A small Java 21 CLI to transform JSON files.

Built as a compact “vitrine” project: executable fat jar (java -jar), real file I/O, clean structure, unit tests, and explicit exit codes.

Requirements

• Java 21
• Maven 3.8+

Build

mvn clean test
mvn clean package

The executable jar is created in the target/ directory.

Usage

Show help:

java -jar target/java-cli-data-tool-*.jar --help
java -jar target/java-cli-data-tool-*.jar transform --help

Operation: flatten

Flatten nested objects using dot notation. Arrays are kept as-is.

java -jar target/java-cli-data-tool-*.jar transform \
  --in samples/input-nested.json \
  --op flatten

Write to a file:

java -jar target/java-cli-data-tool-*.jar transform \
  --in samples/input-nested.json \
  --op flatten \
  --out build/output.json

Operation: pick

Keep only selected top-level fields (comma-separated list). Missing fields are ignored.

java -jar target/java-cli-data-tool-*.jar transform \
  --in samples/input-nested.json \
  --op pick \
  --fields user,age

Exit codes

• 0 : OK
• 2 : Usage error (invalid or missing CLI arguments — handled by Picocli)
• 3 : Input error (missing/unreadable input file or output I/O error)
• 4 : Parse error (invalid JSON)
• 5 : Processing error (unsupported operation, missing --fields for pick, etc.)

Project structure

• fr.manooweb.cli Entry point and Picocli commands

• fr.manooweb.core Transformation logic (flatten, pick)

• fr.manooweb.io JSON read/write helpers (Jackson)

• fr.manooweb.error Domain exceptions and exit code mapping

Tech stack

• Java 21
• Maven
• Picocli
• Jackson (tree model)
• JUnit 5