How do I parse command line arguments in Java?

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

Check these out:

• http://commons.apache.org/cli/
• http://www.martiansoftware.com/jsap/

Or roll your own:

• http://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html

---

For instance, this is how you use commons-cli to parse 2 string arguments:

import org.apache.commons.cli.*;

public class Main {


    public static void main(String[] args) throws Exception {

        Options options = new Options();

        Option input = new Option("i", "input", true, "input file path");
        input.setRequired(true);
        options.addOption(input);

        Option output = new Option("o", "output", true, "output file");
        output.setRequired(true);
        options.addOption(output);

        CommandLineParser parser = new DefaultParser();
        HelpFormatter formatter = new HelpFormatter();
        CommandLine cmd = null;//not a good practice, it serves it purpose 

        try {
            cmd = parser.parse(options, args);
        } catch (ParseException e) {
            System.out.println(e.getMessage());
            formatter.printHelp("utility-name", options);

            System.exit(1);
        }

        String inputFilePath = cmd.getOptionValue("input");
        String outputFilePath = cmd.getOptionValue("output");

        System.out.println(inputFilePath);
        System.out.println(outputFilePath);

    }

}

usage from command line:

$> java -jar target/my-utility.jar -i asd                                                                                       
Missing required option: o

usage: utility-name
 -i,--input <arg>    input file path
 -o,--output <arg>   output file

Take a look at the more recent JCommander.

I created it. I’m happy to receive questions or feature requests.

I have been trying to maintain a list of Java CLI parsers.

• Airline
  • Active Fork: https://github.com/rvesse/airline
• argparse4j
• argparser
• args4j
• clajr
• cli-parser
• CmdLn
• Commandline
• DocOpt.java
• dolphin getopt
• DPML CLI (Jakarta Commons CLI2 fork)
• Dr. Matthias Laux
• Jakarta Commons CLI
• jargo
• jargp
• jargs
• java-getopt
• jbock
• JCLAP
• jcmdline
• jcommander
• jcommando
• jewelcli (written by me)
• JOpt simple
• jsap
• naturalcli
• Object Mentor CLI article (more about refactoring and TDD)
• parse-cmd
• ritopt
• Rop
• TE-Code Command
• picocli has ANSI colorized usage help and autocomplete

> It is 2022, 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 (<command> -xvfInputFile as well as <command> -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.

Someone pointed me to args4j lately which is annotation based. I really like it!

I've used JOpt and found it quite handy: http://jopt-simple.sourceforge.net/

The front page also provides a list of about 8 alternative libraries, check them out and pick the one that most suits your needs.

This is Google's command line parsing library open-sourced as part of the Bazel project. Personally I think it's the best one out there, and far easier than Apache CLI.

<https://github.com/pcj/google-options>

Installation

Bazel

maven_jar(
    name = "com_github_pcj_google_options",
    artifact = "com.github.pcj:google-options:jar:1.0.0",
    sha1 = "85d54fe6771e5ff0d54827b0a3315c3e12fdd0c7",
)

Gradle

dependencies {
  compile 'com.github.pcj:google-options:1.0.0'
}

Maven

<dependency>
  <groupId>com.github.pcj</groupId>
  <artifactId>google-options</artifactId>
  <version>1.0.0</version>
</dependency>

Usage

Create a class that extends OptionsBase and defines your @Option(s).

package example;

import com.google.devtools.common.options.Option;
import com.google.devtools.common.options.OptionsBase;

import java.util.List;

/**
 * Command-line options definition for example server.
 */
public class ServerOptions extends OptionsBase {

  @Option(
      name = "help",
      abbrev = 'h',
      help = "Prints usage info.",
      defaultValue = "true"
    )
  public boolean help;

  @Option(
      name = "host",
      abbrev = 'o',
      help = "The server host.",
      category = "startup",
      defaultValue = ""
  )
  public String host;

  @Option(
    name = "port",
    abbrev = 'p',
    help = "The server port.",
    category = "startup",
    defaultValue = "8080"
    )
    public int port;

  @Option(
    name = "dir",
    abbrev = 'd',
    help = "Name of directory to serve static files.",
    category = "startup",
    allowMultiple = true,
    defaultValue = ""
    )
    public List<String> dirs;

}

Parse the arguments and use them.

package example;

import com.google.devtools.common.options.OptionsParser;
import java.util.Collections;

public class Server {

  public static void main(String[] args) {
    OptionsParser parser = OptionsParser.newOptionsParser(ServerOptions.class);
    parser.parseAndExitUponError(args);
    ServerOptions options = parser.getOptions(ServerOptions.class);
    if (options.host.isEmpty() || options.port < 0 || options.dirs.isEmpty()) {
      printUsage(parser);
      return;
    }

    System.out.format("Starting server at %s:%d...\n", options.host, options.port);
    for (String dirname : options.dirs) {
      System.out.format("\\--> Serving static files at <%s>\n", dirname);
    }
  }

  private static void printUsage(OptionsParser parser) {
    System.out.println("Usage: java -jar server.jar OPTIONS");
    System.out.println(parser.describeOptions(Collections.<String, String>emptyMap(),
                                              OptionsParser.HelpVerbosity.LONG));
  }

}

<https://github.com/pcj/google-options>

I know most people here are going to find 10 million reasons why they dislike my way, but nevermind. I like to keep things simple, so I just separate the key from the value using a '=' and store them in a HashMap like this:

Map<String, String> argsMap = new HashMap<>();
for (String arg: args) {
    String[] parts = arg.split("=");
    argsMap.put(parts[0], parts[1]);
} 

You could always maintain a list with the arguments you are expecting, to help the user in case he forgot an argument or used a wrong one... However, if you want too many features this solution is not for you anyway.

