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I want to access a log file that is locked by a third party Java application. The file is locked for the whole day and will be released the next day. However, my objective is to read it now using RandomAccessFile (must use this class because I need to start/store the last position while reading) without waiting until tomorrow.

Currently, I can read the log only if I unlock it with a file Unlocker software. Can anyone suggest any jar/utilities that I can use in my Java program to meet my objective?

Brock Adams
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Joe Ijam
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    A little similar to your problem? http://stackoverflow.com/questions/1555883/locking-and-unlocking-files-using-the-java-api Though the solution provided isn't very elegant, it will work for the time being. – darnir Nov 10 '11 at 09:51
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    Also, can you identify what application is locking your log file? Is it an external 3rd party app? Or some other piece of your own code? If it is your own code somewhere, this might be of some help: http://www.javaspecialists.eu/archive/Issue190.html – darnir Nov 10 '11 at 09:53
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    Another similar problem: http://stackoverflow.com/questions/5680588/in-java-is-there-a-way-to-read-a-file-when-that-file-is-locked-by-other-thread – Xeno Lupus Nov 13 '11 at 12:13
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    It is 3rd party apps who blocks the file. I have to use Unblocker Assistant Software to kill the block then i can access the file at the same time the 3rd party software still running as usual. So is there any libs/software that can integrate with my apps to unblock it? – Joe Ijam Nov 18 '11 at 02:49
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    Just out of curiosity. Doesn't it work to open the file i read only mode? Like new RandomAccessFile(file, "r") Then you can store the position when reading. You can open it later in "rw" if you need to write once the lock has been released. – Oystein Aadland Dec 02 '11 at 08:25
  • Of cause it is working if it realize the lock. The problem is, it will be unlock on the next day(daily pattern file). What i want is immediately. – Joe Ijam Dec 05 '11 at 04:01

3 Answers3

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Assuming you're using a Microsoft operating system:
The software Shadow Copy is using using Microsoft's (VSS) to copy locked files. You could use the software from within the Java Runtime Environment or perhaps make use of the VSS-API yourself via Java Native Interface.

My approach would be to the file and then access the content through it's copy. The downside is that you're possibly reading outdated information if the file has been updated since your copy operation.

However, this is just a guess as I'm not familiar with this topic.

xmoex
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  • I agreed with you this can solve my objective only if the file is small. However to copy a huge file is not a good idea within every 3 seconds. That why i am using RandamAccessFile just to access the specific location in the file. – Joe Ijam Dec 12 '11 at 03:17
  • @JoeIjam: The VSS uses copy-on-write, so it only writes a bit of header information. The file's contents aren't copied until one of the files is written to, and then only the changed areas are copied. Even so, using shadow copies for this is complete overkill. It's better to just break the original process' lock, unpleasant though that is. – Sam Hanes Jan 24 '12 at 11:08
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You can lock/unlock files and folders in Java but only by application that locked them (you programmed). However there is no Java method/class which can unlock file used by other process.

You should bundle your application with another (native) software. For example you could create shell script for Linux systems and execute it. In Java application detect in which OS it is running so you can execute proper script/software.

When application requires RW lock, system must ensure that no one else have rights to modify it, thats why you need to kill process that is using it.

If you have access to source code of that 3rd party Java application (that is actually locking file you need), then you could implement server side which will listen requests for unlocking file and approval for locking it back again.

By my opinion better approach would be to transfer file by that application to yours, then do what you want and 3rd party app can run without interruption (shouldn't be noticeable). If you need to modify it, then 3rd should wait, your modifies and send back an updated version, 3rd continue to work.

DRAX
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I don't see any reliable tool to do such job, my first mind is to try to avoid the lock while exposing the file as a service through any servlet or any other mechanism. The servlet reads the file once then deliver its contents as plain text (or stream)... No more lock contention

HTH Jerome

romje
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