Most efficient way to tail/poll a log file in Java

Question

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?

Cratylus · Accepted Answer · 2012-04-12T06:33:17.953

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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.

edited Apr 12 '12 at 06:33

answered Apr 12 '12 at 06:18

Cratylus

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I suspect File Notifications is only for adding and removing entries. There is an ENTRY_MODIFY but I thought it was only changes to the directory entry rather than file length. – Peter Lawrey Apr 12 '12 at 07:22

score 3 · Answer 2 · answered Feb 27 '13 at 20:55

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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.

answered Feb 27 '13 at 20:55

saravanan07

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Java 7, cant use it in this case but good suggestion. – NightWolf Feb 27 '13 at 22:41

score 1 · Answer 3 · answered Apr 12 '12 at 07:24

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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.

answered Apr 12 '12 at 07:24

Peter Lawrey

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score 0 · Answer 4 · answered Apr 12 '12 at 06:19

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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)

answered Apr 12 '12 at 06:19

bugs_

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Most efficient way to tail/poll a log file in Java

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