You can use the good old read method like this:

   public static byte[] readAllBytes(InputStream inputStream) throws IOException {
    final int bufLen = 1024;
    byte[] buf = new byte[bufLen];
    int readLen;
    IOException exception = null;

    try {
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();

        while ((readLen = inputStream.read(buf, 0, bufLen)) != -1)
            outputStream.write(buf, 0, readLen);

        return outputStream.toByteArray();
    } catch (IOException e) {
        exception = e;
        throw e;
    } finally {
        if (exception == null) inputStream.close();
        else try {
            inputStream.close();
        } catch (IOException e) {
            exception.addSuppressed(e);
        }
    }
}

You need to read each byte from your InputStream and write it to a ByteArrayOutputStream.

You can then retrieve the underlying byte array by calling toByteArray():

InputStream is = ...
ByteArrayOutputStream buffer = new ByteArrayOutputStream();

int nRead;
byte[] data = new byte[16384];

while ((nRead = is.read(data, 0, data.length)) != -1) {
  buffer.write(data, 0, nRead);
}

return buffer.toByteArray();