Roastd / Minecraft

Java is too new for this modpack

What it looks like in your log

java.lang.IllegalArgumentException: Unsupported class file major version 65

What causes it

The Java running this server is newer than the modpack can load, so its mods fail before the server starts.

How to fix it

Run the server with the Java version its Minecraft needs (1.18-1.20.4 packs need Java 17, 1.20.5+ needs Java 21), from adoptium.net.

Why it happens

The mirror image of Java being too old. Older modloaders read class files with a library that has a hard-coded ceiling on the class version it understands, and a Java newer than that ceiling breaks them before the server starts. Forge for 1.16 and 1.18 packs is the usual case: it needs Java 8 or 17 respectively and falls over on Java 21.

Fix it step by step

1. 1Find the pack's Minecraft versionIt is in the pack name, the manifest, or the server jar name.
2. 2Install the matching Java1.16 and earlier: Java 8. 1.17: Java 16. 1.18 to 1.20.4: Java 17. 1.20.5 and later: Java 21. Adoptium has every one.
3. 3Point the server at itA panel Java setting, or the start script calling the right java by path. Keep the newer Java installed for other servers.
4. 4RestartThe loader should now get past mod discovery.

If that didn't work

If the Java matches the game and a single mod still fails with this error, the mod is built for a newer game version than the pack runs. Remove it or find its build for your version.

Errors that travel with this one

• Java is too old for this softwarehas been compiled by a more recent version of the Java Runtime (class file version 65.0), this version of the Java Runtime only recognizes class file versions up to 61.0
• Java is too old for this Minecraft versionjava.lang.UnsupportedClassVersionError: net/minecraft/server/Main has been compiled by a more recent version of the Java Runtime
• The Java runtime itself crashed# A fatal error has been detected by the Java Runtime Environment:
• The operating system killed the server for using too much memoryMinecraft server exited with code 137
• The server died with a segmentation faultMinecraft server exited with code 139
• The server was told to stopMinecraft server exited with code 143