Skip to content


Folders and files

Last commit message
Last commit date

Latest commit


Repository files navigation


docs-badge Java CI

External-Memory Sorting in Java: useful to sort very large files using multiple cores and an external-memory algorithm.

The versions 0.1 of the library are compatible with Java 6 and above. Versions 0.2 and above require at least Java 8.

This code is used in Apache Jackrabbit Oak as well as in Apache Beam and in Spotify scio.

Code sample


//... inputfile: input file name
//... outputfile: output file name
// next command sorts the lines from inputfile to outputfile
int numLinesWritten = ExternalSort.mergeSortedFiles(ExternalSort.sortInBatch(new File(inputfile)), new File(outputfile));
// you can also provide a custom string comparator, see API

Code sample (CSV)

For sorting CSV files, it might be more convenient to use CsvExternalSort.


// provide a comparator
Comparator<CSVRecord> comparator = (op1, op2) -> op1.get(0).compareTo(op2.get(0));
//... inputfile: input file name
//... outputfile: output file name
//...provide sort options
CsvSortOptions sortOptions = new CsvSortOptions
				.Builder(comparator, CsvExternalSort.DEFAULTMAXTEMPFILES, CsvExternalSort.estimateAvailableMemory())
// container to store the header lines
ArrayList<CSVRecord> header = new ArrayList<CSVRecord>();

// next two lines sort the lines from inputfile to outputfile
List<File> sortInBatch = CsvExternalSort.sortInBatch(file, null, sortOptions, header);
// at this point you can access header if you'd like.
int numWrittenLines = CsvExternalSort.mergeSortedFiles(sortInBatch, outputfile, sortOptions, true, header);

The numHeader parameter is the number of lines of headers in the CSV files (typically 1 or 0) and the skipHeader parameter indicates whether you would like to exclude these lines from the parsing.

API Documentation

Maven dependency

You can download the jar files from the Maven central repository:

You can also specify the dependency in the Maven "pom.xml" file:


How to build

  • get the java jdk
  • Install Maven 2
  • mvn install - builds jar (requires signing)
  • or mvn package - builds jar (does not require signing)
  • mvn test - runs tests