Use Logger instead of println

src/main/java/net/fabricmc/example/ExampleMod.java

@@ -1,14 +1,19 @@
 package net.fabricmc.example;
 
 import net.fabricmc.api.ModInitializer;
+import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 
 public class ExampleMod implements ModInitializer {
+	public static final Logger LOGGER = LogManager.getLogger();
+
 	@Override
 	public void onInitialize() {
 		// This code runs as soon as Minecraft is in a mod-load-ready state.
 		// However, some things (like resources) may still be uninitialized.
 		// Proceed with mild caution.
 
-		System.out.println("Hello Fabric world!");
+		LOGGER.log(Level.INFO, "Hello Fabric world!");
 	}
 }

src/main/java/net/fabricmc/example/mixin/ExampleMixin.java

@@ -1,6 +1,8 @@
 package net.fabricmc.example.mixin;
 
+import net.fabricmc.example.ExampleMod;
 import net.minecraft.client.gui.screen.TitleScreen;
+import org.apache.logging.log4j.Level;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
@@ -10,6 +12,6 @@ import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
 public class ExampleMixin {
 	@Inject(at = @At("HEAD"), method = "init()V")
 	private void init(CallbackInfo info) {
-		System.out.println("This line is printed by an example mod mixin!");
+		ExampleMod.LOGGER.log(Level.INFO, "This line is printed by an example mod mixin!");
 	}
 }