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https://issues.apache.org/jira/browse/IO-288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13210186#comment-13210186
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Lars Kolb edited comment on IO-288 at 2/17/12 12:58 PM:
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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 bytes). This actually results in a
Reverse org.apache.hadoop.util.LineReader.
was (Author: weihnachtsmann):
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 bytes). 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!
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