// guide

Which Java version does my Minecraft server need?

Match Java to the Minecraft version your server runs: Java 25 for 26.1 and newer, Java 21 for 1.20.5 to 1.21.11, Java 17 for 1.18 to 1.20.4, Java 16 for 1.17, and Java 8 for 1.16.5 and older. Forge, NeoForge, Fabric and Quilt follow the same rule, because they run on top of Minecraft’s own server.

Updated October 2026  ·  Java Edition  ·  Forge, NeoForge, Fabric, Quilt

The chart

These come from Mojang’s own version files, which list the Java each release was built for. We last checked them on October 12, 2026: the newest release, 26.3, still lists Java 25, and so does the newest 26.4 snapshot. Minecraft moved to year-based version numbers in 2026, so 26.1 is the release after 1.21.11.

Watch the edges. 1.20.4 is a Java 17 version and 1.20.5 is a Java 21 version, even though they look almost the same. A pack’s name often hides its Minecraft version, so check the version on the pack’s page, or look it up in the per-pack table below.

Class file versions: the numbers in the error

When the Java is wrong, the server usually stops with a line like this one:

java.lang.UnsupportedClassVersionError: net/minecraft/bundler/Main has been compiled by a more recent version of the Java Runtime (class file version 69.0), this version of the Java Runtime only recognizes class file versions up to 65.0

Each Java release writes its class files with a format number one higher than the release before it, so the number minus 44 is the Java version. “Class file version 69.0” means the code needs Java 25. “Up to 65.0” means the server is running Java 21. That’s a Minecraft 26.1 or newer server started on Java 21.

No Minecraft release needs Java 26 or 27 (class file 70 or 71). If you see one of those numbers, either a mod was built for that Java or something on the server can’t read the class files of the newer Java you’re running. In both cases, go back to the Java in the chart.

The other common wording comes from ASM, the library loaders and Mixin use to read class files:

java.lang.IllegalArgumentException: Unsupported class file major version 69

This one is ambiguous. Sometimes a mod needs a newer Java than you run. Just as often, the server runs a Java that’s newer than its loader can read, like a 1.20.1 Forge pack started on Java 25 (see below). Our wrong Java version crash guide walks through every form of the error.

Loader notes

Forge 1.16.5 and older

Use Java 8; these servers start with java -jar on the Forge jar. On 1.12.2 and older that’s not just advice: Forge’s launcher fails on Java 9 or newer with java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader.

Forge 1.17+ and NeoForge

The installer writes run.sh and run.bat. Both start the server with a bare java @user_jvm_args.txt @libraries/.../unix_args.txt (win_args.txt on Windows), so the Java you get depends on your PATH. NeoForge’s scripts, and Forge’s for 1.17 to 1.20.2 (1.20.1 included), don’t check it at all. Forge’s scripts for 1.20.4 and newer first run java -jar forge-…-shim.jar --onlyCheckJava, which stops with “Current Java is 17 but we require at least 21” when the Java is too old, but nothing stops a Java that’s too new. user_jvm_args.txt only passes options to a Java that’s already running, so it can’t pick one either. NeoForge’s version numbers follow Minecraft: NeoForge 21.1.x is for Minecraft 1.21.1, and from 26.1 on the version starts with the Minecraft version (26.1.2.x is for 26.1.2). Our NeoForge server guide has the full setup.

Why Java 25 can break a 1.20.1 Forge pack

Mojang lists Java 17 for Minecraft 1.20.1, and that’s what 1.20.1 packs are built and tested on. Newer Java can fail in a specific way. Forge, its mod launcher and Mixin read class files with a library called ASM, and each ASM release can only read class files up to the newest Java it knows about. We read the library list in every Forge 1.20.1 installer from 47.2.0 to 47.4.26 to see which ASM each one uses:

Checked on October 5, 2026, from the version.json inside each installer on Forge’s Maven. 47.4.10 is Forge’s recommended 1.20.1 build and 47.4.26 the latest. ASM’s release notes date Java 25 support to ASM 9.8, in March 2025.

