[jira] [Commented] (IO-288) Supply a ReversedLinesFileReader

["Lars Kolb (Commented) (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/IO-288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13210186#comment-13210186 ] 

Lars Kolb commented on IO-288:
------------------------------

Works like a charm, thanks Georg.

I had a similar requirement. I wanted to "chunk-wise" read a HDFS file backwards to allow file browsing similar to the Hadoop namenode's web interface. By clicking a button a user triggers to fetch the previous N lines starting from a specific offset.

With a few changes to your ReversedLinesFileReader implementation I was able to implement this functionality. I would suggest to extend your ReversedLinesFileReader to be able to operate on InputStreams and to return the number of consumed bytes ((i.e. the number of bytes actually read for "line construction" not the number of buffered)). This actually results in a Reverse org.apache.hadoop.util.LineReader.
                
> Supply a ReversedLinesFileReader 
> ---------------------------------
>
>                 Key: IO-288
>                 URL: https://issues.apache.org/jira/browse/IO-288
>             Project: Commons IO
>          Issue Type: New Feature
>          Components: Utilities
>            Reporter: Georg Henzler
>             Fix For: 2.2
>
>         Attachments: ReversedLinesFileReader0.3.zip
>
>
> I needed to analyse a log file today and I was looking for a ReversedLinesFileReader: 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
> ReversedLinesFileReader reversedLinesFileReader = new ReversedLinesFileReader (myFile, blockSize, "UTF-8"); // encoding is supported
> String line = null;
> while((line=reversedLinesFileReader.readLine())!=null) {
>   ... // use the line
>   if(enoughLinesSeen) {
>      break;  
>   }
> }
> reversedLinesFileReader.close();
> I believe this could be useful for other people as well!

--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators: https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira