Efficient way of handling file pointers in Java? (Using BufferedReader with file pointer)

Question

I have a log file which gets updated every second. I need to read the log file periodically, and once I do a read, I need to store the file pointer position at the end of t

Answer 1

Neil Coffey's solution is good if you are reading fixed length files. However for files that have variable length (data keep coming in) there are some problems with using BufferedReader directly on FileInputStream or FileChannel inputstream via an InputStreamReader. For ex consider the cases

• 1) You want to read data from some offset to current file length. So you use BR on FileInputStream/FileChannel(via an InputStreamReader) and use its readLine method. But while you are busy reading the data let say some data got added which causes BF's readLine to read more data than what you expected(the previous file length)

• 2) You finished readLine stuff but when you try to read the current file length/channel position some data got added suddenly which causes the current file length/channel position to increase but you have already read less data than this.

In both of the above cases it is difficult to know the actual data you have read (you cannot just use the length of data read using readLine because it skips some chars like carriage return)

So it is better to read the data in buffered bytes and use a BufferedReader wrapper around this. I wrote some methods like this

/** Read data from offset to length bytes in RandomAccessFile using BufferedReader
 * @param offset
 * @param length
 * @param accessFile
 * @throws IOException
 */
    public static void readBufferedLines(long offset, long length, RandomAccessFile accessFile) throws IOException{
    if(accessFile == null) return;
    int bufferSize = BYTE_BUFFER_SIZE;// constant say 4096

    if(offset < length && offset >= 0){ 
        int index = 1;
        long curPosition = offset;
        /*
         * iterate (length-from)/BYTE_BUFFER_SIZE times to read into buffer no matter where new line occurs
         */
        while((curPosition + (index * BYTE_BUFFER_SIZE)) <  length){        

            accessFile.seek(offset); // seek to last parsed data rather than last data read in to buffer

            byte[] buf = new byte[bufferSize];
            int read = accessFile.read(buf, 0, bufferSize);
            index++;// Increment whether or not read successful

            if(read > 0){

                int lastnewLine = getLastLine(read,buf);

                if(lastnewLine <= 0){ // no new line found in the buffer reset buffer size and continue
                    bufferSize = bufferSize+read;
                    continue;

                }
                else{
                    bufferSize = BYTE_BUFFER_SIZE;
                }

                readLine(buf, 0, lastnewLine); // read the lines from buffer and parse the line

                offset = offset+lastnewLine; // update the last data read

            }

        }



        // Read last chunk. The last chunk size in worst case is the total file when no newline occurs 
        if(offset < length){

            accessFile.seek(offset); 
            byte[] buf = new byte[(int) (length-offset)];
            int read = accessFile.read(buf, 0, buf.length);

            if(read > 0){

                readLine(buf, 0, read);

                offset = offset+read; // update the last data read


            }
        }


    }

}

private static void readLine(byte[] buf, int from , int lastnewLine) throws IOException{

    String readLine = "";
    BufferedReader reader = new BufferedReader(new InputStreamReader(new ByteArrayInputStream(buf,from,lastnewLine) ));
    while( (readLine =  reader.readLine()) != null){
        //do something with readLine
        System.out.println(readLine);
    }
    reader.close();
}


private static int getLastLine(int read, byte[] buf) {
    if(buf == null ) return -1;
    if(read > buf.length) read = buf.length;
    while( read > 0 && !(buf[read-1] == '\n' || buf[read-1] == '\r')) read--;       
    return read;
}   
 public static void main(String[] args) throws IOException {
    RandomAccessFile accessFile = new RandomAccessFile("C:/sri/test.log",    "r");
    readBufferedLines(0, accessFile.length(), accessFile);
    accessFile.close();

}