When it comes to developing rich command-line applications in Java, Picocli is a star player that you shouldn’t miss out on. This modern library provides a simple and elegant way to create applications that can interact with users through the terminal. In this guide, you’ll learn how to effectively use Picocli.

Why Choose Picocli?

Picocli revolutionizes command-line interface (CLI) development by offering:

• Ease of use with annotations and a programmatic API
• Support for nested subcommands and user-friendly help messages
• Fast startup times with GraalVM native images
• Advanced features like TAB autocompletion and color customization

Installation

To get started, you need to add Picocli as a dependency to your project. Here’s how to set it up:

• Gradle: implementation info.picocli:picocli:4.7.6
• Maven:

  
          <dependency>
              <groupId>info.picocli</groupId>
              <artifactId>picocli</artifactId>
              <version>4.7.6</version>
          </dependency>
      

• Scala SBT: libraryDependencies += "info.picocli" % "picocli" % "4.7.6"
• Grapes (Groovy): @Grab(group="info.picocli", module="picocli", version="4.7.6")

Creating Your First Command Line Application

To create a simple Picocli application, you can start by annotating a class as follows:

import picocli.CommandLine;
import picocli.CommandLine.Option;
import picocli.CommandLine.Parameters;

import java.io.File;

@Command(name = "example", mixinStandardHelpOptions = true, version = "Picocli example 4.0")
public class Example implements Runnable {
    @Option(names = {"-v", "--verbose"}, description = "Verbose mode. Helpful for troubleshooting.")
    private boolean[] verbose = new boolean[0];

    @Parameters(arity = "1..*", paramLabel = "FILE", description = "File(s) to process.")
    private File[] inputFiles;

    public void run() {
        if (verbose.length > 0)
            System.out.println(inputFiles.length + " files to process...");
        if (verbose.length > 1)
            for (File f : inputFiles)
                System.out.println(f.getAbsolutePath());
    }

    public static void main(String[] args) {
        int exitCode = new CommandLine(new Example()).execute(args);
        System.exit(exitCode);
    }
}

Understanding the Code with an Analogy

Think of creating a command-line application like designing a menu for a restaurant. Each command can be thought of as a “dish” offered on your menu:

• The @Command annotation is like the restaurant’s sign, announcing the menu item name.
• The @Option and @Parameters annotations represent ingredients, providing the necessary information to make your dish (command) complete.
• The run method is the chef in the kitchen, executing the recipe when the dish is ordered (command executed).

Just as a chef uses ingredients to create a dish, your application uses command-line arguments to perform actions based on user input.

Troubleshooting Common Issues

If you run into issues while using Picocli, here are some troubleshooting tips:

• Ensure that your class is annotated correctly.
• Double-check that all required libraries are included in your project.
• If you are experiencing issues with argument parsing, utilize the @Spec annotation to debug your command object model.
• For further assistance, check the [FAQ](https://github.com/remkopp/picocli/wiki/FAQ) section of the Picocli documentation.

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Conclusion

In conclusion, Picocli is a powerful tool that simplifies the process of building command-line applications in Java. With its rich features, you can easily manage arguments and create a user-friendly experience.

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