The Minecraft Java Version for Mods

Minecraft requires a specific Java major version depending on which Minecraft version you are running. Mods are compiled against the same target, so a mod for 1.21 will not run on a Java 17 runtime.

Three versions cover essentially everything currently played.

The error is UnsupportedClassVersionError, and it includes a class file version number that tells you exactly what was expected.

Class file version 65.0 means Java 21. 61.0 means Java 17. 60.0 is Java 16, and 52.0 is Java 8. The number is always the Java version plus 44, which is a piece of trivia that saves looking it up.

The message names the class that failed to load, which usually identifies the mod. But the fix is the same regardless of which mod triggered it: change the runtime, not the mod.

Open a terminal and run java -version. It prints the version of whatever Java is first on your system path.

The important caveat: this is not necessarily the Java that Minecraft uses. The official launcher bundles its own runtimes and ignores your system installation entirely, which is why the official launcher rarely has this problem.

To see what the game actually used, look at the crash report's system details section, which records the Java version and the path it came from.

Mojang ships Java runtimes with the launcher and picks the correct one per profile automatically. If you only ever use the official launcher, you may never encounter a Java version problem.

The exception is when a profile has been configured with a custom executable, which some older guides recommend. If the JVM path field in a profile's settings points somewhere specific, that is what will be used, and it may be the wrong version.

Clearing that field returns the profile to the bundled runtime and fixes the problem outright.

Prism Launcher, MultiMC and similar tools do not bundle runtimes by default. They use whatever Java you point them at, per instance.

Prism will usually detect installed Java versions and can download one for you if none is suitable. Open the instance settings, find the Java section, and confirm the selected version matches the table above.

The advantage of this arrangement is that each instance can use a different Java version, which is exactly what you want if you play both a 1.12.2 pack and a current one. The disadvantage is that it is one more thing to get wrong.

Download from Adoptium, which publishes free, well-maintained OpenJDK builds for every platform. Take the version you need, install it, and point your launcher at it.

You can have several Java versions installed simultaneously. They do not conflict, and instance launchers can select between them per instance. There is no need to uninstall an old one to add a new one.

Avoid the oracle.com consumer Java download unless you specifically want it. It installs a browser-oriented runtime and its licensing has been changed more than once. Adoptium is what the Minecraft community actually uses.

A server has no launcher managing runtimes for it. It uses whatever java command is on the path, which is frequently the wrong one on a machine that has had Java installed for some other purpose.

The fix is to invoke the specific runtime by full path in your start script rather than relying on the path. It is explicit, it survives system updates, and it removes an entire category of confusion.

On a rented server, the host usually offers a Java version selector in the control panel. Set it to match your pack rather than leaving it on the default.

There is a tempting assumption that installing the newest Java available will satisfy everything. It will not.

Minecraft versions are tested against specific Java versions, and running 1.16.5 on Java 21 fails — often with obscure errors rather than a clean version message, because older Forge builds use internals that newer Java removed.

Match the table. Newer is not compatible with older in this direction, and 'I installed the latest Java' is a common reason an old pack stopped working.

If you see UnsupportedClassVersionError, read the class file version number and subtract 44 to get the Java version needed. Compare that with what your launcher is using. Install the correct one from Adoptium if you do not have it, point the instance at it, and relaunch.

If you are on the official launcher, check whether a custom Java path has been set on that profile and clear it. That resolves nearly every case there.

Anyone playing both modern and classic packs will end up with two or three Java installations, and it is worth being deliberate about it.

Install Java 8, 17 and 21 from Adoptium and leave them all in place. Configure each instance to use the correct one explicitly rather than relying on a system default, which can change when something else updates.

Note the choice somewhere. When a pack breaks six months later, knowing which Java it was working under is the fastest route back.

Adoptium and similar sites offer both a JRE and a JDK, and the distinction confuses people who only want to play a game.

A JRE is the runtime alone: enough to run Java programs, nothing more. A JDK is the runtime plus development tools — a compiler, a debugger and various utilities.

For playing Minecraft, either works. The JRE is smaller. For running a server, either works too.

Take the JDK if you are unsure, because some launchers and tools expect utilities that ship only with it, and the extra disk space is trivial. The only situation where it genuinely matters is if you intend to write mods yourself, where a JDK is required.

Occasionally someone installs a 32-bit Java build, usually by taking the default download on an older page, and then cannot work out why they are unable to allocate more than about 1.5GB of memory.

A 32-bit process cannot address more than roughly 2GB regardless of how much physical memory the machine has. Setting -Xmx4G on a 32-bit runtime either fails to start or silently caps.

Every modern machine should be running a 64-bit build. Check with java -version, which prints '64-Bit Server VM' on a correct installation.

If you find a 32-bit runtime, uninstall it and take the x64 build. This is rare in 2026 but it still catches people using older hardware or following outdated guides.

A specific and confusing case: the game launches, most mods load, and one fails with a version error while everything else is fine.

This means that single mod was compiled for a newer Java than the rest. It happens when an author updates their build tooling ahead of the ecosystem, and it is a bug on their side rather than a misconfiguration on yours.

Check the mod's page and issue tracker — this kind of thing is usually reported within days and fixed shortly after. In the meantime, an older build of that mod will normally work.

Do not raise your Java version to accommodate one mod if the rest of your pack targets an older one. You will fix one failure and create several.