minecraft/src/main/resources/frog.mod.toml

@@ -10,7 +10,7 @@ credits = [
     { name = "You", roles = ["author", "other_role"] }
 ]
 
-[frog.extensions]
+[frog.extensions.frogloader]
 prelaunch = "dev.frogmc.frogloader.example.ExamplePreLaunchExtension"
 mixin = "example_mod.mixins.json"
 accesswidener = "example_mod.accesswidener"

minecraft/src/test/resources/frog.mod.toml

@@ -10,6 +10,6 @@ credits = [
     { name = "You", roles = ["author", "other_role"] }
 ]
 
-[frog.extensions]
+[frog.extensions.frogloader]
 prelaunch="dev.frogmc.example.ExampleTestMod"
 

src/main/java/dev/frogmc/frogloader/api/exception/ModExtensionResolutionException.java

@@ -0,0 +1,17 @@
+package dev.frogmc.frogloader.api.exception;
+
+public class ModExtensionResolutionException extends Exception {
+
+	public ModExtensionResolutionException(String message) {
+		super(message);
+	}
+
+	public ModExtensionResolutionException(Throwable cause) {
+		super(cause);
+	}
+
+	public ModExtensionResolutionException(String message, Throwable cause) {
+		super(message, cause);
+	}
+
+}

src/main/java/dev/frogmc/frogloader/api/extensions/PreLaunchExtension.java

@@ -1,7 +1,7 @@
 package dev.frogmc.frogloader.api.extensions;
 
 import dev.frogmc.frogloader.api.mod.ModExtensions;
-import dev.frogmc.frogloader.impl.mod.BuiltinExtensions;
+import dev.frogmc.frogloader.impl.Constants;
 
 /**
  * The Pre-Launch Extension.
@@ -13,7 +13,7 @@ public interface PreLaunchExtension {
 	/**
 	 * This extension's id. This is the key to use in your frog.mod.toml.
 	 */
-	String ID = BuiltinExtensions.PRE_LAUNCH;
+	String ID = Constants.EXTENSION_PRE_LAUNCH;
 
 	/**
 	 * The initializer. This method will be invoked when this extension is run.

src/main/java/dev/frogmc/frogloader/api/mod/ModExtensions.java

@@ -1,150 +1,74 @@
 package dev.frogmc.frogloader.api.mod;
 
-import java.lang.invoke.MethodHandle;
-import java.lang.invoke.MethodHandleProxies;
-import java.lang.invoke.MethodHandles;
-import java.lang.invoke.MethodType;
-import java.lang.reflect.Field;
-import java.lang.reflect.Method;
-import java.lang.reflect.Modifier;
-import java.util.Collection;
-import java.util.Map;
 import java.util.function.Consumer;
 
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
+import dev.frogmc.frogloader.api.exception.ModExtensionResolutionException;
 
 /**
  * This class stores a mod's extensions.
- * <p>An extension is a simple key-value mapping of a string name to any value.</p>
- * <p>This class further provides utility methods to easily work with extension values of various types,
- * especially for extensions providing some form of class or method reference.</p>
+ * <p>
+ * An extension is a simple key-value mapping of a string name to any value.
+ * </p>
+ * <p>
+ * This class further provides utility methods to easily work with extension
+ * values of various types, especially for extensions providing some form of
+ * class or method reference.
+ * </p>
  *
  * @see ModProperties
  */
-public final class ModExtensions {
-
-	private static final Logger LOGGER = LoggerFactory.getLogger(ModExtensions.class);
-	private final Map<String, Object> extensions;
-
-	private ModExtensions(Map<String, Object> entries) {
-		extensions = entries;
-	}
-
-	/**
-	 * Retrieve a new instance of this class.
-	 * <p><strong>Internal use only.</strong></p>
-	 *
-	 * @param entries the entries read from the mod's properties file
-	 * @return an instance of this class
-	 */
-	public static ModExtensions of(Map<String, Object> entries) {
-		return new ModExtensions(entries);
-	}
+public interface ModExtensions {
 
