Remove log4j from compile classpath starting with Minecraft 22w03a.

This is done to force modders to use SLF4J as Mojang have the ability to remove it at anytime. An option is provided to disable this.

src/main/java/net/fabricmc/loom/api/LoomGradleExtensionAPI.java

@@ -152,4 +152,6 @@ public interface LoomGradleExtensionAPI {
 	default void splitMinecraftJar() {
 		getMinecraftJarConfiguration().set(MinecraftJarConfiguration.SPLIT);
 	}
+
+	Property<Boolean> getRuntimeOnlyLog4j();
 }

src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java

@@ -63,6 +63,7 @@ public final class CompileConfiguration {
 		extension.createLazyConfiguration(Constants.Configurations.MOD_COMPILE_CLASSPATH_MAPPED, configuration -> configuration.setTransitive(false));
 		extension.createLazyConfiguration(Constants.Configurations.MINECRAFT_NAMED, configuration -> configuration.setTransitive(false)); // The launchers do not recurse dependencies
 		NamedDomainObjectProvider<Configuration> serverDeps = extension.createLazyConfiguration(Constants.Configurations.MINECRAFT_SERVER_DEPENDENCIES, configuration -> configuration.setTransitive(false));
+		extension.createLazyConfiguration(Constants.Configurations.MINECRAFT_RUNTIME_DEPENDENCIES, configuration -> configuration.setTransitive(false));
 		extension.createLazyConfiguration(Constants.Configurations.MINECRAFT_DEPENDENCIES, configuration -> {
 			configuration.extendsFrom(serverDeps.get());
 			configuration.setTransitive(false);
@@ -126,6 +127,8 @@ public final class CompileConfiguration {
 		extendsFrom(JavaPlugin.RUNTIME_CLASSPATH_CONFIGURATION_NAME, Constants.Configurations.LOOM_DEVELOPMENT_DEPENDENCIES, project);
 		extendsFrom(JavaPlugin.TEST_RUNTIME_CLASSPATH_CONFIGURATION_NAME, Constants.Configurations.LOOM_DEVELOPMENT_DEPENDENCIES, project);
 
+		extendsFrom(JavaPlugin.RUNTIME_CLASSPATH_CONFIGURATION_NAME, Constants.Configurations.MINECRAFT_RUNTIME_DEPENDENCIES, project);
+
 		// Add the dev time dependencies
 		project.getDependencies().add(Constants.Configurations.LOOM_DEVELOPMENT_DEPENDENCIES, Constants.Dependencies.DEV_LAUNCH_INJECTOR + Constants.Dependencies.Versions.DEV_LAUNCH_INJECTOR);
 		project.getDependencies().add(Constants.Configurations.LOOM_DEVELOPMENT_DEPENDENCIES, Constants.Dependencies.TERMINAL_CONSOLE_APPENDER + Constants.Dependencies.Versions.TERMINAL_CONSOLE_APPENDER);

src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftLibraryProvider.java

@@ -38,9 +38,11 @@ public class MinecraftLibraryProvider {
 	private static final Pattern NATIVES_PATTERN = Pattern.compile("^(?<group>.*)/(.*?)/(?<version>.*)/((?<name>.*?)-([0-9].*?)-)(?<classifier>.*).jar$");
 
 	public void provide(MinecraftProvider minecraftProvider, Project project) {
-		final MinecraftJarConfiguration jarConfiguration = LoomGradleExtension.get(project).getMinecraftJarConfiguration().get();
+		final LoomGradleExtension extension = LoomGradleExtension.get(project);
+		final MinecraftJarConfiguration jarConfiguration = extension.getMinecraftJarConfiguration().get();
 		final MinecraftVersionMeta versionInfo = minecraftProvider.getVersionInfo();
 		final BundleMetadata serverBundleMetadata = minecraftProvider.getServerBundleMetadata();
+		final boolean runtimeOnlyLog4j = versionInfo.isVersionOrNewer(Constants.MinecraftReleaseTimes.MC_20W03A) && extension.getRuntimeOnlyLog4j().get();
 
 		final boolean overrideLWJGL = LWJGLVersionOverride.overrideByDefault() || LWJGLVersionOverride.forceOverride(project) || Boolean.getBoolean("loom.test.lwjgloverride");
 
