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+# 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