Overview (picocli 2.3.0 API)

Packages
Package	Description
picocli	Provides classes and interfaces for the main picocli command line parsing and `autocompletion` functionality.
picocli.groovy	Provides classes and annotations to give Groovy scripts convenient access to picocli functionality.

Picocli is a one-file framework for creating Java command line applications with almost zero code. It supports a variety of command line syntax styles including POSIX, GNU, MS-DOS and more. Generates highly customizable usage help messages with ANSI colors and styles. Picocli-based applications can have command line TAB completion showing available options, option parameters and subcommands, for any level of nested subcommands.

How it works: annotate your class and picocli initializes it from the command line arguments, converting the input to strongly typed values in the fields of your class. Then invoke CommandLine.parse or CommandLine.populateCommand with the command line parameters and an object you want to initialize.

Picocli provides a number of convenience methods that allow you to omit error handling and other boilerplate code for common use cases. Here is a small example application that uses the CommandLine.call convenience method to do parsing and error handling in one line of code.

The full user manual is hosted at http://picocli.info.

@Command(name = "checksum", description = "Prints the checksum (MD5 by default) of a file to STDOUT.")
class CheckSum implements Callable<Void> {

    @Parameters(index = "0", description = "The file whose checksum to calculate.")
    private File file;

    @Option(names = {"-a", "--algorithm"}, description = "MD5, SHA-1, SHA-256, ...")
    private String algorithm = "MD5";

    @Option(names = {"-h", "--help"}, usageHelp = true, description = "Show this help message and exit.")
    private boolean helpRequested;

    public static void main(String[] args) {
        // CheckSum implements Callable, so parsing, error handling and handling user
        // requests for usage help or version help can be done with one line of code.
        CommandLine.call(new CheckSum(), System.err, args);
    }

    @Override
    public Void call() throws Exception {
        // your business logic goes here
        byte[] fileContents = Files.readAllBytes(file.toPath());
        byte[] digest = MessageDigest.getInstance(algorithm).digest(fileContents);
        System.out.println(javax.xml.bind.DatatypeConverter.printHexBinary(digest));
        return null;
    }
}