 	/**
 	 * Get the value of the provided extension name.
 	 *
+	 * @param modId The mod ID this extension is stored under
 	 * @param key The extension name to query
 	 * @param <T> The type of the value of this extension
+	 *
 	 * @return The value of the extension, or null
 	 */
-	@SuppressWarnings("unchecked")
-	public <T> T get(String key) {
-		return (T) extensions.get(key);
-	}
+	<T> T get(String modId, String key);
 
 	/**
 	 * Get the value of the provided extension name.
 	 *
+	 * @param modId The mod ID this extension is stored under
 	 * @param key The extension name to query
-	 * @param defaultValue a default value
+	 * @param defaultValue A default value
 	 * @param <T> The type of the value of this extension
+	 *
 	 * @return The value of the extension, or the default value if it isn't present
 	 */
-	@SuppressWarnings("unchecked")
-	public <T> T getOrDefault(String key, T defaultValue) {
-		return (T) extensions.getOrDefault(key, defaultValue);
-	}
+	<T> T getOrDefault(String modId, String key, T defaultValue);
 
 	/**
 	 * Run the given action on this extension if it is present.
 	 *
-	 * @param key    the extension name to query
-	 * @param action the action to run on the value of the extension if it is present
+	 * @param modId The mod ID this extension is stored under
+	 * @param key The extension name to query
+	 * @param action The action to run on the value of the extension if it is present
 	 * @param <T> The type of the value of this extension
 	 */
-	@SuppressWarnings({"rawtypes", "unchecked"})
-	public <T> void runIfPresent(String key, Consumer<T> action) {
-		Object value = get(key);
-		if (value == null) {
-			return;
-		}
-		if (value instanceof Collection c) {
-			((Collection<T>) c).forEach(action);
-		} else {
-			action.accept((T) value);
-		}
-	}
+	<T> void runIfPresent(String modId, String key, Consumer<T> action);
 
 	/**
 	 * Run the given action on this extension if it is present.
-	 * <p>This method simplifies handling references to classes or methods.</p>
-	 * It will first query the string value of the extension and, if it is found, create a new instance of the referenced class
-	 * or invoke the referenced method to retrieve an instance of the provided class. Then the provided action is run on this
-	 * object.
+	 * <p>
+	 * This method simplifies handling references to classes or methods.
+	 * </p>
+	 * It will first query the string value of the extension and, if it is found,
+	 * create a new instance of the referenced class or invoke the referenced method
+	 * to retrieve an instance of the provided class. Then the provided action is
+	 * run on this object.
 	 *
+	 * @param modId The mod ID this extension is stored under
 	 * @param key The name of the extension
 	 * @param type The class type of the extension (The class the extension class is extending/implementing)
 	 * @param action The action to run on the newly retrieved instance of the provided class
 	 * @param <T> The type of the class
+	 *
+	 * @throws ModExtensionResolutionException If the reference could not be resolved
 	 */
-	@SuppressWarnings({"unchecked"})
-	public <T> void runIfPresent(String key, Class<T> type, Consumer<T> action) {
-		runIfPresent(key, (Consumer<String>) s -> {
-			try {
-				T object;
-				if (s.contains("::")) {
-					String[] parts = s.split("::");
-					Class<?> extension = Class.forName(parts[0]);
-					object = (T) handleReference(extension, type, parts[1]);
-				} else {
-					object = (T) MethodHandles.lookup().findConstructor(Class.forName(s), MethodType.methodType(void.class)).invoke();
-				}
-				if (object != null) {
-					action.accept(object);
-				}
-			} catch (Throwable e) {
-				LOGGER.warn("Failed to instantiate Extension: ", e);
-			}
-		});
-	}
-
-	private Object handleReference(Class<?> clazz, Class<?> type, String ref) throws Throwable {
-		try {
-			Field found = clazz.getDeclaredField(ref);
-			found.setAccessible(true);
-			return found.get(null);
-		} catch (Exception ignored) {
-		}
-
-		Method found = null;
-		for (Method method : clazz.getDeclaredMethods()) {
-			if (method.getName().equals(ref)) {
-				if (found != null) {
-					throw new IllegalArgumentException("Ambiguous method reference: "+ref+" in "+clazz.getName());
-				}
-				found = method;
-			}
-		}
-
-		if (found != null) {
-			MethodHandle method = MethodHandles.lookup().unreflect(found);
-			if (!Modifier.isStatic(found.getModifiers())) {
-				method = method.bindTo(MethodHandles.lookup().findConstructor(clazz, MethodType.methodType(void.class)).invoke());
-			}
-			return MethodHandleProxies.asInterfaceInstance(type, method);
-		}
-		throw new IllegalArgumentException("Could not find either a static field or a method named '" + ref + "' in class " + clazz.getName() + "!");
-	}
+	<T> void runIfPresent(String modId, String key, Class<T> type, Consumer<T> action) throws ModExtensionResolutionException;
 }

