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Trying to read a log file line-by-line (in Java). This log file is being written to simultaneously by another process (non-java program).

I have 2 approaches -

  1. BufferedReader (BufferedReader br = new BufferedReader(new FileReader(logFile));)
  2. RandomAccessFile (RandomAccessFile accessFile = new RandomAccessFile(logFile.getAbsolutePath(), "r");)

Do both these approaches cause the file to be locked till i call the 'close' method on the BufferedReader/RandomAccessFile object ?

Are there any other ways (Java) to read a file in such a way that the file is not locked/blocked for other processes/programs ?

PS - in all my searches, I have come across multiple answers/solutions (old and new) to this problem. I just wish to seek clarification/closure on this issue.

Quest Monger
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    (terse, but perhaps useful) http://stackoverflow.com/questions/2537306/java-opening-and-reading-from-a-file-without-locking-it , (also related) http://stackoverflow.com/questions/1073274/in-java-what-is-the-best-safest-pattern-for-monitoring-a-file-being-appended-to –  Nov 18 '12 at 22:47
  • yea saw that. that post throws up good options, but no definitive answers. plus, its 2 years old. maybe java-7 has something new ? – Quest Monger Nov 18 '12 at 22:51
  • Refer this answer https://stackoverflow.com/a/53424839/4090550 – Anil Agrawal Nov 22 '18 at 06:15

2 Answers2

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Take a look at this: Java: opening and reading from a file without locking it

If the non-Java process has an exclusive lock on the file when it is writing, no other process may be able to read it. Of course, this may vary slightly with operating system. You might want to look at what the operating system says about the file lock when the non-Java process is running.

If a shared lock is available, then java.nio may allow more control over the locking behaviour: http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html

Community
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Akber Choudhry
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  • Windows introduces an additional state: *exclusive read* (when not FILE_SHARE_READ), so how [does] the standard Java IO work on Windows when "reading" a file? Will it try for shared read or not? :( –  Nov 19 '12 at 01:10
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BufferedReader is preferable to RandomAccessFile on performance grounds, but neither of them will lock the file at all unless the operating system kindly does so for you, in which case closing the file will release it.

However the operating system may also kindly prevent you reading the file at all if someone else is writing to it.

Reading a sequential file while another process is writing to it is not good design. You shouldn't really be reading log files at all. Log files are for humans. You should be using a database.

user207421
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  • So in particular: what about "read exclusive" (or "no flag") and "read shared" modes in, say, Windows? (I bring up Windows because common "*IX" operating systems generally don't boast such access modes where special flags need to be set for non "exclusive write" files.) –  Nov 19 '12 at 01:05
  • i guess this response answers a part of my question. If possible, can someone quote any references for this statement - ' but neither of them will lock the file at all unless the operating system kindly does so for you'. Btw, i know i shouldn't be messing with locked files, but i am trying to understand the bigger problem here, which is the locking of files in Java during I/O. – Quest Monger Nov 22 '12 at 06:28
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    @QuestMonger Windows locks a file you have open for output so no-one else can read it. Java doesn't lock *any* file other than via explicit NIO locks, but I believe it also doesn't do anything to prevent what Windows does by default. – user207421 Nov 22 '12 at 06:55