Android :: How To Split File Into Chunks While Writing Into It?

I tried to create byte array blocks from file whil the process was still using the file for writing. Actually I am storing video into file and I would like to create chunks from the same file while recording. The following method was supposed to read blocks of bytes from file:

private byte[] getBytesFromFile(File file) throws IOException{
InputStream is = new FileInputStream(file);
long length = file.length(); int numRead = 0;
byte[] bytes = new byte[(int)length - mReadOffset];
numRead = is.read(bytes, mReadOffset, bytes.length - mReadOffset);
if(numRead != (bytes.length - mReadOffset)){
throw new IOException("Could not completely read file " + file.getName());
} mReadOffset += numRead; is.close(); return bytes;
}

But the problem is that all array elements are set to 0 and I guess it is because the writing process locks the file. Any other way to create file chunks while writing into file.