src/main/java/dev/frogmc/frogloader/api/plugin/ModProvider.java

@@ -65,8 +65,9 @@ public interface ModProvider {
 	/**
 	 * This method will be invoked just before the game is launched. It may be used for a pre-launch entrypoint for mods.
 	 * @param mods The mods loaded by this provider
+	 * @throws Exception If an exception occurs during pre-launch. It will be handled by the loader.
 	 */
-	default void preLaunch(Collection<ModProperties> mods) {
+	default void preLaunch(Collection<ModProperties> mods) throws Exception {
 	}
 
 	/**

src/main/java/dev/frogmc/frogloader/impl/Constants.java

@@ -0,0 +1,14 @@
+package dev.frogmc.frogloader.impl;
+
+import lombok.experimental.UtilityClass;
+
+@UtilityClass
+public class Constants {
+	public final String MOD_ID = "frogloader";
+	public final String EXTENSION_MIXIN_CONFIG = "mixin";
+	public final String EXTENSION_INCLUDED_JARS = "included_jars";
+	public final String EXTENSION_PRE_LAUNCH = "prelaunch";
+	public final String EXTENSION_ACCESSWIDENER = "accesswidener";
+	public final String EXTENSION_LOADING_TYPE = "loading_type";
+	public final String EXTENSION_MOD_PROVIDER = "modprovider";
+}

src/main/java/dev/frogmc/frogloader/impl/FrogLoaderImpl.java

@@ -78,7 +78,8 @@ public class FrogLoaderImpl implements FrogLoader {
 			modIds = collectModIds();
 			LOGGER.info(ModUtil.getModList(getMods()));
 			LOGGER.info("Launching...");
-			modProviders.forEach(plugin -> plugin.preLaunch(mods.get(plugin.id()).values()));
+			for (ModProvider plugin : modProviders)
+				plugin.preLaunch(mods.get(plugin.id()).values());
 			gamePlugin.run();
 		} catch (Throwable t) {
 			LoaderGui.execReport(CrashReportGenerator.writeReport(t, getMods()), false);

src/main/java/dev/frogmc/frogloader/impl/PluginLoader.java

@@ -11,7 +11,6 @@ import dev.frogmc.frogloader.api.plugin.GamePlugin;
 import dev.frogmc.frogloader.api.plugin.ModProvider;
 import dev.frogmc.frogloader.impl.gui.LoaderGui;
 import dev.frogmc.frogloader.impl.mixin.AWProcessor;
-import dev.frogmc.frogloader.impl.mod.BuiltinExtensions;
 import dev.frogmc.frogloader.impl.mod.ModDependencyResolver;
 import dev.frogmc.frogloader.impl.util.CrashReportGenerator;
 import org.slf4j.Logger;
@@ -47,10 +46,12 @@ public class PluginLoader {
 				properties.put(plugin.id(), loadedMods);
 
