All The Mods 10 requires Java 21. This guide explains the required Java version, whether Java 25 works, how to set Java correctly on your server, and which JVM arguments to use.
ATM10 Java Version Requirement
ATM10 requires Java 21.
ATM10 runs on Minecraft 1.21.1 with NeoForge. The NeoForge loader for 1.21.1 targets the Java 21 LTS release. Running any other Java version will cause launch failures or instability.
| Java Version | ATM10 Compatible | Notes |
|---|---|---|
| Java 8 | No | Too old, will not launch |
| Java 11 | No | Not supported by NeoForge 1.21.1 |
| Java 17 | No | Works for 1.20.x only, not 1.21.1 |
| Java 21 | Yes | Required and recommended |
| Java 25 | Partially | See section below |
Does ATM10 Work on Java 25?
Java 25 is the next LTS release. As of mid-2026:
- Singleplayer: ATM10 on Java 25 works for most players but some mods report compatibility issues with newer JDK internals
- Server: Java 25 is not officially recommended for ATM10 servers. Some mods use internal Java APIs that changed in Java 25, causing warnings or rare crashes
- Recommendation: Use Java 21 for all ATM10 servers until NeoForge and major mods publish explicit Java 25 support
If you are seeing errors like InaccessibleObjectException or ClassNotFoundException on Java 25 with ATM10, downgrade to Java 21.
How to Check Your Java Version
Run this command:
java -version
Correct output for ATM10:
openjdk version "21.0.x"
If you see Java 17 or lower, update before running ATM10.
Installing Java 21
Linux (Ubuntu/Debian)
sudo apt update
sudo apt install openjdk-21-jdk -y
java -version
Set as default if multiple Java versions are installed:
sudo update-alternatives --config java
Select Java 21 from the menu.
Linux (Fedora/RHEL)
sudo dnf install java-21-openjdk-devel -y
Windows
Download OpenJDK 21 from adoptium.net, run the installer, ensure PATH is updated, then verify with java -version in a new terminal.
ATM10 JVM Arguments for Java 21
Aikar Flags (Most Common, Stable)
-Xms10G -Xmx14G -XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:+AlwaysPreTouch -XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40 -XX:G1HeapRegionSize=8M -XX:G1ReservePercent=20 -XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4 -XX:InitiatingHeapOccupancyPercent=15 -XX:G1MixedGCLiveThresholdPercent=90 -XX:G1RSetUpdatingPauseTimePercent=5 -XX:SurvivorRatio=32 -XX:+PerfDisableSharedMem -XX:MaxTenuringThreshold=1 -Daikars.new.flags=true
Generational ZGC (Java 21+, Lower Pause Times)
For servers with 16 GB or more RAM:
-Xms14G -Xmx16G -XX:+UseZGC -XX:+ZGenerational -XX:+UnlockExperimentalVMOptions -XX:+AlwaysPreTouch -XX:+DisableExplicitGC -XX:+ParallelRefProcEnabled
ZGC with generational mode reduces garbage collection pauses significantly. Use Aikar flags if you experience instability.
How Much RAM to Allocate for ATM10
| Server Size | -Xmx Setting | Leave Free |
|---|---|---|
| 1-3 players | 10G | 2 GB for OS |
| 3-6 players | 12G | 2-4 GB for OS |
| 6-10 players | 14G | 4 GB for OS |
| 10-15 players | 16G | 4 GB for OS |
| 15-20 players | 20G | 4-6 GB for OS |
Never allocate 100% of available RAM to -Xmx. The operating system needs headroom.
Common Java Errors with ATM10
Error: Unsupported class file major version - Wrong Java version. ATM10 needs Java 21.
Error: OutOfMemoryError: Java heap space - Your -Xmx value is too low. Increase it and restart.
Error: Could not reserve enough space for object heap - Machine does not have enough RAM for the -Xms value set. Lower -Xms to 8G.
Server starts but immediately crashes with NeoForge - Usually wrong Java version. Check with java -version.
FAQ
What Java version does ATM10 use in 2026? Java 21. NeoForge on Minecraft 1.21.1 requires Java 21. Java 17 and older will not work.
Does ATM10 work with Java 25? Partially. Some mods have compatibility issues with Java 25 internals. Stick with Java 21 for stable server operation.
What is the best JVM flag setup for ATM10? Aikar flags with G1GC are the standard recommendation for most ATM10 servers. If you have 16+ GB RAM and Java 21+, try generational ZGC for lower pause times.
How much RAM should I set with -Xmx for ATM10? Set -Xmx to your total RAM minus 2-4 GB. For a 16 GB server, use -Xmx14G.
Can I use Adoptium OpenJDK 21 for ATM10? Yes. Adoptium Temurin 21 is a well-tested choice for Minecraft servers.
Related guides: ATM10 server requirements, ATM10 RAM requirements, ATM9 vs ATM10
