# Singleplayer Server Crash: fastutil ClassNotFoundException

## Status: Open — likely outdated HytaleServer.jar (Feb 24-28 2026)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
3. **7645754** (Feb 28) — Standalone server on localhost, **FIXED by updating HytaleServer.jar**

- Reproduces 100% on singleplayer, every attempt (users 1-2)
- Multiplayer works fine for users 1-2
- 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. |

## Key Finding: Outdated HytaleServer.jar (Feb 28)

User `7645754` had the **exact same error** on their standalone localhost server but NOT on their VPS. **Fixed by replacing `HytaleServer.jar` with the current version.** The old JAR used to work but stopped — likely the bundled JRE was updated and is now incompatible with older JAR versions.

This strongly suggests the root cause for F2P launcher users is also a **stale/mismatched `HytaleServer.jar`**. The launcher may report the correct version but the actual file on disk could be from an older download.

## What We Know

- `fastutil` is bundled inside `HytaleServer.jar` (fat/shaded JAR)
- 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`
- **Replacing `HytaleServer.jar` with a fresh copy fixes the issue** (confirmed by user 3)
- The issue is NOT caused by the DualAuth agent or any launcher modification

## Fix for Users

### F2P Launcher users (Asentrix, JAYED)
1. **Delete the entire game folder**: `%LOCALAPPDATA%\HytaleF2P\release\package\game\`
2. Relaunch — launcher will re-download everything fresh
3. NOT just "repair" — full delete to ensure no stale files remain

### Standalone server users
1. Download fresh `HytaleServer.jar` from current game version
2. Replace the old JAR file

## 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

### Feb 28: Third user (7645754) — FIXED by replacing HytaleServer.jar
- Standalone server user had same crash on localhost, VPS worked fine
- **Fixed by updating `HytaleServer.jar` to match VPS version**
- Root cause likely: outdated JAR incompatible with current/updated JRE
- For F2P launcher users: need to delete game folder and force fresh re-download

## 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)