feat: Add Java and Kotlin support to sandbox

[itigges22]

Description

The sandbox currently supports Python, JavaScript, TypeScript, Go, Rust, C, C++, and Bash. Java and Kotlin are widely used and would broaden ATLAS's usefulness for more developers.

Requirements

• Add Java compilation and execution (javac + java)
• Add Kotlin compilation and execution (kotlinc + kotlin or kotlinc to JAR)
• Add language detection in sandbox/executor_server.py
• Add language aliases (e.g., java, kt, kotlin)
• Install JDK and Kotlin compiler in sandbox Dockerfile
• Add tests for both languages
• Update docs/TROUBLESHOOTING.md supported languages list

Context

The sandbox architecture is straightforward — each language needs a compile step and a run step. See existing implementations in sandbox/executor_server.py for the pattern.

[itigges22]

Status — half-shipped. The sandbox restructure in V3.1.0 (PC-191/192/193) added language-agnostic layout detection that recognizes Java and C/C++ project layouts and picks the right runner. From the CHANGELOG: "Detects Python, Node, Go, Rust, Java, C/C++ project layouts and uses the appropriate runner."

What's still missing for this ticket: actual Java / Kotlin runtime install in the sandbox container, and the executor entries. sandbox/executor_server.py still gates on python | javascript | typescript | go | rust | c | cpp | bash, and the Dockerfile doesn't install openjdk or kotlinc. Layout detection is the harder half; runtime install is mostly a Dockerfile addition + executor wiring once someone wants to claim it.

[Anuj-72]

Hi @itigges22, I'd like to pick this up!

I've read through the codebase and your status update — layout detection is already in place from V3.1.0, so this is the Dockerfile + executor wiring half.

My plan:

I'll split this into two focused PRs for cleaner review:

PR 1 — Java support:

• sandbox/Dockerfile: install openjdk-21-jdk-headless
• sandbox/executor_server.py: add "java" to SUPPORTED_LANGUAGES, normalize_language(), /languages check, /execute gate, _syntax_check_impl (javac), new execute_java() (javac → java -cp), LANGUAGE_HANDLERS registration
• v3-service/main.py: add "java" to verified_languages + .java to _ext_to_lang
• docs/TROUBLESHOOTING.md + docs/ARCHITECTURE.md: update supported language lists
• tests/infrastructure/test_sandbox_java_kotlin.py: Java execution + syntax check + error handling tests

PR 2 — Kotlin support (follow-up):

• Same pattern, using kotlinc -include-runtime -d program.jar → java -jar for the compile/run step
• Kotlin compiler installed from official GitHub release ZIP
• "kotlin" / "kt" aliases

Both PRs follow the existing execute_go / execute_rust compile-then-run pattern exactly.

Happy to start on PR 1 right away if this approach works!

[itigges22]

@Anuj-72 Approved - the two-PR split works, and your file list is right. Three tips before you start:

1. javac needs the file named after its public class (the other handlers use fixed filenames) - have execute_java() extract the class name, falling back to Main.
2. CI boots the executor on the host runner, which has no JDK - gate your tests with skipif(shutil.which("javac") is None), and include docker compose exec sandbox test output in the PR description.
3. Use --no-install-recommends for the JDK, and pin the Kotlin ZIP by version + SHA-256 (everything fetched at build time here is checksum-pinned).

Base both PRs on a fresh pull of dev. Assigning you now!