ForgeMC 1.7.xMC 1.12.x

Legacy Forge launchwrapper ClassCastException (URLClassLoader)

Legacy Forge (Minecraft 1.12 and older) starts mods through launchwrapper, which casts the classloader to a URLClassLoader. Java 9+ replaced that classloader hierarchy, so any modern Java crashes legacy Forge at boot with a ClassCastException naming URLClassLoader. The pack is fine; the JVM is simply too new.

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How to fix it

  1. 1

    Switch the server to Java 8

    Minecraft 1.7 through 1.12 with Forge requires Java 8 specifically. On CoalHosting that is Settings → Runtime; on a self-hosted box install a Java 8 JDK and point the startup script at it.

  2. 2

    Verify the runtime actually in use

    Run java -version from the server's working directory. Panel settings and PATH can disagree — the version in the boot log header is the truth.

  3. 3

    Do not 'upgrade to be safe'

    Newer Java is not a free improvement for old packs. RLCraft, old Feed The Beast packs and most 1.12-era servers break on Java 9+ by design of launchwrapper.

  4. 4

    Re-test after the switch

    Boot once with no changes beyond Java. If a different error appears, that is the next real problem — fix them one at a time, not all at once.

Alternative causes

These can produce the same error message — worth ruling out if the steps above don't resolve it.

A single mod compiled for newer Java inside an old pack

That produces UnsupportedClassVersionError instead of a ClassCastException. Different error, different fix — see the related article.

Corrupted launchwrapper jar

Rare, but a half-copied launchwrapper produces the same cast error on the correct Java. Redownload the Forge installer jar if Java 8 still fails.

Frequently asked

Can I run a 1.12 Forge server on Java 17?

No. launchwrapper's URLClassLoader cast was removed in Java 9's module system. Legacy Forge requires Java 8 regardless of available memory or performance flags.

Does this affect Minecraft 1.16 and newer?

No. Forge dropped launchwrapper for modern versions; 1.17+ runs on Java 17 and 1.20.5+ on Java 21. The cast error only affects the 1.7–1.12 era.

Why do old packs break on new Java at all?

Java 9 changed the internal classloader APIs launchwrapper relied on. Mojang and Forge rewrote loading for newer versions, but 1.12-era Forge was never backported.

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Last reviewed 2026-09-06. If a step is wrong or out of date, tell us — we'll fix the article and the auto-pattern at the same time.