 				LOGGER.debug("Loaded {} mod(s) from provider: {}", loadedMods.size(), plugin.id());
-				loadedMods.forEach(p -> p.extensions().runIfPresent(BuiltinExtensions.MOD_PROVIDER, ModProvider.class, provider -> {
+				for (ModProperties p : loadedMods) {
+					p.extensions().runIfPresent(Constants.MOD_ID, Constants.EXTENSION_MOD_PROVIDER, ModProvider.class, provider -> {
 						providers.add(provider);
 						size[0]++;
-				}));
+					});
+				}
 				modProviders.add(plugin);
 			} catch (Throwable e) {
 				LOGGER.error("Error during plugin initialisation: ", e);
@@ -78,7 +79,7 @@ public class PluginLoader {
 	private static void initializeModMixins(Collection<ModProperties> loadedMods) {
 		Map<String, ModProperties> configs = new HashMap<>();
 		loadedMods.forEach(props -> {
-			Object o = props.extensions().get(BuiltinExtensions.MIXIN_CONFIG);
+			Object o = props.extensions().get(Constants.MOD_ID, Constants.EXTENSION_MIXIN_CONFIG);
 			if (o instanceof String name) {
 				configs.put(name, props);
 				Mixins.addConfiguration(name);

src/main/java/dev/frogmc/frogloader/impl/mixin/AWProcessor.java

@@ -14,12 +14,11 @@ import java.util.regex.Pattern;
 import java.util.stream.Stream;
 
 import dev.frogmc.frogloader.api.mod.ModProperties;
+import dev.frogmc.frogloader.impl.Constants;
 import dev.frogmc.frogloader.impl.launch.transformer.AccessWidener;
-import dev.frogmc.frogloader.impl.mod.BuiltinExtensions;
 
 public class AWProcessor {
 
-	private static final String AW_EXTENSION_NAME = BuiltinExtensions.ACCESSWIDENER;
 	private static final Predicate<String> HEADER = Pattern.compile("accessWidener\\s+v[12]\\s+.*").asMatchPredicate();
 	private static final String SEPARATOR = "[\\t ]+";
 
@@ -30,7 +29,7 @@ public class AWProcessor {
 		Map<String, Map<String, AccessWidener.Entry>> mutations = new ConcurrentHashMap<>();
 		Set<String> classNames = new ConcurrentSkipListSet<>();
 
-		mods.stream().map(ModProperties::extensions).map(e -> e.get(AW_EXTENSION_NAME)).flatMap(o -> {
+		mods.stream().map(ModProperties::extensions).map(e -> e.get(Constants.MOD_ID, Constants.EXTENSION_ACCESSWIDENER)).flatMap(o -> {
 					if (o instanceof Collection<?> c) {
 						return c.stream().map(Object::toString);
 					}

src/main/java/dev/frogmc/frogloader/impl/mod/BuiltinExtensions.java

@@ -1,13 +0,0 @@
-package dev.frogmc.frogloader.impl.mod;
-
-import lombok.experimental.UtilityClass;
-
-@UtilityClass
-public class BuiltinExtensions {
-	public final String MIXIN_CONFIG = "mixin";
-	public final String INCLUDED_JARS = "included_jars";
-	public final String PRE_LAUNCH = "prelaunch";
-	public final String ACCESSWIDENER = "accesswidener";
-	public final String LOADING_TYPE = "loading_type";
-	public final String MOD_PROVIDER = "modprovider";
-}

src/main/java/dev/frogmc/frogloader/impl/mod/JavaModProperties.java

@@ -25,7 +25,7 @@ public class JavaModProperties {
 					"",
 					Map.of(System.getProperty("java.vm.vendor"), Collections.singleton("Vendor")),
 					new ModDependencies(Collections.emptySet(), Collections.emptySet(), Collections.emptySet(), Collections.emptySet()),
-					ModExtensions.of(Collections.emptyMap()), Collections.emptySet());
+					new ModExtensionsImpl(Collections.emptyMap()), Collections.emptySet());
 		}
 		return INSTANCE;
 	}

