args4j-helpers

larry ogrodnek - 26 Feb 2014

I really like using args4j for command-line parsing in both java, and scala, but I found myself writing the same boilerplately code to parse options, deal with help, deal with parsing issues, etc. args4j-helpers is a project that simplifies parsing with args4j.

It provides typical option parsing error handling:

  • If help (provided via OptionWithHelp base trait bound to --help and -h) is requested, print usage information to stderr, exit with code 0.
  • If a required option is missing (required=true), print usage information to stderr, exit with code 1 (unless help was requested).

This typical parsing code:

class Options {
  import org.kohsuke.args4j.Option

  @Option(name="--help", aliases=Array("-h"), usage="show this message")
  var help = false

  @Option(name="--blah", aliases=Array("-b"), usage="some val", metaVar="BLAH")
  var blah: Int = 0
}

def main(args: Array[String]) {
  val options = new Options
  val parser = new CmdLineParser(options)

  try {
    parser.parseArgument(args : _*)

    if (options.help) {
      parser.printUsage(System.err)
      sys.exit(0)
    }
  } catch {
    case e: CmdLineException => {
      System.err.println(e.getMessage)
      parser.printUsage(System.err)
      sys.exit(1)
    }
  }
}

can be simplified as:

class Options extends OptionsWithHelp {
  import org.kohsuke.args4j.Option

  @Option(name="--blah", aliases=Array("-b"), usage="some val", metaVar="BLAH")
  var blah: Int = 0
}

def main(args: Array[String]) {
  val options = optionsOrExit(args, new Options)
}

Additionally, the helper class handles the case where help is requested and required arguments are missing (which is missing from the simplified boilerplate code).

The code is in scala, and available on github.

In the future, I'd like to extend it to add better support for more scala-ish types (Option, Seq, etc., which should be mostly possible by implementing additional args4j OptionHandlers).

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