them into a ByteArrayOutputStream until I encounter an EOL, then converting the resulting byte array into a String using the String constructor with the appropriate encoding.

Note that while you can't use a BufferedInputReader, nothing stops you from using a BufferedInputStream from the very beginning, which will make byte-by-byte reads more efficient.

Update

In fact, I am doing something like this right now, only a bit more complicated. It is an application protocol that involves exchanging some data structures that are nicely represented in XML, but they sometimes have binary data attached to them. We implemented this by having two attributes in the root XML: fragmentLength and isLastFragment. The first one indicates how much bytes of binary data follow the XML part and isLastFragment is a boolean attribute indicating the last fragment so the reading side knows that there will be no more binary data. XML is null-terminated so we don't have to deal with readLine(). The code for reading looks like this:

    InputStream ins = new BufferedInputStream(socket.getInputStream());
    while (!finished) {
      ByteArrayOutputStream buf = new ByteArrayOutputStream();
      int b;
      while ((b = ins.read()) > 0) {
        buf.write(b);
      }
      if (b == -1)
        throw new EOFException("EOF while reading from socket");
      // b == 0
      Document xml = readXML(new ByteArrayInputStream(buf.toByteArray()));
      processAnswers(xml);
      Element root = xml.getDocumentElement();
      if (root.hasAttribute("fragmentLength")) {
        int length = DatatypeConverter.parseInt(
                root.getAttribute("fragmentLength"));
        boolean last = DatatypeConverter.parseBoolean(
                root.getAttribute("isLastFragment"));
        int read = 0;
        while (read < length) {
          // split incoming fragment into 4Kb blocks so we don't run 
          // out of memory if the client sent a really large fragment
          int l = Math.min(length - read, 4096);
          byte[] fragment = new byte[l];
          int pos = 0;
          while (pos < l) {
            int c = ins.read(fragment, pos, l - pos);
            if (c == -1)
              throw new EOFException(
                      "Preliminary EOF while reading fragment");
            pos += c;
            read += c;
          }
          // process fragment
        }

Using null-terminated XML for this turned out to be a really great thing as we can add additional attributes and elements without changing the transport protocol. At the transport level we also don't have to worry about handling UTF-8 because XML parser will do it for us. In your case you're probably fine with those two lines, but if you need to add more metadata later you may wish to consider null-terminated XML too.

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Here is your problem. The first few lines of the program your using in.readLine() which is probably some sort of BufferedReader. BufferedReaders will read data off the socket in 8K chunks. So when you did the first readLine() it read the first 8K into the buffer. The first 8K contains your two numbers followed by newlines, then some portion of the head of the VOB file (that's the missing chunk). Now when you switched to using the getInputStream() off the socket you are 8K into the transmission assuming your starting at zero.

socket.getInputStream().read(buffer);  // you can't do this without losing data.

While the BufferedReader is nice for reading character data, switching between binary and character data in a stream is not possible with it. You'll have to switch to using InputStream instead of Reader and convert the first few portions by hand to character data. If you read the file using a buffered byte array you can read the first chunk, look for your newlines and convert everything to the left of that to character data. Then write everything to the right to your file, then start reading the rest of the file.

This used to be easier with DataInputStream, but it doesn't do a good job handling character conversion for you (readLine is deprecated with BufferedReader being the only replacement - doh). Probably should write a DataInputStream replacement that under the covers uses Charset to properly handle string conversion. Then switching between characters and binary would be easier.

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Your basic problem is that BufferedReader will read as much data is available and place in its buffer. It will give you the data as you ask for it. This is the whole point of buffereing i.e. to reduce the number of calls to the OS. The only safe way to use an buffered input is to use the same buffer over the life of the connection.

In your case, you only use the buffer to read two lines, however it is highly likely that 8192 bytes has been read into the buffer. (The default size of the buffer) Say the first two lines consist of 32 bytes, this leaves 8160 waiting for you to read, however you by-pass the buffer to perform the read() on the socket directly leading to 8160 bytes left in the buffer you end up discarding. (the amount you are missing)

BTW: You should be able to see this in a debugger if you inspect the contents of your buffered reader.

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Sergei may have been right about data being lost inside the buffer, but I'm not sure about his explanation. (BufferedReaders don't usually hold onto data inside their buffers. He may be thinking of a problem with BufferedWriters, which can lose data if the underlying stream is shut down prematurely.) [Never mind; I had misread Sergei's answer. The rest of this is valid AFAIK.]

I think you have a problem that's specific to your application. In your client code, you start reading as follows:

public static void recv(Socket socket){
    try {
        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        //...
        int numFiles = Integer.parseInt(in.readLine());

... and you proceed to use in for the start of the exchange. But then you switch to using the raw socket stream:

            while(bytesRead > fileSize){
                read = socket.getInputStream().read(buffer);

Because in is a BufferedReader, it's already going to have filled its buffer with up to 8192 bytes from the socket input stream. Any bytes that are in that buffer, and which you don't read from in, will be lost. Your app is hanging because it believes that the server is holding onto some bytes, but the server doesn't have them.

The solution is not to do byte-by-byte reads from the socket (ouch! your poor CPU!), but to use the BufferedReader consistently. Or, to use buffering with binary data, change the BufferedReader to a BufferedInputStream that wraps the socket's InputStream.

By the way, TCP is not as reliable as many people assume it to be. For example, when the server socket closes, it's possible for it to have written data into the socket which then gets lost as the socket connection is shutdown. Calling Socket.setSoLinger can help to prevent this problem.

EDIT: Also BTW, you're playing with fire by treating byte and character data as if they're interchangeable, as you do below. If the data really is binary, then the conversion to String risks corrupting the data. Perhaps you want to be writing into a BufferedOutputStream?

                // Java is retarded and reading and writing operate with
                // fundamentally different types. So we write a String of
                // binary data.
                fileWriter.write(new String(buffer));
                bytesRead += read;

EDIT 2: Clarified (or attempted to clarify :-} the handling of binary vs. String data.