src/main/java/dev/frogmc/frogloader/impl/mod/ModDependencyResolver.java

@@ -11,6 +11,7 @@ import com.google.common.collect.MultimapBuilder;
 import dev.frogmc.frogloader.api.mod.ModDependencies;
 import dev.frogmc.frogloader.api.mod.ModProperties;
 import dev.frogmc.frogloader.api.mod.SemVer;
+import dev.frogmc.frogloader.impl.Constants;
 import dev.frogmc.frogloader.impl.SemVerParseException;
 import lombok.AllArgsConstructor;
 import lombok.Getter;
@@ -121,7 +122,7 @@ public class ModDependencyResolver {
 			DependencyEntry value = entry.getValue();
 			if (!(presentMods.containsKey(s) && value.range.versionMatches(presentOrProvided.get(s)))) { // The dependency isn't fulfilled by any present mods
 				if (!(provides.containsKey(s) && provides.get(s).stream().map(ProvidedMod::version).allMatch(value.range::versionMatches))) { // The dependency also isn't fulfilled by any provided mods
-					if (value.origin.extensions().getOrDefault(BuiltinExtensions.LOADING_TYPE, "required").equals("required")) {
+					if (value.origin.extensions().getOrDefault(Constants.MOD_ID, Constants.EXTENSION_LOADING_TYPE, "required").equals("required")) {
 						unfulfilled.add(new UnfulfilledDependencyException.Entry(value.origin, s, value.range, presentOrProvided.get(s), value.link(), value.name()));
 					} else {
 						LOGGER.debug("Skipping optional mod: {} ({})", value.origin().id(), value.origin().name());

src/main/java/dev/frogmc/frogloader/impl/mod/ModExtensionsImpl.java

@@ -0,0 +1,136 @@
+package dev.frogmc.frogloader.impl.mod;
+
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandleProxies;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
+import java.lang.invoke.WrongMethodTypeException;
+import java.lang.reflect.AccessibleObject;
+import java.lang.reflect.Field;
+import java.lang.reflect.Member;
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Consumer;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import dev.frogmc.frogloader.api.exception.ModExtensionResolutionException;
+import dev.frogmc.frogloader.api.mod.ModExtensions;
+
+public class ModExtensionsImpl implements ModExtensions {
+
+	private final Map<Key, Object> map;
+
+	public ModExtensionsImpl(Map<Key, Object> map) {
+		this.map = map;
+	}
+
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> T get(String modId, String key) {
+		return (T) map.get(new Key(modId, key));
+	}
+
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> T getOrDefault(String modId, String key, T defaultValue) {
+		return (T) map.getOrDefault(new Key(modId, key), defaultValue);
+	}
+
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> void runIfPresent(String modId, String key, Consumer<T> action) {
+		Object value = get(modId, key);
+
+		if (value == null)
+			return;
+
+		if (value instanceof Collection)
+			((Collection<T>) value).forEach(action);
+		else
+			action.accept((T) value);
+	}
+
+	@Override
+	public <T> void runIfPresent(String modId, String key, Class<T> type, Consumer<T> action)
+			throws ModExtensionResolutionException {
+		String s = get(modId, key);
+
+		if (s == null)
+			return;
+
+		T object;
+		if (s.contains("::")) {
+			String[] parts = s.split("::", 2);
+			try {
+				Class<?> extension = Class.forName(parts[0]);
+				object = (T) handleReference(extension, type, parts[1]);
+			} catch (ModExtensionResolutionException e) {
+				throw e;
+			} catch (Throwable e) {
+				throw new ModExtensionResolutionException(e);
+			}
+		} else {
+			try {
+				Class<?> extension = Class.forName(s);
+
+				if (!type.isAssignableFrom(extension))
+					throw new ModExtensionResolutionException(extension + " does not inherit from " + type);
+
+				object = (T) MethodHandles.lookup().findConstructor(extension, MethodType.methodType(void.class))
+						.invoke();
+			} catch (ModExtensionResolutionException e) {
+				throw e;
+			} catch (Throwable e) {
+				throw new ModExtensionResolutionException(e);
+			}
+		}
+
+		if (object != null) {
+			action.accept(object);
+		}
+	}
+
+	private Object handleReference(Class<?> owner, Class<?> type, String name)
+			throws ModExtensionResolutionException, IllegalAccessException {
+		try {
+			Field found = owner.getDeclaredField(name);
+
+			if (Modifier.isStatic(found.getModifiers())) {
+				found.setAccessible(true);
+				return found.get(null);
+			}
+		} catch (NoSuchFieldException ignored) {
+		}
+
+		Method found = null;
+		for (Method method : owner.getDeclaredMethods()) {
+			if (!Modifier.isStatic(method.getModifiers()))
+				continue;
+
+			if (!method.getName().equals(name))
+				continue;
+
+			if (found != null)
+				throw new ModExtensionResolutionException("Ambiguous method reference: " + name + " in " + owner.getName());
+
+			found = method;
+		}
+
+		if (found != null) {
+			found.setAccessible(true);
+
+			MethodHandle method = MethodHandles.lookup().unreflect(found);
+			return MethodHandleProxies.asInterfaceInstance(type, method);
+		}
+
+		throw new ModExtensionResolutionException("Could not find a static field or method named '" + name + "' in class " + owner.getName() + "!");
+	}
+
+	public record Key(String modId, String key) {
+	}
+}

