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https://issues.apache.org/jira/browse/IO-288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13130101#comment-13130101
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Georg Henzler commented on IO-288:
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I will provide you with Unit tests if you're interested - I tested the class
with a main class and different input files but it's easy to make Unit tests
from the main class.
Regarding your questions:
- The Tailer class listens to file changes (as the unix tail does) and notifies
a provided Listener passing the added line. The ReverseFileReader starts at the
last line of a file and moves towards the start of the file (ignoring added
lines after it has instantiated).
- We could subclass FileReader but I'm not sure how to implement e.g.
read(char[] cbuf, int off, int len)... implementing this going backward would
be hard. Mixing going forward and backward is probably not really intuitive. I
would suggest that if we implement FileReader, we throw a
UnsupportedOperationException for most of the Reader inferface's methods.
- I'm not sure of the Filename... is BufferedReverseFileReader a better name to
emphasize on the fact that it's all about the method readLine()? Any other name
suggestions?
> Supply a ReverseFileReader
> --------------------------
>
> Key: IO-288
> URL: https://issues.apache.org/jira/browse/IO-288
> Project: Commons IO
> Issue Type: New Feature
> Components: Utilities
> Reporter: Georg Henzler
> Attachments: ReverseFileReader.java
>
>
> I needed to analyse a log file today and I was looking for a
> ReverseFileReader: A class that behaves exactly like BufferedReader except
> that it goes from bottom to top when readLine() is called. I didn't find it
> in IOUtils and the internet didn't help a lot either, e.g.
> http://www.java2s.com/Tutorial/Java/0180__File/ReversingaFile.htm is a fairly
> inefficient - the log files I'm analysing are huge and it is not a good idea
> to load the whole content in the memory.
> So I ended up writing an implementation myself using little memory and the
> class RandomAccessFile - see attached file. It's used as follows:
> int blockSize = 4096; // only that much memory is needed, no matter how big
> the file is
> ReverseFileReader reverseFileReader = new ReverseFileReader(myFile,
> blockSize, "UTF-8"); // encoding is supported
> String line = null;
> while((line=reverseFileReader.readLine())!=null) {
> ... // use the line
> if(enoughLinesSeen) {
> break;
> }
> }
> reverseFileReader.close();
> I believe this could be useful for other people as well!
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