Loom allows semi-automatic updating of the mappings used in a Java codebase. Due to frequent changes in Yarn, this can be a useful tool for keeping a codebase up-to-date with the newest changes.

Note: This automated process currently does not handle Mixins or reflection, instances of these will need to be manually updated.

Say you want to migrate from 1.16.5 yarn to 1.17.1 yarn.

1. Go here, select the version to migrate to, and copy the Gradle command under “Mappings Migration”, for example gradlew migrateMappings --mappings "1.17.1+build.40". DO NOT modify your gradle.properties or build.gradle yet.
2. Run the command in the root of your Gradle project.
3. Your migrated sources will appear in remappedSrc. Verify that the migration produced valid migrated code.
4. Copy the sources from remappedSrc to the original folder. Keep the original sources backed up just in case.
5. Update your gradle.properties file according to the instructions in the first site.
6. Refresh the Gradle project in your IDE.
7. Check and update any Mixin targets that may be outdated.

If you want to go from Mojang's official mappings, AKA mojmap, to yarn, make sure your mappings in build.gradle is set to loom.officialMojangMappings() before running migrateMappings. For more information, check out the dedicated Mappings page.

• Specify from where to take your Java files with --input path/to/source. Default: src/main/java.
• Specify where to output the remapped source with --output path/to/output. Default: 'remappedSrc'. You can use src/main/java here to avoid having to copy the remapped classes, but make sure you have a backup.
• Specify a custom place to retrieve the mappings from with --mappings some_group:some_artifact:some_version:some_qualifier. Default: net.fabricmc:yarn:<version-you-inputted>:v2. Use net.minecraft:mappings:<minecraft-version> to migrate to official Mojang mappings.

Loom uses Mercury to remap Java source code, for problems with remapping please report issues to their issue tracker, or discuss it through their communications channel (irc.esper.net #cadix).

Some assembly required.

1. Figure out your target mappings version. For example, “net.fabricmc:yarn:1.14.1 Pre-Release 2+build.2”.
2. Make sure the mappings for this version get created. This is the hacky part, as currently the only way to do it is to edit the “minecraft” and “mappings” fields in a build.gradle to the new version, run any Gradle command (“gradle build” will do, even if it crashes), then change the fields back.
3. Run the following magical wizardry command: gradle migrateMappings -PtargetMappingsArtifact="net.fabricmc:yarn:1.14.1 Pre-Release 2+build.2" -PinputDir=src/main/java -PoutputDir=remappedSrc, where:
   • “targetMappingsArtifact” refers to the target mappings version. It is imperative that the build.gradle be set to the current mappings version of the mod when running this command!
   • “inputDir” is the input directory, containing Java source code,
   • “outputDir” is the output directory. It will be created if it is missing.
4. Copy the remapped source code to the input directory, if everything's fine.
5. Hope for the best.

Note: You may need to specify the full paths in quotes, try this if you get file not found issues.

This should work across Minecraft versions as well, provided we haven't massively broken Intermediaries or done something equally silly (aka: most of the time).