Cree un programa de línea de comandos de Java con Picocli

1. Introducción

En este tutorial, nos acercaremos a la biblioteca picocli , que nos permite crear fácilmente programas de línea de comandos en Java.

Primero, comenzaremos creando un comando Hello World. Luego, profundizaremos en las características clave de la biblioteca reproduciendo, parcialmente, el comando git .

2. Hello World Command

Comencemos con algo fácil: ¡un comando Hello World!

Lo primero es lo primero, necesitamos agregar la dependencia al proyecto picocli :

 info.picocli picocli 3.9.6 

Como podemos ver, usaremos la versión 3.9.6 de la biblioteca, aunque se está construyendo una versión 4.0.0 (actualmente disponible en prueba alfa).

Ahora que la dependencia está configurada, creemos nuestro comando Hello World. Para hacer eso, usaremos la anotación @Command de la biblioteca :

@Command( name = "hello", description = "Says hello" ) public class HelloWorldCommand { }

Como podemos ver, la anotación puede tomar parámetros. Solo usamos dos de ellos aquí. Su propósito es proporcionar información sobre el comando actual y el texto del mensaje de ayuda automático.

Por el momento, no podemos hacer mucho con este comando. Para hacer que haga algo, tenemos que añadir un principal método de llamar a la conveniencia CommandLine.run (Ejecutable, String []) método . Esto toma dos parámetros: una instancia de nuestro comando, que por lo tanto tiene que implementar la interfaz Runnable , y una matriz de cadenas que representa los argumentos del comando (opciones, parámetros y subcomandos):

public class HelloWorldCommand implements Runnable { public static void main(String[] args) { CommandLine.run(new HelloWorldCommand(), args); } @Override public void run() { System.out.println("Hello World!"); } }

Ahora, cuando ejecutamos el método principal , veremos que la consola genera "¡Hola mundo!"

Cuando se empaqueta en un jar, podemos ejecutar nuestro comando Hello World usando el comando java :

java -cp "pathToPicocliJar;pathToCommandJar" com.baeldung.picoli.helloworld.HelloWorldCommand

Sin sorpresa, eso también genera el mensaje "¡Hola mundo!" cadena a la consola.

3. Un caso de uso concreto

Ahora que hemos visto los conceptos básicos, profundizaremos en la biblioteca picocli . Para hacer eso, vamos a reproducir, parcialmente, un comando popular: git .

Por supuesto, el propósito no será implementar el comportamiento del comando git , sino reproducir las posibilidades del comando git : qué subcomandos existen y qué opciones están disponibles para un subcomando peculiar.

Primero, tenemos que crear una clase GitCommand como lo hicimos para nuestro comando Hello World:

@Command public class GitCommand implements Runnable { public static void main(String[] args) { CommandLine.run(new GitCommand(), args); } @Override public void run() { System.out.println("The popular git command"); } }

4. Agregar subcomandos

El comando git ofrece muchos subcomandos: agregar, confirmar, remoto y muchos más. Nos centraremos aquí en agregar y confirmar .

Entonces, nuestro objetivo aquí será declarar esos dos subcomandos al comando principal. Picocli ofrece tres formas de lograrlo.

4.1. Uso de la anotación @Command en clases

La anotación @Command ofrece la posibilidad de registrar subcomandos a través del parámetro de subcomandos :

@Command( subcommands = { GitAddCommand.class, GitCommitCommand.class } )

En nuestro caso, agregamos dos nuevas clases: GitAddCommand y GitCommitCommand . Ambos están anotados con @Command e implementan Runnable . Es importante darles un nombre, ya que picocli usará los nombres para reconocer qué subcomando (s) ejecutar:

@Command( name = "add" ) public class GitAddCommand implements Runnable { @Override public void run() { System.out.println("Adding some files to the staging area"); } }
@Command( name = "commit" ) public class GitCommitCommand implements Runnable { @Override public void run() { System.out.println("Committing files in the staging area, how wonderful?"); } }

Por lo tanto, si ejecutamos nuestro comando principal con add como argumento, la consola dará como resultado "Agregar algunos archivos al área de preparación" .

4.2. Uso de la anotación @Command en métodos

Otra forma de declarar subcomandos es crear @Command, métodos anotados que representen esos comandos en la clase GitCommand :

@Command(name = "add") public void addCommand() { System.out.println("Adding some files to the staging area"); } @Command(name = "commit") public void commitCommand() { System.out.println("Committing files in the staging area, how wonderful?"); }

De esa manera, podemos implementar directamente nuestra lógica de negocios en los métodos y no crear clases separadas para manejarlo.

