Restarting Minecraft can take a hefty amount of time. Thankfully, there are tools that allow you to apply some changes while the game is running, also called hotswapping.

In Eclipse or IntelliJ IDEA, run Minecraft in debug mode. To apply changes in code, click the “Build” button in IntelliJ IDEA or save in Eclipse. Note: this only allows you to change method bodies. If you do any other kind of change, you will have to restart. However, if you use a special JDK, DCEVM (up to Java 11), you will be able to do most changes, including adding and removing methods and classes. For Java 11+ use the JetBrains Runtime and add -XX:+AllowEnhancedClassRedefinition as a VM option to your run configuration:

After you make changes to assets such as textures and block/item models, you can rebuild the project and press F3 + T to apply changes without restarting Minecraft. More specifically, this is how to reload anything the mod provides as a resource pack.

You can apply any changes made in the data/ directory such as recipes, loot tables and tags by rebuilding the project and then using the in-game command /reload. More specifically, this reloads anything the mod provides as a data pack.

You can also hotswap mixins by using Mixin Java agent. For how to achieve that, see Fabric Docs page and mixin_hotswaps.