Take a look at the Commons CLI project, lots of good stuff in there.

Yeap.

I think you're looking for something like this: <http://commons.apache.org/cli>

>The Apache Commons CLI library provides an API for processing command line interfaces.

Maybe these

• JArgs command line option parsing suite for Java - this tiny project provides a convenient, compact, pre-packaged and comprehensively documented suite of command line option parsers for the use of Java programmers. Initially, parsing compatible with GNU-style 'getopt' is provided.

• ritopt, The Ultimate Options Parser for Java - Although, several command line option standards have been preposed, ritopt follows the conventions prescribed in the opt package.

If you are already using Spring Boot, argument parsing comes out of the box.

If you want to run something after startup, implement the ApplicationRunner interface:

@SpringBootApplication
public class Application implements ApplicationRunner {

  public static void main(String[] args) {
    SpringApplication.run(Application.class, args);
  }

  @Override
  public void run(ApplicationArguments args) {
    args.containsOption("my-flag-option"); // test if --my-flag-option was set
    args.getOptionValues("my-option");     // returns values of --my-option=value1 --my-option=value2 
    args.getOptionNames();                 // returns a list of all available options
    // do something with your args
  }
}

Your run method will be invoked after the context has started up successfully.

If you need access to the arguments before you fire up your application context, you can just simply parse the application arguments manually:

@SpringBootApplication
public class Application implements ApplicationRunner {

  public static void main(String[] args) {
    ApplicationArguments arguments = new DefaultApplicationArguments(args);
    // do whatever you like with your arguments
    // see above ...
    SpringApplication.run(Application.class, args);
  }

}

And finally, if you need access to your arguments in a bean, just inject the ApplicationArguments:

@Component
public class MyBean {

   @Autowired
   private ApplicationArguments arguments;

   // ...
}

I wrote another one: http://argparse4j.sourceforge.net/

Argparse4j is a command line argument parser library for Java, based on Python's argparse.

If you are familiar with gnu getopt, there is a Java port at: http://www.urbanophile.com/arenn/hacking/download.htm.

There appears to be a some classes that do this:

• http://docs.sun.com/source/816-5618-10/netscape/ldap/util/GetOpt.html
• http://xml.apache.org/xalan-j/apidocs/org/apache/xalan/xsltc/cmdline/getopt/GetOpt.html

[airline @ Github][1] looks good. It is based on annotation and is trying to emulate Git command line structures.

[1]: https://github.com/airlift/airline "Airline is a Java annotation-based framework for parsing Git like command line structures."

Argparse4j is best I have found. It mimics Python's argparse libary which is very convenient and powerful.

I want to show you my implementation: ReadyCLI

Advantages:

• for lazy programmers: a very small number of classes to learn, just see the two small examples on the README in the repository and you are already at 90% of learning; just start coding your CLI/Parser without any other knowledge; ReadyCLI allows coding CLIs in the most natural way;
• it is designed with Developer Experience in mind; it largely uses the Builder design pattern and functional interfaces for Lambda Expressions, to allow a very quick coding;
• it supports Options, Flags and Sub-Commands;
• it allows to parse arguments from command-line and to build more complex and interactive CLIs;
• a CLI can be started on Standard I/O just as easily as on any other I/O interface, such as sockets;
• it gives great support for documentation of commands.

I developed this project as I needed new features (options, flag, sub-commands) and that could be used in the simplest possible way in my projects.

If you want something lightweight (jar size ~ 20 kb) and simple to use, you can try [argument-parser][1]. It can be used in most of the use cases, supports specifying arrays in the argument and has no dependency on any other library. It works for Java 1.5 or above. Below excerpt shows an example on how to use it:

public static void main(String[] args) {
	String usage = "--day|-d day --mon|-m month [--year|-y year][--dir|-ds directoriesToSearch]";
    ArgumentParser argParser = new ArgumentParser(usage, InputData.class);
    InputData inputData = (InputData) argParser.parse(args);
    showData(inputData);
    
    new StatsGenerator().generateStats(inputData);
}

More examples can be found [here][2] [1]: https://github.com/shekhar-himanshu/argument-parser [2]: https://easy-develop.github.io/argument-parser/1.0.0/examples

As one of the comments mentioned earlier (https://github.com/pcj/google-options) would be a good choice to start with.

One thing I want to add-on is:

1. If you run into some parser reflection error, please try use a newer version of the guava. in my case:

   maven_jar( name = "com_google_guava_guava", artifact = "com.google.guava:guava:19.0", server = "maven2_server", )

   maven_jar( name = "com_github_pcj_google_options", artifact = "com.github.pcj:google-options:jar:1.0.0", server = "maven2_server", )

   maven_server( name = "maven2_server", url = "http://central.maven.org/maven2/";, )

2. When running the commandline:

   bazel run path/to/your:project -- --var1 something --var2 something -v something

3. When you need the usage help, just type:

   bazel run path/to/your:project -- --help

For Spring users, we should mention also https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/core/env/SimpleCommandLinePropertySource.html and his twin brother https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/core/env/JOptCommandLinePropertySource.html (JOpt implementation of the same functionality). The advantage in Spring is that you can directly bind the command line arguments to attributes, there is an example here https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/core/env/CommandLinePropertySource.html