There are many approaches, one could use create a Program and just call the Unix tail command, however the problem with this is that the process must be installed on a Unix machine with tail installed.
The other option is to just read the file, line by line until there are no more lines, sleep for a period of time and then try to read the next line and repeat.
The question is what/how do I go about polling a file which is actively being written to. AFAIK there is no way in Java to get async notifications when a file has been modified as this is too low level (something that comes from the OS). I am also aware of no libs which support this.
I was thinking a better approach may be to sleep/poll on the modified date of a file..?
Any solutions?
If you can use Java 7 there are File Watchers and you could implement what you need.
File Notifications
For Java 6 (and previous) you would have to use Apache Commons VFS (File Monitor) lib to create your own file watcher.
If you can't go to Java 7 or use third party libs (e.g commons or jnotify) you would have to write a file watchdog your self using polling threads.
You can use java nio's WatchService to get asynchronous notifications if a file/directory has changed. This tutorial explains the way to use it.
What you can do to simplify this is to create a Thread which poll the size of the file and only reads that much i.e. it never reaches the end as such. This data read can be written to a pipe allowing your main thread to read a continous pipe.
I don't know answer. Just two tips
1. Maybe RandomAccessFile would help you.
2. Try find another open source program, which do this. (e.g. IntelliJ IDEA do exactly this)
来源：https://stackoverflow.com/questions/10118264/most-efficient-way-to-tail-poll-a-log-file-in-java