@@ -54,11 +56,16 @@ public class MinecraftLibraryProvider {
 			}
 
 			if (library.isValidForOS() && !library.hasNatives() && library.artifact() != null) {
-				if (serverBundleMetadata != null && isLibraryInBundle(serverBundleMetadata, library)) {
+				if (runtimeOnlyLog4j && library.name().startsWith("org.apache.logging.log4j")) {
+					// Make log4j a runtime only dep in 20w03a or later. Modders should use SLF4J.
+					project.getDependencies().add(Constants.Configurations.MINECRAFT_RUNTIME_DEPENDENCIES, library.name());
+				} else if (serverBundleMetadata != null && isLibraryInBundle(serverBundleMetadata, library)) {
 					project.getDependencies().add(Constants.Configurations.MINECRAFT_SERVER_DEPENDENCIES, library.name());
 				} else if (jarConfiguration.getSupportedEnvironments().contains("client")) {
 					// Client only library, or legacy version
 					project.getDependencies().add(Constants.Configurations.MINECRAFT_DEPENDENCIES, library.name());
+				} else {
+					throw new RuntimeException("Library %s was not added to a configuration".formatted(library.name()));
 				}
 			}
 

src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftVersionMeta.java

@@ -51,6 +51,10 @@ public record MinecraftVersionMeta(
 		return downloads().get(key);
 	}
 
+	public boolean isVersionOrNewer(String releaseTime) {
+		return this.releaseTime().compareTo(releaseTime) >= 0;
+	}
+
 	public record AssetIndex(String id, long totalSize, String path, String sha1, long size, String url) {
 		public String fabricId(String version) {
 			return id.equals(version) ? version : version + "-" + id;

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionApiImpl.java

@@ -63,6 +63,7 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 	protected final Property<Boolean> transitiveAccessWideners;
 	protected final Property<String> intermediary;
 	protected final Property<Boolean> enableInterfaceInjection;
+	private final Property<Boolean> runtimeOnlyLog4j;
 	private final Property<MinecraftJarConfiguration> minecraftJarConfiguration;
 
 	private final ModVersionParser versionParser;
@@ -104,6 +105,9 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 
 		this.accessWidener.finalizeValueOnRead();
 		this.getGameJarProcessors().finalizeValueOnRead();
+
+		this.runtimeOnlyLog4j = project.getObjects().property(Boolean.class).convention(true);
+		this.runtimeOnlyLog4j.finalizeValueOnRead();
 	}
 
 	@Override
@@ -213,6 +217,11 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 		return minecraftJarConfiguration;
 	}
 
+	@Override
+	public Property<Boolean> getRuntimeOnlyLog4j() {
+		return runtimeOnlyLog4j;
+	}
+
 	// This is here to ensure that LoomGradleExtensionApiImpl compiles without any unimplemented methods
 	private final class EnsureCompile extends LoomGradleExtensionApiImpl {
 		private EnsureCompile() {

src/main/java/net/fabricmc/loom/util/Constants.java

@@ -68,6 +68,7 @@ public class Constants {
 		 */
 		public static final String MINECRAFT_SERVER_DEPENDENCIES = "minecraftServerLibraries";
 		public static final String MINECRAFT_DEPENDENCIES = "minecraftLibraries";
+		public static final String MINECRAFT_RUNTIME_DEPENDENCIES = "minecraftRuntimeOnlyLibraries";
 		public static final String MINECRAFT_NATIVES = "minecraftNatives";
 		public static final String MINECRAFT_NAMED = "minecraftNamed";
 		public static final String MAPPINGS = "mappings";
@@ -141,4 +142,8 @@ public class Constants {
 		private TaskGroup() {
 		}
 	}
+
+	public static final class MinecraftReleaseTimes {
+		public static final String MC_20W03A = "2022-01-19T16:04:59+00:00";
+	}
 }

src/test/groovy/net/fabricmc/loom/test/util/GradleProjectTestTrait.groovy

@@ -107,10 +107,12 @@ trait GradleProjectTestTrait {
             throw new FileNotFoundException("Failed to find project directory at: $projectSourceDir.absolutePath")
         }
 
+        def settingsGradle = new File(projectDir, "settings.gradle")
+
         // Cleanup some basic things if they already exists
         new File(projectDir, "src").deleteDir()
         new File(projectDir, "build.gradle").delete()
-        new File(projectDir, "settings.gradle").delete()
+        settingsGradle.delete()
 
         projectSourceDir.eachFileRecurse { file ->
             if (file.isDirectory()) {
@@ -128,6 +130,10 @@ trait GradleProjectTestTrait {
             tempFile.parentFile.mkdirs()
             tempFile.bytes = file.bytes
         }
+
+        if (!settingsGradle.exists()) {
+            settingsGradle.createNewFile()
+        }
     }
 
     @Immutable

src/test/resources/projects/simple/src/main/java/net/fabricmc/example/ExampleMod.java

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