# Singleplayer Server Crash: fastutil ClassNotFoundException

## Status: Open — NO SOLUTION (Feb 24-27 2026)

## Symptom

Singleplayer server crashes immediately on boot:

```
Exception in thread "main" java.lang.NoClassDefFoundError: it/unimi/dsi/fastutil/objects/ObjectArrayList
    at com.hypixel.hytale.plugin.early.EarlyPluginLoader.<clinit>(EarlyPluginLoader.java:34)
    at com.hypixel.hytale.Main.main(Main.java:36)
Caused by: java.lang.ClassNotFoundException: it.unimi.dsi.fastutil.objects.ObjectArrayList
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(Unknown Source)
```

Server exits with code 1. Multiplayer works fine for the same user.

## Affected Users

1. **ヅ𝚃 JAYED !** (Feb 24) — Windows x86_64, had AOT cache errors before fastutil crash
2. **Asentrix** (Feb 27) — Windows x86_64 (NT 10.0.26200.0), RTX 4060, Launcher v2.4.4, NO AOT cache errors

- Reproduces 100% on singleplayer, every attempt
- Multiplayer works fine for both users
- macOS/Linux users are NOT affected

## Ruled Out (confirmed via debug builds)

| Suspect | Tested | Result |
|---------|--------|--------|
| **DualAuth Agent** | Debug build with agent completely disabled (`debug-no-agent` tag) | **Same crash.** Agent is innocent. |
| **`-Xshare:off` (CDS)** | Added to `JAVA_TOOL_OPTIONS` in launcher code (`debug-xshare-off` tag) | **Did not help.** CDS is not the cause. |
| **`-XX:+UseCompactObjectHeaders`** | Stripped via wrapper | **Did not help.** Server has `-XX:+IgnoreUnrecognizedVMOptions` anyway. |
| **Corrupted game files** | User did repair + full reinstall | **Same crash.** |
| **Java wrapper** | Logs confirm wrapper works correctly | Not the cause. |
| **ARM64/Parallels** | User is on standard Windows x86_64 | Not applicable. |
| **AOT cache** | Asentrix has no AOT errors (JAYED did), both crash the same way | Not the root cause. |

## What We Know

- `fastutil` is bundled inside `HytaleServer.jar` (fat/shaded JAR) — same JAR for all users
- JVM's `BuiltinClassLoader` cannot find `it.unimi.dsi.fastutil.objects.ObjectArrayList` despite it being in the JAR
- Crash happens at `EarlyPluginLoader` static initializer (line 34) which imports `ObjectArrayList`
- The bundled JRE is identical for all users (downloaded by launcher)
- The issue is **not** caused by anything the F2P launcher adds — it's the vanilla server JVM failing to load its own classes

## Remaining Theories

1. **Antivirus/security software** — Windows Defender or third-party AV intercepting JAR file reads. Real-time scanning + fat JAR = known conflict. **Untested** — user should try disabling AV temporarily.
2. **Windows Insider build** — Asentrix is on NT 10.0.26200.0 (Windows 11 Dev/Insider). Bleeding-edge Windows may have JVM compatibility issues.
3. **File locking** — Stalled `java.exe` processes holding `HytaleServer.jar` open (Asentrix had stalled processes killed at every launch).
4. **Corrupted JRE on disk** — Despite being the same download, filesystem or AV may have corrupted specific JRE files on their system.

## Next Steps to Try

1. **Disable Windows Defender** temporarily — the only quick test left
2. **Delete bundled JRE** and let launcher re-download — rules out local JRE corruption
3. **Ask if official Hytale singleplayer works** — if it also crashes, it's their system (but F2P users may not have access)

## Update History

### Feb 24: First report (JAYED)
User reported singleplayer crash. Initial investigation found AOT cache errors + fastutil ClassNotFoundException. Stripping `-XX:+UseCompactObjectHeaders` did not help.

### Feb 27: Second report (Asentrix), extensive debugging
- Asentrix hit same crash, no AOT errors — ruled out AOT as root cause
- Built `debug-xshare-off`: added `-Xshare:off` to `JAVA_TOOL_OPTIONS` — **did not help**
- Built `debug-no-agent`: completely disabled DualAuth agent — **same crash**
- **Conclusion**: Neither the agent nor CDS is the cause. The JVM itself cannot load classes from the fat JAR on these specific Windows systems.
- Note: wrapper `injectArgs` append AFTER `-jar`, so they cannot inject JVM flags — only `JAVA_TOOL_OPTIONS` works for JVM flags

## Related

- Java wrapper config: `backend/core/config.js` (stripFlags / injectArgs)
- DualAuth Agent: v1.1.12, package `ws.sanasol.dualauth`
- Game version at time of reports: `2026.02.19-1a311a592`
- Debug tags: `debug-xshare-off`, `debug-no-agent`
- Log submission IDs: `c88e7b71` (Asentrix initial), `0445e4dc` (xshare test), `748dceeb` (no-agent test)