I have the following code snippet where I'm picking up a file from an FTP server and then streaming it to OpenCSV. I have very little experience with streams, so I'd like to have someone review this code for efficiency. I use this on the cloud, so I/O and memory efficiency is important. Generally my CSV files consist of a few hundred thousand records each.

public InputStream getData(ImportTask importTask) throws IOException {
    //FTP Connection code

    try {
                //enter passive mode

                if (!ftp.setFileType(FTP.BINARY_FILE_TYPE)) {
                    System.out.println("Setting binary file type failed.");

                ByteArrayOutputStream output = new ByteArrayOutputStream();

                String file = importTask.isZipfile() ? importTask.getZipFilename() : importTask.getFilename();

                ftp.retrieveFile(file, output);
                InputStream inputStream = new ByteArrayInputStream(output.toByteArray());

                if (importTask.isZipfile()) {
                    inputStream = importUtils.getZipData(new ZipInputStream(inputStream), importTask.getFilename());


                return inputStream;
            } catch (FileNotFoundException ex) {
                throw new Exception(ex);


InputStream is;

try {
        is = dataSource.getData(importTask);
    } catch (IOException ex) {

final HeaderColumnNameTranslateMappingStrategy<MyObject> strategy = new HeaderColumnNameTranslateMappingStrategy<>();

final CsvToBean csv = new CsvToBean();
List list = csv.parse(strategy, new InputStreamReader(is));

for (Object object : list) {
    MyObject myObject = (MyObject) object;
    //Do something

Switch to uniVocity-parsers as it is twice as fast as OpenCSV and the input is read in parallel.

    final BeanListProcessor<MyObject > clientProcessor = new BeanListProcessor<MyObject>(MyObject .class);

    CsvParserSettings settings = new CsvParserSettings();

    settings.setReadInputOnSeparateThread(true); // this enabled by default

    CsvParser parser = new CsvParser(settings);
    parser.parse(new InputStreamReader(new ZipInputStream(inputStream)));

    List<MyObject> rows = clientProcessor.getBeans();

If you are not using all columns of your CSV input, you can select the rows you want as well to make the process even faster:

    settings.selectIndexes(4, 6, 3);
    settings.selectFields("Field A", "B", "and C");

Disclosure: I am the author of this library. It's open-source and free (Apache V2.0 license).

  • \$\begingroup\$ you should use var on the left hand side of assignations as the type is easily discernable from the right hand side. \$\endgroup\$ – d347hm4n Jun 9 '15 at 9:08

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