4.3. Agregar subcomandos mediante programación

Finalmente, picocli nos ofrece la posibilidad de registrar nuestros subcomandos mediante programación. Este es un poco más complicado, ya que tenemos que crear un objeto CommandLine que envuelva nuestro comando y luego agregarle los subcomandos:

CommandLine commandLine = new CommandLine(new GitCommand()); commandLine.addSubcommand("add", new GitAddCommand()); commandLine.addSubcommand("commit", new GitCommitCommand());

Después de eso, todavía tenemos que ejecutar nuestro comando, pero ya no podemos hacer uso del método CommandLine.run () . Ahora, tenemos que llamar al método parseWithHandler () en nuestro objeto C ommandLine recién creado :

commandLine.parseWithHandler(new RunLast(), args);

Debemos tener en cuenta el uso de la clase RunLast , que le dice a picocli que ejecute el subcomando más específico. Hay otros dos controladores de comandos proporcionados por picocli : RunFirst y RunAll . El primero ejecuta el comando superior, mientras que el segundo los ejecuta todos.

Cuando se usa el método de conveniencia CommandLine.run () , el controlador RunLast se usa por defecto.

5. Gestión de opciones mediante la anotación @Option

5.1. Opción sin argumento

Let's now see how to add some options to our commands. Indeed, we would like to tell our add command that it should add all modified files. To achieve that, we'll add a field annotated with the @Option annotation to our GitAddCommand class:

@Option(names = {"-A", "--all"}) private boolean allFiles; @Override public void run() { if (allFiles) { System.out.println("Adding all files to the staging area"); } else { System.out.println("Adding some files to the staging area"); } }

As we can see, the annotation takes a names parameter, which gives the different names of the option. Therefore, calling the add command with either -A or –all will set the allFiles field to true. So, if we run the command with the option, the console will show “Adding all files to the staging area”.

5.2. Option with an Argument

As we just saw, for options without arguments, their presence or absence is always evaluated to a boolean value.

However, it's possible to register options that take arguments. We can do this simply by declaring our field to be of a different type. Let's add a message option to our commit command:

@Option(names = {"-m", "--message"}) private String message; @Override public void run() { System.out.println("Committing files in the staging area, how wonderful?"); if (message != null) { System.out.println("The commit message is " + message); } }

Unsurprisingly, when given the message option, the command will show the commit message on the console. Later in the article, we'll cover which types are handled by the library and how to handle other types.

5.3. Option with Multiple Arguments

But now, what if we want our command to take multiple messages, as is done with the real git commit command? No worries, let's make our field be an array or a Collection, and we're pretty much done:

@Option(names = {"-m", "--message"}) private String[] messages; @Override public void run() { System.out.println("Committing files in the staging area, how wonderful?"); if (messages != null) { System.out.println("The commit message is"); for (String message : messages) { System.out.println(message); } } }

Now, we can use the message option multiple times:

commit -m "My commit is great" -m "My commit is beautiful"

However, we might also want to give the option only once and separate the different parameters by a regex delimiter. Hence, we can use the split parameter of the @Option annotation:

@Option(names = {"-m", "--message"}, split = ",") private String[] messages;

Now, we can pass -m “My commit is great”,”My commit is beautiful” to achieve the same result as above.

5.4. Required Option

Sometimes, we might have an option that is required. The required argument, which defaults to false, allows us to do that:

@Option(names = {"-m", "--message"}, required = true) private String[] messages;

Now it's impossible to call the commit command without specifying the message option. If we try to do that, picocli will print an error:

Missing required option '--message=' Usage: git commit -m= [-m=]... -m, --message=

6. Managing Positional Parameters

6.1. Capture Positional Parameters

Now, let's focus on our add command because it's not very powerful yet. We can only decide to add all files, but what if we wanted to add specific files?

We could use another option to do that, but a better choice here would be to use positional parameters. Indeed, positional parameters are meant to capture command arguments that occupy specific positions and are neither subcommands nor options.

In our example, this would enable us to do something like:

add file1 file2

In order to capture positional parameters, we'll make use of the @Parameters annotation:

@Parameters private List files; @Override public void run() { if (allFiles) { System.out.println("Adding all files to the staging area"); } if (files != null) { files.forEach(path -> System.out.println("Adding " + path + " to the staging area")); } }

Now, our command from earlier would print:

Adding file1 to the staging area Adding file2 to the staging area

6.2. Capture a Subset of Positional Parameters

It's possible to be more fine-grained about which positional parameters to capture, thanks to the index parameter of the annotation. The index is zero-based. Thus, if we define:

@Parameters(index="2..*")

This would capture arguments that don't match options or subcommands, from the third one to the end.

