Java — How to read an unknown number of bytes from an inputStream (socket/socketServer)?

Question

Looking to read in some bytes over a socket using an inputStream. The bytes sent by the server may be of variable quantity, and the client doesn\'t know in advance the lengt

Answer 1

Here is a simpler example using ByteArrayOutputStream...

        socketInputStream = socket.getInputStream();
        int expectedDataLength = 128; //todo - set accordingly/experiment. Does not have to be precise value.
        ByteArrayOutputStream baos = new ByteArrayOutputStream(expectedDataLength);
        byte[] chunk = new byte[expectedDataLength];
        int numBytesJustRead;
        while((numBytesJustRead = socketInputStream.read(chunk)) != -1) {
            baos.write(chunk, 0, numBytesJustRead);
        }
        return baos.toString("UTF-8");

However, if the server does not return a -1, you will need to detect the end of the data some other way - e.g., maybe the returned content always ends with a certain marker (e.g., ""), or you could possibly solve using socket.setSoTimeout(). (Mentioning this as it is seems to be a common problem.)

Answer 2

Without re-inventing the wheel, using Apache Commons:

IOUtils.toByteArray(inputStream);

For example, complete code with error handling:

    public static byte[] readInputStreamToByteArray(InputStream inputStream) {
    if (inputStream == null) {
        // normally, the caller should check for null after getting the InputStream object from a resource
        throw new FileProcessingException("Cannot read from InputStream that is NULL. The resource requested by the caller may not exist or was not looked up correctly.");
    }
    try {
        return IOUtils.toByteArray(inputStream);
    } catch (IOException e) {
        throw new FileProcessingException("Error reading input stream.", e);
    } finally {
        closeStream(inputStream);
    }
}

private static void closeStream(Closeable closeable) {
    try {
        if (closeable != null) {
            closeable.close();
        }
    } catch (Exception e) {
        throw new FileProcessingException("IO Error closing a stream.", e);
    }
}

Where FileProcessingException is your app-specific meaningful RT exception that will travel uninterrupted to your proper handler w/o polluting the code in between.

Answer 3

Stream all Input data into Output stream. Here is working example:

    InputStream inputStream = null;
    byte[] tempStorage = new byte[1024];//try to read 1Kb at time
    int bLength;
    try{

        ByteArrayOutputStream outputByteArrayStream =  new ByteArrayOutputStream();     
        if (fileName.startsWith("http"))
            inputStream = new URL(fileName).openStream();
        else
            inputStream = new FileInputStream(fileName);            

        while ((bLength = inputStream.read(tempStorage)) != -1) {
                outputByteArrayStream.write(tempStorage, 0, bLength);
        }
        outputByteArrayStream.flush();
        //Here is the byte array at the end
        byte[] finalByteArray = outputByteArrayStream.toByteArray();
        outputByteArrayStream.close();
        inputStream.close();
    }catch(Exception e){
        e.printStackTrace();
        if (inputStream != null) inputStream.close();
    }

Answer 4

Either:

1. Have the sender close the socket after transferring the bytes. Then at the receiver just keep reading until EOS.

2. Have the sender prefix a length word as per Chris's suggestion, then read that many bytes.

3. Use a self-describing protocol such as XML, Serialization, ...

Answer 5

Assuming the sender closes the stream at the end of the data:

ByteArrayOutputStream baos = new ByteArrayOutputStream();

byte[] buf = new byte[4096];
while(true) {
  int n = is.read(buf);
  if( n < 0 ) break;
  baos.write(buf,0,n);
}

byte data[] = baos.toByteArray();