src/main/java/net/fabricmc/example/client/ExampleClientMod.java

@@ -0,0 +1,14 @@
+package net.fabricmc.example.client;
+
+import net.fabricmc.api.ClientModInitializer;
+
+public class ExampleClientMod implements ClientModInitializer {
+	@Override
+	public void onInitializeClient() {
+		// This code runs on the client side only, and should be used to set up
+		// client-side logic, such as render or integrated-server tweaks.
+		// Proceed with mild caution.
+
+		System.out.println("Hello client world!");
+	}
+}

src/main/resources/fabric.mod.json

@@ -20,6 +20,9 @@
   "entrypoints": {
     "main": [
       "net.fabricmc.example.ExampleMod"
+    ],
+    "client": [
+      "net.fabricmc.example.client.ExampleClientMod"
     ]
   },
   "mixins": [