Java IO implementation of unix/linux "tail -f"

Question

I'm wondering what techniques and/or library to use to implement the functionality of the linux command "tail -f ". I'm essentially looking for a drop in add-on/replacement for java.io.FileReader. Client code could look something like this:

TailFileReader lft = new TailFileReader("application.log");
BufferedReader br = new BufferedReader(lft);
String line;
try {
  while (true) {
    line= br.readLine();
    // do something interesting with line
  }
} catch (IOException e) {
  // barf
}

The missing piece is a reasonable implementation of TailFileReader. It should be able to read parts of the file that exist before the file is opened as well as the lines that are added.

Answer 1

The ability to continue to read a file, and wait around until the file has some more updates for you shouldn't be that hard to accomplish in code yourself. Here's some pseudo-code:

BufferedReader br = new BufferedReader(...);
String line;
while (keepReading) {
    line = reader.readLine();
    if (line == null) {
        //wait until there is more of the file for us to read
        Thread.sleep(1000);
    }
    else {
        //do something interesting with the line
    }
}

I would assume that you would want to put this type of functionality in its own Thread, so that you can sleep it and not affect any other areas of your application. You would want to expose keepReading in a setter so that your main class / other parts of the application can safely shut the thread down without any other headaches, simply by calling stopReading() or something similar.

Answer 2

Here's a short story which you could use as a pointer:

I've coded TailingInputStream at work for the very same reason. It basically uses File and refreshed its contents on demand and checked against internal buffer if it has changed significantly (4kB memory stamp IIRC) and then did what the tail -f does. A bit hacky, yes, but it works perfectly and doesn't mess with Threads or anything fancy like that - it's compatible all the way back to 1.4.2 at least.

That said, it was a lot easier to do than ReverseInputStream which went from file's end to start and didn't die if the file was updated on the fly...

Answer 3

Check JLogTailer, which does this logic.