Java & JVM Setup
The recommended Java runtime, memory settings and safe launcher guidance for Fabric 1.20.1.
Recommended Java version
Use Eclipse Temurin 17, 64-bit, from the Eclipse Adoptium project. Fabric 1.20.1 uses Java 17 as its normal compatibility baseline. A newer Java version is not necessarily faster or more compatible with a large 1.20.1 modpack.
Download Temurin 17 from Eclipse Adoptium ↗
Install it on Windows
- Open the official Temurin download page and select Java 17.
- Choose the Windows 64-bit installer appropriate for your processor architecture; most desktop PCs use x64.
- Install the runtime, then restart your launcher if it was already open.
- In your launcher settings, choose the installed Java 17 executable for the PotA profile.
- Verify the profile is using Java 17 (64-bit) before launching.
Memory and JVM arguments
For most players, the default HotSpot garbage collector is a sensible choice. Java 17 uses G1GC by default on typical desktop systems, so you do not need a long list of untested JVM optimisation flags.
| System RAM | Example JVM arguments | Meaning |
|---|---|---|
| 16 GB | -Xms4G -Xmx8G | Heap may grow from 4 GB to 8 GB. |
| 32 GB | -Xms4G -Xmx10G | Heap may grow from 4 GB to 10 GB. |
| 32 GB+ (alternative) | -Xms4G -Xmx12G | A 12 GB ceiling if the pack actually needs it. |
Finding the correct Java executable
A Temurin installation usually places javaw.exe under its installation directory on Windows. Use the launcher’s browse button to locate it. The exact folder name depends on the installed Java update.
Problems caused by Java configuration
| Symptom | Check first |
|---|---|
| Game closes before the main menu | Confirm 64-bit Java 17 and remove obsolete custom JVM flags. |
| Out-of-memory error | Check the launcher memory allocation and available system RAM. |
| Launcher points to an old Java path | Re-select the Java executable after reinstalling or moving the runtime. |
| Poor performance with very high allocation | Reduce unnecessary allocation; more heap does not automatically mean higher FPS. |
What not to do
Avoid copying large “ultimate JVM flag” strings from unrelated modpacks. Keep the runtime and settings simple, then diagnose specific issues from logs if necessary. The Performance Guide covers visual settings and RAM in more detail.