Every modpack pins its own Forge build. The latest All the Mods 9 (1.1.1) pins Forge 47.4.0, and 12 of the 24 Forge 1.20.1 packs from Modrinth on our list pin a build older than 47.4.9. On Java 25, their ASM can’t read Java 25’s own class files, so the server can stop with “Unsupported class file major version 69”. A newer Forge build removes that one problem, but the pack’s mods were still only tested on Java 17. Run 1.20.1 packs on Java 17.

Fabric and Quilt

Fabric starts from a small launcher jar (java -jar fabric-server-mc.1.21.1-loader.0.18.4-launcher.1.1.2.jar nogui), so the java you type is the Java you get. Fabric mods can name the Java they need in their fabric.mod.json, and many do: in Cobbleverse 1.7.42, 67 of the 135 mods it downloads list java as a dependency. When the version doesn’t fit, Fabric Loader stops with a dependency error that names the mod and the Java it wants, instead of a crash halfway through loading. Quilt reads the same field.

The exception: GregTech: New Horizons

GT New Horizons runs on Minecraft 1.7.10 but ships two server zips: one for Java 8 and one for Java 17 to 25. Use the Java the zip was made for. GTNH’s wiki calls Java 17 the most widely tested.

How to switch Java

On a Pelican or Pterodactyl panel

Each Java is its own Docker image. On the server’s Startup page, choose the image for the Java you need under Docker Image, then restart. Nothing in your server folder changes. Hosts decide which images are offered, so if yours isn’t listed, ask them. DSN runs Pelican, so this is also how you switch Java on a DSN server.

On Windows

1. Install the Java you need. Eclipse Temurin is a free build of OpenJDK: download the JRE .msi for your version from adoptium.net. A JRE is enough to run a server; a JDK also works. The installer puts it in C:\Program Files\Eclipse Adoptium\ and can add it to your PATH.
2. Open Command Prompt and run java -version. The first line names the version that java starts. where java lists every java.exe on the PATH, in the order Windows tries them.
3. With more than one Java installed, don’t fight over the PATH. Put the full path in your start script instead. In run.bat, find the line that starts with java @user_jvm_args.txt and replace that first java with the full path in quotes, for example "C:\Program Files\Eclipse Adoptium\jre-21...\bin\java.exe" (copy the real folder name from that directory).

On Linux (Debian or Ubuntu)

1. Install the Java you need. Adoptium’s apt repository has temurin-8-jre, temurin-17-jre, temurin-21-jre and temurin-25-jre; add the repository as shown on adoptium.net, then run sudo apt install temurin-21-jre (or the version you need).
2. java -version shows the default. update-alternatives --list java lists every installed java, and sudo update-alternatives --config java changes the default.
3. For a server that should always use one Java, put its full path in run.sh or your systemd unit instead of plain java, so a system update can’t switch it for you.

In Docker

The itzg/minecraft-server image has a tag per Java: :java25, :java21, :java17 and :java8.

Check it worked

Run java -version exactly the way your start script calls Java (or check the image on the Startup page), then start the server. It’s up when the console prints Done (…)! For help, type "help". If it crashes later for some other reason, the System Details section of the crash report has a Java Version line showing the Java that actually ran.

Java version by modpack

Every modpack we have a guide for, grouped by the Java its latest version needs (pack data from October 4, 2026). Where a pack also has versions for older Minecraft releases, the notes say which Java those need. Each name goes to that pack's setup guide, with its RAM needs and server files.

Jump to: Java 25 · Java 21 · Java 17 · Java 8

Java 25 (6 packs)

Java 21 (49 packs)

Java 17 (65 packs)

Java 8 (11 packs)

Pack not here? Check its Minecraft version on CurseForge or Modrinth and use the chart at the top. Our guide to making a modded server covers the rest of the setup.

FAQ