And that's why fabric api is important

build.gradle

@@ -8,16 +8,30 @@ apply plugin: "java"
 sourceCompatibility = 1.8
 targetCompatibility = 1.8
 
+sourceSets {
+	testmod
+}
+
 dependencies {
 	minecraft "com.mojang:minecraft:21w08b"
 	mappings "net.fabricmc:yarn:21w08b+build.8:v2"
 	modImplementation "net.fabricmc:fabric-loader:0.11.1"
+	testmodImplementation sourceSets.main.output
+	testmodCompileOnly sourceSets.main.compileClasspath
+	testmodRuntimeOnly sourceSets.main.runtimeClasspath
+	modRuntime("net.fabricmc.fabric-api:fabric-api:0.31.2+1.17") {
+		exclude module: "fabric-loader"
+	}
 }
 
 version = "1.0.0"
 archivesBaseName = "Matrix"
 group = "org.dimdev"
 
+license {
+	include '**/*.java'
+}
+
 processResources {
 	filesMatching("fabric.mod.json") {
 		expand "version": project.version

src/main/java/org/dimdev/matrix/Matrix.java

@@ -2,8 +2,9 @@
 
 import java.lang.reflect.Modifier;
 import java.util.Arrays;
-import java.util.Optional;
 
+import net.minecraft.item.BlockItem;
+import net.minecraft.item.Item;
 import net.minecraft.util.Identifier;
 import net.minecraft.util.registry.Registry;
 
@@ -25,28 +26,34 @@ public class Matrix {
 	 *
 	 * @param clazz The class that should be scanned for registry entries.
 	 * @param registry The registry that entries should be registered to.
-	 * @param entryClass The class that each registry entry should be an instance of.
-	 * @param <T> The registry's container type.
 	 */
-	public static <T> void register(Class<?> clazz, Registry<T> registry, Class<T> entryClass) {
-		String modId = Optional.ofNullable(clazz.getAnnotation(Register.class)).orElseThrow(() -> new UnsupportedOperationException(clazz.getName() + " did not have the Register annotation!")).modid();
+	@SuppressWarnings({"unchecked", "rawtypes"})
+	public static void register(Class<?> clazz, Registry<?> registry) {
+		Registrar registrar = clazz.getAnnotation(Registrar.class);
+		if (registrar == null) {
+			return;
+		}
+
+		String modid = registrar.modid();
+		Class<?> element = registrar.element();
+
 		Arrays.stream(clazz.getFields())
-				.filter(field -> {
-					int modifiers = field.getModifiers();
-					return Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers) && Modifier.isFinal(modifiers);
-				})
-				.filter(field -> field.isAnnotationPresent(RegistryEntry.class))
-				.filter(field -> entryClass.isAssignableFrom(field.getType()))
+				.filter(field -> field.isAnnotationPresent(RegistryEntry.class)
+						&& Modifier.isPublic(field.getModifiers())
+						&& Modifier.isStatic(field.getModifiers())
+						&& Modifier.isFinal(field.getModifiers())
+						&& element.isAssignableFrom(field.getType())
+				)
 				.forEach(field -> {
-					T value;
 					try {
-						//noinspection unchecked
-						value = (T) field.get(null);
+						Object value = field.get(null);
+						Registry.register((Registry) registry, new Identifier(modid, field.getAnnotation(RegistryEntry.class).value()), element.cast(value));
+						if (value instanceof BlockItem) {
+							Item.BLOCK_ITEMS.put(((BlockItem) value).getBlock(), (Item) value);
+						}
 					} catch (IllegalAccessException e) {
-						throw new AssertionError(e); // cant happen
+						throw new AssertionError(e);
 					}
-					Identifier id = new Identifier(modId, field.getAnnotation(RegistryEntry.class).value());
-					Registry.register(registry, id, value);
 				});
 	}
 }

src/main/java/org/dimdev/matrix/Register.java → src/main/java/org/dimdev/matrix/Registrar.java

@@ -13,6 +13,8 @@
  */
 @Retention(RetentionPolicy.RUNTIME)
 @Target(ElementType.TYPE)
-public @interface Register {
+public @interface Registrar {
+	Class<?> element();
+
 	String modid();
 }

src/testmod/java/org/dimdev/matrix/test/ModBlocks.java

@@ -0,0 +1,17 @@
+package org.dimdev.matrix.test;
+
+import org.dimdev.matrix.Registrar;
+import org.dimdev.matrix.RegistryEntry;
+
+import net.minecraft.block.AbstractBlock;
+import net.minecraft.block.Block;
+import net.minecraft.block.Blocks;
+import net.minecraft.item.BlockItem;
+import net.minecraft.item.Item;
+import net.minecraft.item.ItemGroup;
+
+@Registrar(element = Block.class, modid = "matrix-test")
+public class ModBlocks {
+	@RegistryEntry("test_block")
+	public static final Block BLOCK = new Block(AbstractBlock.Settings.copy(Blocks.DIRT));
+}

src/testmod/java/org/dimdev/matrix/test/ModItems.java

@@ -0,0 +1,14 @@
+package org.dimdev.matrix.test;
+
+import org.dimdev.matrix.Registrar;
+import org.dimdev.matrix.RegistryEntry;
+
+import net.minecraft.item.BlockItem;
+import net.minecraft.item.Item;
+import net.minecraft.item.ItemGroup;
+
+@Registrar(element = Item.class, modid = "matrix-test")
+public class ModItems {
+	@RegistryEntry("test_item")
+	public static final Item ITEM = new BlockItem(ModBlocks.BLOCK, new Item.Settings().group(ItemGroup.FOOD));
+}

src/testmod/java/org/dimdev/matrix/test/Test.java

@@ -0,0 +1,15 @@
+package org.dimdev.matrix.test;
+
+import org.dimdev.matrix.Matrix;
+
+import net.minecraft.util.registry.Registry;
+
+import net.fabricmc.api.ModInitializer;
+
+public class Test implements ModInitializer {
+	@Override
+	public void onInitialize() {
+		Matrix.register(ModBlocks.class, Registry.BLOCK);
+		Matrix.register(ModItems.class, Registry.ITEM);
+	}
+}

src/testmod/resources/fabric.mod.json

@@ -0,0 +1,12 @@
+{
+  "schemaVersion": 1,
+  "version": "${version}",
+  "id": "matrix-test",
+  "name": "Matrix Test",
+  "description": "Testmod",
+  "entrypoints": {
+    "main": [
+      "org.dimdev.matrix.test.Test"
+    ]
+  }
+}