Allow replacing the default mod directory via fabric.modsFolder (#850)

* Allow replacing the default mod directory via fabric.modDir

* Apply requested changes

* Fix checkstyle failures

src/main/java/net/fabricmc/loader/impl/FabricLoaderImpl.java

@@ -21,6 +21,7 @@
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -207,7 +208,7 @@ private void setup() throws ModResolutionException {
 
 		ModDiscoverer discoverer = new ModDiscoverer(versionOverrides, depOverrides);
 		discoverer.addCandidateFinder(new ClasspathModCandidateFinder());
-		discoverer.addCandidateFinder(new DirectoryModCandidateFinder(gameDir.resolve("mods"), remapRegularMods));
+		discoverer.addCandidateFinder(new DirectoryModCandidateFinder(getModsDirectory0(), remapRegularMods));
 		discoverer.addCandidateFinder(new ArgumentModCandidateFinder(remapRegularMods));
 
 		Map<String, Set<ModCandidate>> envDisabledMods = new HashMap<>();
@@ -592,6 +593,13 @@ public String[] getLaunchArguments(boolean sanitize) {
 		return getGameProvider().getLaunchArguments(sanitize);
 	}
 
+	@Override
+	protected Path getModsDirectory0() {
+		String directory = System.getProperty(SystemProperties.MODS_FOLDER);
+
+		return directory != null ? Paths.get(directory) : gameDir.resolve("mods");
+	}
+
 	/**
 	 * Provides singleton for static init assignment regardless of load order.
 	 */

src/main/java/net/fabricmc/loader/impl/util/SystemProperties.java

@@ -32,6 +32,8 @@ public final class SystemProperties {
 	public static final String LOG_FILE = "fabric.log.file";
 	// minimum log level for builtin log handler
 	public static final String LOG_LEVEL = "fabric.log.level";
+	// a path to a directory to replace the default mod search directory
+	public static final String MODS_FOLDER = "fabric.modsFolder";
 	// additional mods to load (path separator separated paths, @ prefix for meta-file with each line referencing an actual file)
 	public static final String ADD_MODS = "fabric.addMods";
 	// a comma-separated list of mod ids to disable, even if they're discovered. mostly useful for unit testing.

src/main/legacyJava/net/fabricmc/loader/FabricLoader.java

@@ -17,6 +17,7 @@
 package net.fabricmc.loader;
 
 import java.io.File;
+import java.nio.file.Path;
 import java.util.Collection;
 import java.util.List;
 
@@ -37,7 +38,7 @@ public abstract class FabricLoader implements net.fabricmc.loader.api.FabricLoad
 	public static final FabricLoader INSTANCE = FabricLoaderImpl.InitHelper.get();
 
 	public File getModsDirectory() {
-		return getGameDir().resolve("mods").toFile();
+		return getModsDirectory0().toFile();
 	}
 
 	@Override
@@ -52,4 +53,6 @@ public Collection<ModContainer> getModContainers() {
 	public List<ModContainer> getMods() {
 		return (List) getAllMods();
 	}
+
+	protected abstract Path getModsDirectory0();
 }