Crea un programma a riga di comando Java con Picocli

1. Introduzione

In questo tutorial, ci avvicineremo alla libreria picocli , che ci consente di creare facilmente programmi a riga di comando in Java.

Per prima cosa inizieremo creando un comando Hello World. Faremo quindi un'immersione profonda nelle funzionalità chiave della libreria riproducendo, parzialmente, il comando git .

2. Hello World Command

Cominciamo con qualcosa di semplice: un comando Hello World!

Per prima cosa, dobbiamo aggiungere la dipendenza al progetto picocli :

 info.picocli picocli 3.9.6 

Come possiamo vedere, useremo la versione 3.9.6 della libreria, sebbene sia in costruzione una versione 4.0.0 (attualmente disponibile in alpha test).

Ora che la dipendenza è impostata, creiamo il nostro comando Hello World. Per fare ciò, useremo l' annotazione @Command dalla libreria :

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

Come possiamo vedere, l'annotazione può assumere parametri. Ne stiamo usando solo due qui. Il loro scopo è fornire informazioni sul comando corrente e sul testo per il messaggio di aiuto automatico.

Al momento, non c'è molto che possiamo fare con questo comando. Per fargli fare qualcosa, abbiamo bisogno di aggiungere un principale metodo di chiamare la comodità (Runnable, String []) CommandLine.run metodo . Questo richiede due parametri: un'istanza del nostro comando, che quindi deve implementare l' interfaccia Runnable , e un array String che rappresenta gli argomenti del comando (opzioni, parametri e sottocomandi):

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!"); } }

Ora, quando eseguiamo il metodo principale , vedremo che la console restituisce "Hello World!"

Quando impacchettato in un vaso, possiamo eseguire il nostro comando Hello World usando il comando java :

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

Senza sorprese, viene visualizzato anche il messaggio "Hello World!" stringa alla console.

3. Un caso d'uso concreto

Ora che abbiamo visto le basi, ci immergeremo nella libreria picocli . Per fare ciò, riprodurremo, parzialmente, un popolare comando: git .

Naturalmente, lo scopo non sarà implementare il comportamento del comando git ma riprodurre le possibilità del comando git - quali sottocomandi esistono e quali opzioni sono disponibili per un particolare sottocomando.

Per prima cosa, dobbiamo creare una classe GitCommand come abbiamo fatto per il nostro 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. Aggiunta di sottocomandi

Il comando git offre molti sottocomandi: add, commit, remote e molti altri. Ci concentreremo qui sull'aggiunta e il commit .

Quindi, il nostro obiettivo qui sarà dichiarare quei due sottocomandi al comando principale. Picocli offre tre modi per ottenere questo risultato.

4.1. Utilizzo dell'annotazione @Command sulle classi

L' annotazione @Command offre la possibilità di registrare sottocomandi tramite il parametro subcommands :

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

Nel nostro caso, aggiungiamo due nuove classi: GitAddCommand e GitCommitCommand . Entrambi sono annotati con @Command e implementano Runnable . È importante dare loro un nome, poiché i nomi saranno usati da picocli per riconoscere quali sottocomandi eseguire:

@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?"); } }

Quindi, se eseguiamo il nostro comando principale con add come argomento, la console mostrerà "Aggiunta di alcuni file all'area di staging" .

4.2. Utilizzo dell'annotazione @Command sui metodi

Un altro modo per dichiarare i sottocomandi è creare metodi annotati con @Command che rappresentino quei comandi nella classe 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?"); }

That way, we can directly implement our business logic into the methods and not create separate classes to handle it.

4.3. Adding Subcommands Programmatically

Finally, picocli offers us the possibility to register our subcommands programmatically. This one's a bit trickier, as we have to create a CommandLine object wrapping our command and then add the subcommands to it:

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

After that, we still have to run our command, but we can't make use of the CommandLine.run() method anymore. Now, we have to call the parseWithHandler() method on our newly created CommandLine object:

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

We should note the use of the RunLast class, which tells picocli to run the most specific subcommand. There are two other command handlers provided by picocli: RunFirst and RunAll. The former runs the topmost command, while the latter runs all of them.

When using the convenience method CommandLine.run(), the RunLast handler is used by default.

5. Managing Options Using the @Option Annotation

5.1. Option with No Argument

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.