How would I erase the beginning of a file?

Question

I want to be able to erase the beginning of a file, while writing to the end...WITHOUT reading the whole file and writing it again because that would be a waste of resourses. Obviously I know that option is available.
My situation is that I am adding lines to a log file, and I want to remove any entries that are X hours old. Logging a lot of information would mean that every few seconds, an entry is older than X hours and needs to be removed. It doesnt have a set number of lines so I can't just recycle the file by doing something like: if at line number Y, start at line 1 and remember the current line number.
Adding to the top of the file, pushing the others lines down, and making cuts at the end would also work, but the same situation arises, how do I add to the beginning of the file, without having to copy the entire file to add the entry to the top.
The reason I don't want to read the whole file is because having to read the whole file and write the whole file when an entry is made is time consuming and wastes CPU and memory, especially when the logs are coming in several lines a second.

Answer 1

I would think it could easily be done. say you want to erase the first 10 bytes, why cant you just say move the start of the file up 10 bytes. Somewhere in the filesystem the start of the file is stored. I found underlying file copying that claims this in FileChannel's transferTo and transferFrom:
This method is potentially much more efficient than a simple loop that reads from this channel and writes to the target channel. Many operating systems can transfer bytes directly from the filesystem cache to the target channel without actually copying them.
If I can't just chop, I'll have to copy quickly at scheduled intervals.
This is code I ran to test the above quote:
    public static void main(String[] args) throws Exception{
        args = new String[2];
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Enter File Name:");
        args[0] = input.readLine();
        System.out.println("Enter test (0-2):");
        args[1] = input.readLine();
        long time = System.currentTimeMillis();
        int version = Integer.parseInt(args[1]);
        switch (version) {
            case 0:
                BufferedReader in = new BufferedReader(new FileReader(args[0]));
                PrintWriter out = new PrintWriter(new FileWriter("tempfile.txt"));
                in.readLine();//skip line.
                for (String line = in.readLine();line != null;line = in.readLine()) {
                    out.println(line);
                in.close();
                out.close();
                break;
            case 1:
                RandomAccessFile raf = new RandomAccessFile(args[0],"rw");
                String line = raf.readLine();
                long offset = raf.getFilePointer();
                byte[] b = new byte[1024];
                int bytes = 1024;
                long index = 0;
                System.out.println("index = " + raf.getFilePointer());
                System.out.println("index = " + line.length());
                while(bytes == 1024) {
                    bytes = raf.read(b);
                    index = raf.getFilePointer();
                    raf.seek(index - bytes - offset);
                    raf.write(b,0,bytes);
                    raf.seek(index);
                raf.setLength(raf.length()-offset);
                raf.close();
                break;
            case 2:
                RandomAccessFile raf2 = new RandomAccessFile(args[0],"rw");
                String line2 = raf2.readLine();
                long offset2 = raf2.getFilePointer();
                FileChannel f = raf2.getChannel();
                FileChannel g = f.position(0);
                g.transferTo(offset2,raf2.length() - offset2,f);
                raf2.setLength(raf2.length()-offset2);
                raf2.close();
                break;
        System.out.println(System.currentTimeMillis() - time);
    }and had these results
166MB Log File
Linux Mandrake 900Mhz
Case 0 = 32914 ms
Case 1 (1k byte[]) = 15043 ms
Case 1 (16k byte[]) = 6988 ms
Case 1 (1MB byte[] = 6113 ms
Case 2 = 6188 ms
:-/ guess this proves that the FileChannel can't chop