# PIP-343: Use picocli instead of jcommander

# Motivation

We use the [jcommander](https://github.com/cbeust/jcommander) to build the CLI tool, which is a good library, and is
stable, but it misses modern CLI features likes autocompletion, flag/command suggestion, native image, etc.

These features are very important because there are many commands in the CLI, but the jcommander doesn't give friendly
hints when we use incorrect flags/commands, which makes the user experience not very friendly.

In modern times, the [picocli](https://github.com/remkop/picocli) supports these features, which is a popular library.

The following is some comparison between jcommander and picocli:

- Error prompt:
  ```
  bin/pulsar-admin clusters update cluster-a -b

  # jcommander
  Need to provide just 1 parameter

  # picocli
  Unknown option: '-b'
  ```

- Command suggestion:
  ```
  bin/pulsar-admin cluste

  # jcommander
  Expected a command, got cluste

  # picocli
  Unmatched argument at index 0: 'cluste'
  Did you mean: pulsar-admin clusters?
  ```

# Goals

## In Scope

Use the picocli instead of the jcommander in our CLI tool:

- bin/pulsar
- bin/pulsar-admin
- bin/pulsar-client
- bin/pulsar-shell
- bin/pulsar-perf

I'm sure this will greatly improve the user experience, and in the future we can also consider using native images to
reduce runtime, and improve the CLI document based on picocli.

## Out Scope

This PR simply replaces jcommander and does not introduce any enhancements.

In the CLI, [autocomplete](https://picocli.info/autocomplete.html) is an important feature, and after this PIP is
complete I will make a new PIP to support this feature.

# Detailed Design

## Design & Implementation Details

The jcommander and picocli have similar APIs, this will make the migration task very simple.

This is [utility argument syntax](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html):

```
utility_name[-a][-b][-c option_argument]
    [-d|-e][-f[option_argument]][operand...]
```

1. Use `@Command` instead of `@Parameters` to define the class as a command:

  ```java
  @Command(name = "my-command", description = "Operations on persistent topics")
  public class MyCommand {

  }
  ```

2. Use `@Option` instead of `@Parameter` to defined the option of command:

  ```java
  @Option(names = {"-r", "--role"})
  private String role;
  ```

3. Use `@Parameters` to get the operand of command:

  ```java
  @Parameters(description = "persistent://tenant/namespace/topic", arity = "1")
  private String topicName;
  ```

4. Migrate jcommander converter to picocli converter:

  ```java
  public class TimeUnitToMillisConverter implements ITypeConverter<Long> {
    @Override
    public Long convert(String value) throws Exception {
      return TimeUnit.SECONDS.toMillis(RelativeTimeUtil.parseRelativeTimeInSeconds(value));
    }
  }
  ```

5. Add the picocli entrypoint:

  ```java
  @Command
  public class MyCommand implements Callable<Integer> {
    // Picocli entrypoint.
    @Override
    public Integer call() throws Exception {
        // TODO
        // run();
        return 0;
    }
  }
  ```

The above is a common migration approach, and then we need to consider pulsar-shell and custom command separately.

- pulsar-shell

  This is an interactive shell based on jline3 and jcommander, which includes pulsar-admin and pulsar-client commands.
  The jcommander does not provide autocompletion because we have implemented it ourselves. In picocli, they
  have [picocli-shell-jline3](https://github.com/remkop/picocli/blob/main/picocli-shell-jline3) to help us quickly build
  the interactive shell.

- custom command:

  This is an extension of pulsar-admin, and the plugin's implementation does not depend on jcommander. Since the bridge
  is used, we only need to change the generator code based on picocli.

# Backward & Forward Compatibility

Fully compatible.

# Links

* Mailing List discussion thread: https://lists.apache.org/thread/ydg1q064cd11pxwz693frtk4by74q32f
* Mailing List voting thread: https://lists.apache.org/thread/1bpsr6tkgm00bb66dt2s74r15o4b37s3
