Switch Impl types to Holderable

paper-server/src/main/java/io/papermc/paper/util/Holderable.java

@@ -1,8 +1,21 @@
 package io.papermc.paper.util;
 
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.mojang.serialization.Codec;
+import com.mojang.serialization.JsonOps;
+import net.kyori.adventure.key.Key;
 import net.minecraft.core.Holder;
+import net.minecraft.resources.RegistryOps;
+import org.bukkit.Keyed;
+import org.bukkit.Registry;
+import org.bukkit.craftbukkit.CraftRegistry;
 import org.bukkit.craftbukkit.util.Handleable;
+import org.intellij.lang.annotations.Subst;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public interface Holderable<M> extends Handleable<M> {
 
     Holder<M> getHolder();
@@ -11,4 +24,55 @@ public interface Holderable<M> extends Handleable<M> {
     default M getHandle() {
         return this.getHolder().value();
     }
+
+    static <T extends Keyed, M> @Nullable T fromBukkitSerializationObject(final Object deserialized, final Codec<? extends Holder<M>> codec, final Registry<T> registry) { // TODO remove Keyed
+        return switch (deserialized) {
+            case @Subst("key:value") final String string -> {
+                if (!(Key.parseable(string))) {
+                    yield null;
+                }
+                yield registry.get(Key.key(string));
+            }
+            case JsonObjectWrapper(final JsonObject element) -> {
+                if (!(registry instanceof final CraftRegistry<?, ?> craftRegistry) || !craftRegistry.supportsDirectHolders()) {
+                    throw new IllegalArgumentException("Cannot deserialize direct holders for " + registry);
+                }
+                final RegistryOps<JsonElement> ops = CraftRegistry.getMinecraftRegistry().createSerializationContext(JsonOps.INSTANCE);
+                final Holder<M> holder = codec.decode(ops, element).getOrThrow().getFirst();
+                yield ((CraftRegistry<T, M>) registry).convertDirectHolder(holder);
+            }
+            default -> throw new IllegalArgumentException("Cannot deserialize " + deserialized);
+        };
+    }
+
+    default Object toBukkitSerializationObject(final Codec<? super Holder<M>> codec) {
+        return switch (this.getHolder()) {
+            case final Holder.Direct<M> direct -> {
+                final RegistryOps<JsonElement> ops = CraftRegistry.getMinecraftRegistry().createSerializationContext(JsonOps.INSTANCE);
+                yield new JsonObjectWrapper(codec.encodeStart(ops, direct).getOrThrow().getAsJsonObject());
+            }
+            case final Holder.Reference<M> reference -> reference.key().location().toString();
+            default -> throw new IllegalArgumentException("Cannot serialize " + this.getHolder());
+        };
+    }
+
+    /**
+     * All implementations should use this as their hashCode implementation
+     */
+    default int implHashCode() {
+        return this.getHolder().hashCode();
+    }
+
+    /**
+     * All implementations should use this as their equals implementation
+     */
+    default boolean implEquals(final @Nullable Object o) {
+        if (o == null || this.getClass() != o.getClass()) return false;
+        final Holderable<?> that = (Holderable<?>) o;
+        return this.getHolder().equals(that.getHolder());
+    }
+
+    default String implToString() {
+        return "%s{holder=%s}".formatted(this.getClass().getSimpleName(), this.getHolder().toString());
+    }
 }

paper-server/src/main/java/io/papermc/paper/util/JsonObjectWrapper.java

@@ -0,0 +1,34 @@
+package io.papermc.paper.util;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParser;
+import java.util.Map;
+import org.bukkit.configuration.serialization.ConfigurationSerializable;
+import org.bukkit.configuration.serialization.ConfigurationSerialization;
+import org.jspecify.annotations.NullMarked;
+
+@NullMarked
+public record JsonObjectWrapper(JsonObject element) implements ConfigurationSerializable {
+
+    private static final String KEY = "value";
+    private static final Gson GSON = new Gson();
+
+    static {
+        ConfigurationSerialization.registerClass(JsonObjectWrapper.class);
+    }
+
+    public JsonObjectWrapper(final JsonElement element) {
+        this(element.getAsJsonObject());
+    }
+
+    public JsonObjectWrapper(final Map<String, Object> input) {
+        this(JsonParser.parseString((String) input.get(KEY)).getAsJsonObject());
+    }
+
+    @Override
+    public Map<String, Object> serialize() {
+        return Map.of(KEY, GSON.toJson(this.element));
+    }
+}