src/main/java/dev/frogmc/frogloader/impl/mod/ModPropertiesReader.java

@@ -153,16 +153,23 @@ public class ModPropertiesReader {
 			var suggests = parseDependencies.apply("suggests");
 			var provides = parseDependencies.apply("provides");
 
+			UnmodifiableConfig extensionsConfig = config.get("frog.extensions");
+			Map<ModExtensionsImpl.Key, Object> extensions = new HashMap<>();
+			if (extensionsConfig != null) {
+				for (var modEntry : extensionsConfig.entrySet()) {
+					UnmodifiableConfig modExtensionsConig = modEntry.getValue();
+
+					for (var entry : modExtensionsConig.entrySet())
+						extensions.put(new ModExtensionsImpl.Key(modEntry.getKey(), entry.getKey()), entry.getValue());
+				}
+			}
+
 			if (!badProperties.isEmpty())
 				throw new InvalidModPropertiesException(id, sources, badProperties);
 
-			UnmodifiableConfig extensionsConfig = config.get("frog.extensions");
-			Map<String, Object> extensions = new HashMap<>();
-			if (extensionsConfig != null)
-				extensionsConfig.entrySet().forEach(entry -> extensions.put(entry.getKey(), entry.getValue()));
 
 			return new ModPropertiesImpl(id, name, icon, semVer, license, Collections.unmodifiableMap(credits),
-					new ModDependencies(depends, breaks, suggests, provides), ModExtensions.of(extensions), sources);
+					new ModDependencies(depends, breaks, suggests, provides), new ModExtensionsImpl(extensions), sources);
 		});
 
 		private static final Map<String, Parser> versions = Arrays.stream(values()).collect(Collectors.toMap(v -> v.version, v -> v.parser));

src/main/java/dev/frogmc/frogloader/impl/mod/ModUtil.java

@@ -6,6 +6,7 @@ import java.util.stream.Collectors;
 
 import com.electronwill.nightconfig.core.UnmodifiableConfig;
 import dev.frogmc.frogloader.api.mod.ModProperties;
+import dev.frogmc.frogloader.impl.Constants;
 
