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Troubleshooting: JVM Arguments: Aikar's Flags, G1GC & Generational ZGC

1 minHow-to
How-tofix aikars flags and g1gc zgc jvm arguments errorprism launcher java crash modded 1.12.2 – 1.21.4+unsupportedclassversionerror prism launcher

Step-by-step fix for Java version mismatches and JVM launch errors on Modded 1.12.2 – 1.21.4+.

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Target Runtime
JVM Arguments: Aikar's Flags, G1GC & Generational ZGC
Minecraft Target
Modded 1.12.2 – 1.21.4+
Vendor
Prism Launcher Java Settings → JVM Arguments

Diagnosing JVM Arguments: Aikar's Flags, G1GC & Generational ZGC startup failures

If your instance on **Modded 1.12.2 – 1.21.4+** exits immediately after clicking Launch, open the **Minecraft Log** tab. An `UnsupportedClassVersionError` or `Incompatible magic value` means the instance is overriding the global runtime with the wrong Java version.

Follow our JVM Arguments: Aikar's Flags, G1GC & Generational ZGC reference to point the instance at a verified 64-bit `javaw.exe`.

Prism Launcher screenshot showing the instance management user-interface
Each instance has its own version, mods, settings and worlds.

3-step repair in Prism Launcher

  1. Right-click the failing instance → **Edit** → **Settings** → **Java**.
  2. Tick **Java installation**, click **Auto-detect** (or download via Prism), and choose **Prism Launcher Java Settings → JVM Arguments**.
  3. Click **Test** to confirm the runtime starts cleanly, then launch your world. Need a fresh launcher install? Visit /download.

Frequently asked questions

What if clicking Test says 'Could not reserve enough space for object heap'?

You either selected a 32-bit Java installation or set Maximum Memory higher than the free physical RAM currently available on Windows.

Should I check 'Skip Java compatibility checks'?

Only enable that toggle for special backported modpacks (such as GTNH Java 17/21 builds on 1.7.10); for normal packs, switch to the matching Java version instead.

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Prism Launcher on Windows showing several modpacks and instances