Detecting file being reopened in Java

Question

I'm working on a small Java application (Java 1.6, Solaris) that will use multiple background threads to monitor a series of text files for output lines that match a particular regex pattern and then make use of those lines. I have one thread per file; they write the lines of interest into a queue and another background thread simply monitors the queue to collect all the lines of interest across the whole collection of files being monitored.

One problem I have is when one of the files I'm monitoring is reopened. Many of the applications that create the files I'm monitoring will simply restart their logfile when they are restarted; they don't append to what's already there.

I need my Java application to detect that the file has been reopened and restart following the file.

How can I best do this?

Answer 1

You could indicate somewhere on the filesystem that indicates you are reading a given file. Suppose next to the file being read (a.txt), you create a file next to it (a.txt.lock) that indicates a.txt is being read. When your process is done with it, a.txt.lock is deleted. Every time a process goes to open a file to read it, it will check for the lock file beforehand. If there is no lockfile, its not being used. I hope that makes sense and answers your question. cheers!

Answer 2

Could you keep a record of each of the length of each file? When the current length subsequently goes back to zero or is smaller than the last time you recorded the length, you know the file has been restarted by the app?

Answer 3

using a lockfile is a solution as Jurassic mentioned.

Another way is to try and open the file while you're reading the pattern and find if the file has a new size and create time. If the create time is NOT same as when you found it, then you can be sure that it has been recreated.