 public class ModUtil {
 
@@ -66,7 +67,7 @@ public class ModUtil {
 	private static Map<ModProperties, Collection<String>> getParentMods(Collection<ModProperties> mods) {
 		Map<ModProperties, Collection<String>> children = new HashMap<>();
 		for (ModProperties mod : mods) {
-			List<UnmodifiableConfig> entries = mod.extensions().get(BuiltinExtensions.INCLUDED_JARS);
+			List<UnmodifiableConfig> entries = mod.extensions().get(Constants.MOD_ID, Constants.EXTENSION_INCLUDED_JARS);
 			if (entries != null) {
 				for (var jar : entries) {
 					String id = jar.get("id");

src/main/java/dev/frogmc/frogloader/impl/plugin/game/minecraft/MinecraftGamePlugin.java

@@ -21,6 +21,7 @@ import dev.frogmc.frogloader.api.mod.ModExtensions;
 import dev.frogmc.frogloader.api.mod.ModProperties;
 import dev.frogmc.frogloader.api.plugin.GamePlugin;
 import dev.frogmc.frogloader.impl.FrogLoaderImpl;
+import dev.frogmc.frogloader.impl.mod.ModExtensionsImpl;
 import dev.frogmc.frogloader.impl.mod.ModPropertiesImpl;
 import dev.frogmc.frogloader.impl.util.SystemProperties;
 import dev.frogmc.thyroxine.HttpHelper;
@@ -204,6 +205,6 @@ public class MinecraftGamePlugin implements GamePlugin {
 				MinecraftSemVerImpl.get(version), "MC-EULA",
 				Map.of("Mojang AB", Collections.singleton("Author")),
 				new ModDependencies(Collections.emptySet(), Collections.emptySet(), Collections.emptySet(), Collections.emptySet()),
-				ModExtensions.of(Collections.emptyMap()), Collections.emptySet());
+				new ModExtensionsImpl(Collections.emptyMap()), Collections.emptySet());
 	}
 }

src/main/java/dev/frogmc/frogloader/impl/plugin/mod/FrogModProvider.java

@@ -10,10 +10,11 @@ import java.util.function.Consumer;
 
 import com.electronwill.nightconfig.core.UnmodifiableConfig;
 import dev.frogmc.frogloader.api.FrogLoader;
+import dev.frogmc.frogloader.api.exception.ModExtensionResolutionException;
 import dev.frogmc.frogloader.api.extensions.PreLaunchExtension;
 import dev.frogmc.frogloader.api.mod.ModProperties;
 import dev.frogmc.frogloader.api.plugin.ModProvider;
-import dev.frogmc.frogloader.impl.mod.BuiltinExtensions;
+import dev.frogmc.frogloader.impl.Constants;
 import dev.frogmc.frogloader.impl.mod.ModPropertiesReader;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -115,7 +116,7 @@ public class FrogModProvider implements ModProvider {
 		if (opt.isPresent()) {
 			ModProperties p = opt.get();
 			modPaths.put(mod, p);
-			List<UnmodifiableConfig> entries = p.extensions().getOrDefault(BuiltinExtensions.INCLUDED_JARS, Collections.emptyList());
+			List<UnmodifiableConfig> entries = p.extensions().getOrDefault(Constants.MOD_ID, Constants.EXTENSION_INCLUDED_JARS, Collections.emptyList());
 			if (entries.isEmpty()) {
 				return;
 			}
@@ -135,7 +136,8 @@ public class FrogModProvider implements ModProvider {
 	}
 
 	@Override
-	public void preLaunch(Collection<ModProperties> mods) {
-		mods.forEach(mod -> mod.extensions().runIfPresent(PreLaunchExtension.ID, PreLaunchExtension.class, PreLaunchExtension::onPreLaunch));
+	public void preLaunch(Collection<ModProperties> mods) throws ModExtensionResolutionException {
+		for (ModProperties mod : mods)
+			mod.extensions().runIfPresent(Constants.MOD_ID, Constants.EXTENSION_PRE_LAUNCH, PreLaunchExtension.class, PreLaunchExtension::onPreLaunch);
 	}
 }