1.21.3 [SOLVED] Use Forge classes in a library

[JimiIT92]

EDIT: I had to declare the library like a mod (using the @Mod annotation and adding the mods.toml + pack.mcmeta files). By doing this it works now

Hi everyone  I'm trying to create an internal library for all my mods, and inside it I have a method that registers an Item

public static RegistryObject<Item> registerItem(final DeferredRegister<Item> registry, final String name, final Supplier<Item> itemSupplier) {
  return registry.register(name, itemSupplier);
}

When the method is called from a mod, Forge raises this error
 

Exception message: java.lang.LinkageError: loader constraint violation: loader 'SECURE-BOOTSTRAP' @6bedbc4d wants to load class net.minecraftforge.registries.DeferredRegister. A different class with the same name was previously loaded by 'TRANSFORMER' @6d672bd4. (net.minecraftforge.registries.DeferredRegister is in module [email protected] of loader 'TRANSFORMER' @6d672bd4, parent loader 'bootstrap')

Both the version of Forge included in the library's build.gradle and the mod's build.gradle are the same.
I've never done this, so there's a good chance I'm configuring something wrong.
This is the build.gradle file of the library (which is part of a multi-module project)
 

plugins {
    id 'net.minecraftforge.gradle' version '[6.0.24,6.2)'
}

repositories {
    gradlePluginPortal()
    maven {
        name = 'MinecraftForge'
        url = 'https://maven.minecraftforge.net/'
    }
}

java.toolchain.languageVersion = JavaLanguageVersion.of(21)

minecraft {
    mappings channel: 'official', version: minecraft_version
    reobf = false
    copyIdeResources = true
}

dependencies {
    minecraft "net.minecraftforge:forge:${minecraft_version}-${forge_version}"
    implementation('net.sf.jopt-simple:jopt-simple:5.0.4') { version { strictly '5.0.4' } }
}

tasks.withType(JavaCompile).configureEach {
    options.encoding = 'UTF-8'
}

 

Edited November 18 by JimiIT92
Solved