How do I parse command line arguments in Java?

Question

What is a good way of parsing command line arguments in Java?

Answer 1

It is 2020, time to do better than Commons CLI... :-)

Should you build your own Java command line parser, or use a library?

Many small utility-like applications probably roll their own command line parsing to avoid the additional external dependency. picocli may be an interesting alternative.

Picocli is a modern library and framework for building powerful, user-friendly, GraalVM-enabled command line apps with ease. It lives in 1 source file so apps can include it as source to avoid adding a dependency.

It supports colors, autocompletion, subcommands, and more. Written in Java, usable from Groovy, Kotlin, Scala, etc.

Features:

• Annotation based: declarative, avoids duplication and expresses programmer intent
• Convenient: parse user input and run your business logic with one line of code
• Strongly typed everything - command line options as well as positional parameters
• POSIX clustered short options ( -xvfInputFile as well as -x -v -f InputFile)
• Fine-grained control: an arity model that allows a minimum, maximum and variable number of parameters, e.g, "1..*", "3..5"
• Subcommands (can be nested to arbitrary depth)
• Feature-rich: composable arg groups, splitting quoted args, repeatable subcommands, and many more
• User-friendly: usage help message uses colors to contrast important elements like option names from the rest of the usage help to reduce the cognitive load on the user
• Distribute your app as a GraalVM native image
• Works with Java 5 and higher
• Extensive and meticulous documentation

The usage help message is easy to customize with annotations (without programming). For example:

(source)

I couldn't resist adding one more screenshot to show what usage help messages are possible. Usage help is the face of your application, so be creative and have fun!

Disclaimer: I created picocli. Feedback or questions very welcome.