Java code to tail -N a text file

This code allows you to go over the last N lines of a specified file. It has a "head" method, which simply allows you to go over the file from the beginning.

import java.io.*;
import java.nio.channels.FileChannel;
import java.nio.CharBuffer;
import java.nio.ByteBuffer;
import java.util.Iterator;
import java.util.NoSuchElementException;public class MMapFile {

    public class MMapIterator implements Iterator<String> {
        private int offset;

        public MMapIterator(int offset) {
            this.offset = offset;
        }
        
        public boolean hasNext() {
            return offset < cb.limit();
        }

        public String next() {
            ByteArrayOutputStream sb = new ByteArrayOutputStream();
            if (offset >= cb.limit())
                throw new NoSuchElementException();
            for (; offset < cb.limit(); offset++) {
                byte c = (cb.get(offset));
                if (c == '\n') {
                    offset++;
                    break;
                }
                if (c != '\r') {
                    sb.write(c);
                }

            }
            try {
                return sb.toString("UTF-8");
            } catch (UnsupportedEncodingException e) {}
            return sb.toString();
        }

        public void remove() {

        }
    }


    private ByteBuffer cb;
    long size;
    private long numLines = -1;
    public MMapFile(String file) throws FileNotFoundException, IOException {
        FileChannel fc = new FileInputStream(new File(file)).getChannel();
        size = fc.size();
        cb = fc.map(FileChannel.MapMode.READ_ONLY, 0, fc.size());
    }

    public long getNumLines() {
        if (numLines != -1) return numLines;  //cache number of lines
        long cnt = 0;
        for (int i=0; i <size; i++) {
            if (cb.get(i) == '\n')
                cnt++;
        }
        numLines = cnt;
        return cnt;
    }

    public Iterator<String> tail(long lines) {
        long cnt=0;
        long i=0;
        for (i=size-1; i>=0; i--) {
            if (cb.get((int)i) == '\n') {
                cnt++;
                if (cnt == lines+1)
                    break;
            }
        }
        return new MMapIterator((int)i+1);
    }

    public Iterator<String> head() {
        return new MMapIterator(0);
    }

    static public void main(String[] args) {
        try {
            Iterator<String> it = new MMapFile("/test.txt").head();
            while (it.hasNext()) {
                System.out.println(it.next());
            }
        } catch (Exception e) {
            
        }

        System.out.println();

        try {
            Iterator<String> it = new MMapFile("/test.txt").tail(2);
            while (it.hasNext()) {
                System.out.println(it.next());
            }
        } catch (Exception e) {

        }

        System.out.println();

        try {
            System.out.println("lines: "+new MMapFile("/test.txt").getNumLines());
        } catch (Exception e) {

        }

    }

}

The technique is to simply map the file into memory using java.nio.channels.FileChannel.map in the Java NIO library and manipulate the file data using memory techniques.

For the "tail" function, we walk back the mapped bytes counting newlines. The MMapIterator class conveniently provides a way to iterate over lines once we find the starting line.

There is a point where care must be taken in the MMapIterator.next() implementation. That is making sure that bytes are converted to the appropriate string encoding. We use "UTF-8" but if you are dealing with a different encoding in the input file, this should be changed.