Recognize a Java version failure
UnsupportedClassVersionError and “compiled by a more recent version of the Java Runtime” are strong signals. The report may include class-file numbers rather than familiar Java release names. The analyzer highlights this pattern and displays the runtime version when the log includes it.
A generic Exit Code 1 does not prove Java is wrong. Confirm the exception before changing runtimes, because loader and dependency failures can produce the same launcher code.
Change Java in the instance, not only Windows
Installing Java does not guarantee that a launcher will select it. Open the affected instance's Java settings and choose the intended executable, then verify the detected version. Modpack launchers can override a global setting per instance.
Use a 64-bit runtime for modern modpacks. A 32-bit runtime cannot address enough memory even if the launcher slider allows a larger number. Download Java from a reputable vendor and avoid bundled installers from unrelated download sites.
Memory and Java are separate settings
Selecting a newer Java runtime does not automatically allocate more RAM, and allocating more RAM does not solve an unsupported class version. Treat them as separate checks. Excessive allocation can also increase system swapping and make performance worse.
After changing the executable path, fully restart the launcher so child processes do not reuse old configuration. Analyze the newly generated latest.log to confirm the reported Java version changed.
Quick troubleshooting checklist
- ✓Confirm UnsupportedClassVersionError
- ✓Check the Java version in latest.log
- ✓Select Java inside the affected instance
- ✓Use a 64-bit runtime
- ✓Restart the launcher
- ✓Verify the version in a fresh log
Every launch creates a new result. After making one change, use the newly generated latest.log rather than continuing from an older report.