Java 21 (64-Bit) for Minecraft 1.20.5 & 1.21+

Mojang upgraded Minecraft 1.20.5 and 1.21+ to require a 64-bit Java 21 runtime; launching with Java 17 or Java 8 causes an immediate incompatible class version crash. Applies to Minecraft 1.20.5, 1.20.6, 1.21 – 1.21.4+.

Configuration

Java 21 (64-Bit) for Minecraft 1.20.5 & 1.21+

Minecraft Range

Minecraft 1.20.5, 1.20.6, 1.21 – 1.21.4+

Recommended Build

Eclipse Adoptium Temurin 21 / Microsoft OpenJDK 21

JVM Flags

-XX:+UseG1GC -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20

Why Java 21 (64-Bit) for Minecraft 1.20.5 & 1.21+ matters for Minecraft 1.20.5, 1.20.6, 1.21 – 1.21.4+

Mojang upgraded Minecraft 1.20.5 and 1.21+ to require a 64-bit Java 21 runtime; launching with Java 17 or Java 8 causes an immediate incompatible class version crash.

Inside Prism Launcher, open **Settings** → **Java** (or override per instance under **Edit Instance** → **Settings**) and click **Auto-detect** or **Download** to select `Eclipse Adoptium Temurin 21 / Microsoft OpenJDK 21`.

Recommended JVM flags and memory checks

• **JVM arguments:** `-XX:+UseG1GC -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20`. Always click **Test** in the Java settings panel to verify the runtime passes Prism's architecture check.
• **64-bit verification:** Ensure the architecture column reads `x86_64` (or `arm64`), never 32-bit `x86`, so you can allocate more than 2 GB of RAM.
• **Download launcher:** Get the latest Windows build of Prism Launcher from /download.