"Unsupported class file major version", "UnsupportedClassVersionError" or a server that closes right after you start it? Nine times out of ten it is the wrong Java version.
Note the jump in names: after 1.21.11 Mojang switched to year-based numbers, so 26.1 is the first version of 2026.
More: Paper vs Spigot vs Purpur vs Fabric · How to make a Minecraft server
The table
| Minecraft version | Java you need |
|---|---|
| 1.16.5 and older | Java 8 |
| 1.17 – 1.17.1 | Java 16 |
| 1.18 – 1.20.4 | Java 17 |
| 1.20.5 – 1.21.11 | Java 21 |
| 26.1 and newer (26.1, 26.2, 26.3 …) | Java 25 |
How to check and fix it
- Run
java -versionin the same terminal or panel that starts your server. - If it is too old, install Java 25 (for example Eclipse Temurin or another OpenJDK build) and point your start script at it, e.g.
/usr/lib/jvm/temurin-25/bin/java -Xms4G -Xmx4G -jar server.jar nogui. - On a host panel, pick Java 25 in the startup settings ("Docker image" or "Java version").
- Running several versions? Keep each Java in its own folder and use the full path in each start script.
Plugins and mods
Updating Java is safe for old versions too (a 1.20.4 server runs fine on Java 21), but very old plugins can break on new Java. Update Paper, plugins and mods together, and make a backup first.More: Paper vs Spigot vs Purpur vs Fabric · How to make a Minecraft server