The index can be either a range or a single number, representing a single position.

7. A Word About Type Conversion

As we've seen earlier in this tutorial, picocli handles some type conversion by itself. For example, it maps multiple values to arrays or Collections, but it can also map arguments to specific types like when we use the Path class for the add command.

As a matter of fact, picocli comes with a bunch of pre-handled types. This means we can use those types directly without having to think about converting them ourselves.

However, we might need to map our command arguments to types other than those that are already handled. Fortunately for us, this is possible thanks to the ITypeConverter interface and the CommandLine#registerConverter method, which associates a type to a converter.

Let's imagine we want to add the config subcommand to our git command, but we don't want users to change a configuration element that doesn't exist. So, we decide to map those elements to an enum:

public enum ConfigElement { USERNAME("user.name"), EMAIL("user.email"); private final String value; ConfigElement(String value) { this.value = value; } public String value() { return value; } public static ConfigElement from(String value) { return Arrays.stream(values()) .filter(element -> element.value.equals(value)) .findFirst() .orElseThrow(() -> new IllegalArgumentException("The argument " + value + " doesn't match any ConfigElement")); } }

Plus, in our newly created GitConfigCommand class, let's add two positional parameters:

@Parameters(index = "0") private ConfigElement element; @Parameters(index = "1") private String value; @Override public void run() { System.out.println("Setting " + element.value() + " to " + value); }

This way, we make sure that users won't be able to change non-existent configuration elements.

Finally, we have to register our converter. What's beautiful is that, if using Java 8 or higher, we don't even have to create a class implementing the ITypeConverter interface. We can just pass a lambda or method reference to the registerConverter() method:

CommandLine commandLine = new CommandLine(new GitCommand()); commandLine.registerConverter(ConfigElement.class, ConfigElement::from); commandLine.parseWithHandler(new RunLast(), args);

This happens in the GitCommand main() method. Note that we had to let go of the convenience CommandLine.run() method.

When used with an unhandled configuration element, the command would show the help message plus a piece of information telling us that it wasn't possible to convert the parameter to a ConfigElement:

Invalid value for positional parameter at index 0 (): cannot convert 'user.phone' to ConfigElement (java.lang.IllegalArgumentException: The argument user.phone doesn't match any ConfigElement) Usage: git config    

8. Integrating with Spring Boot

Finally, let's see how to Springify all that!

Indeed, we might be working within a Spring Boot environment and want to benefit from it in our command-line program. In order to do that, we must create a SpringBootApplicationimplementing the CommandLineRunner interface:

@SpringBootApplication public class Application implements CommandLineRunner { public static void main(String[] args) { SpringApplication.run(Application.class, args); } @Override public void run(String... args) { } }

Plus, let's annotate all our commands and subcommands with the Spring @Component annotation and autowire all that in our Application:

private GitCommand gitCommand; private GitAddCommand addCommand; private GitCommitCommand commitCommand; private GitConfigCommand configCommand; public Application(GitCommand gitCommand, GitAddCommand addCommand, GitCommitCommand commitCommand, GitConfigCommand configCommand) { this.gitCommand = gitCommand; this.addCommand = addCommand; this.commitCommand = commitCommand; this.configCommand = configCommand; }

Note that we had to autowire every subcommand. Unfortunately, this is because, for now, picocli is not yet able to retrieve subcommands from the Spring context when declared declaratively (with annotations). Thus, we'll have to do that wiring ourselves, in a programmatic way:

@Override public void run(String... args) { CommandLine commandLine = new CommandLine(gitCommand); commandLine.addSubcommand("add", addCommand); commandLine.addSubcommand("commit", commitCommand); commandLine.addSubcommand("config", configCommand); commandLine.parseWithHandler(new CommandLine.RunLast(), args); }

And now, our command line program works like a charm with Spring components. Therefore, we could create some service classes and use them in our commands, and let Spring take care of the dependency injection.

9. Conclusion

In this article, we've seen some key features of the picocli library. We've learned how to create a new command and add some subcommands to it. We've seen many ways to deal with options and positional parameters. Plus, we've learned how to implement our own type converters to make our commands strongly typed. Finally, we've seen how to bring Spring Boot into our commands.

Of course, there are many things more to discover about it. The library provides complete documentation.

As for the full code of this article, it can be found on our GitHub.