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A JNI error has occurred

Java application startup error: wrong Java version, corrupted archive, or launching the wrong file.

更新于 2026年8月13日

Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.UnsupportedClassVersionError

The message appears at startup, before the server itself starts.

What it means

Almost always, it's a Java version mismatch: the file was compiled for a newer version than the one running. The second line usually says so directly — UnsupportedClassVersionError.

Less often, it's a corrupted or wrong file: for example, a client .jar is being launched instead of the server one.

What to do

  1. Check which Java your game version needs — Java version. In Millida it's selected automatically, so the error almost always points to your own .jar.
  2. Check that the server file is being launched, not the client one.
  3. Re-upload the file: a corrupted upload produces the same message.

Unable to access jarfile — the file isn't found; here the file is found but incompatible.

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