Read a file that is being written to by another process

Question

I have a file that is being written to by another process (completely different pid). I know it is writing to the file line-by-line.

From Java, I would like to read this file line-by-line as the lines are being written to it. (or as close as I can get to that).

Rather than re-inventing the wheel...is there already a common way to achieve this in Java? It would be nice to have a blocking function called .readLine().

You mean, you want to synchronize the producer and the consumer? Can you modify the sources of the writer, or isn't it under your control? Have you tried with a filesystem-level watcher? What are the supported OS? — Raffaele, Sep 24 '14 at 19:28
I think this is quite like http://stackoverflow.com/questions/557844/java-io-implementation-of-unix-linux-tail-f - I'd personally prefer the Apache Commons IO way. — sfThomas, Sep 24 '14 at 19:33

score 1 · Answer 1 · edited Jun 20 '20 at 09:12

You can use a WatchService to observe events from your operating system.

I prefer the option using the take method since it prevents your system from polling needlessly and waits for events from your operating system.

I have used this successfully on Windows, Linux and OSX - as long as Java 7 is available since this is a new feature since JDK 1.7.

Here is the solution I came up with - running in a different thread than the main thread since I didn't want this to block my UI - but this is up to you and your application architecture.

boolean run = true;
WatchService watchService = FileSystems.getDefault().newWatchService();
Path watchingPath = Paths.get("path/to/your/directory/to/watch");
watchingPath.register(watchService, StandardWatchEventKinds.ENTRY_CREATE, StandardWatchEventKinds.ENTRY_DELETE, StandardWatchEventKinds.ENTRY_MODIFY);

WatchKey key = null;
while(run && !isCancelled()) {
    try {
        key = watchService.take();
        for(WatchEvent<?> event : key.pollEvents()) {
            if(!isCancelled()) {
                if(event.context().toString().equals(fileName)) {
                    //Your file has changed, do something interesting with it.
                }
            }
        }
    } catch (InterruptedException e) {
        //Failed to watch for result file cahnges
        //you might want to log this.
        run = false;
    } catch (ClosedWatchServiceException e1) {
        run = false;
    }
    
    boolean reset = key.reset();
    if(!reset) {
        run = false;
    }
}

Read a file that is being written to by another process

1 Answers1

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