A few comments

src/main/java/net/minestom/server/instance/Instance.java

@@ -80,6 +80,12 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
     // Pathfinder
     private final PFInstanceSpace instanceSpace = new PFInstanceSpace(this);
 
+    /**
+     * Create a new instance
+     *
+     * @param uniqueId      the unique id of the instance
+     * @param dimensionType the dimension type of the instance
+     */
     public Instance(UUID uniqueId, DimensionType dimensionType) {
         this.uniqueId = uniqueId;
         this.dimensionType = dimensionType;

src/main/java/net/minestom/server/instance/InstanceContainer.java

@@ -66,21 +66,28 @@ public class InstanceContainer extends Instance {
     @Setter
     private BiFunction<Integer, Integer, BlockProvider> chunkDecider;
 
+    /**
+     * Create an {@link InstanceContainer}
+     *
+     * @param uniqueId      the unique id of the instance
+     * @param dimensionType the dimension type of the instance
+     * @param storageFolder the {@link StorageFolder} of the instance,
+     *                      can be null if you do not wish to save the instance latter on
+     */
     public InstanceContainer(UUID uniqueId, DimensionType dimensionType, StorageFolder storageFolder) {
         super(uniqueId, dimensionType);
 
         this.storageFolder = storageFolder;
-        chunkLoader = new MinestomBasicChunkLoader(storageFolder);
+        this.chunkLoader = new MinestomBasicChunkLoader(storageFolder);
 
-        if (storageFolder == null) {
-            return;
+        // Get instance data from the saved data if a StorageFolder is defined
+        if (storageFolder != null) {
+            // Retrieve instance data
+            this.uniqueId = storageFolder.getOrDefault(UUID_KEY, UUID.class, uniqueId);
+
+            final Data data = storageFolder.getOrDefault(DATA_KEY, SerializableData.class, null);
+            setData(data);
         }
-
-        // Retrieve instance data
-        this.uniqueId = storageFolder.getOrDefault(UUID_KEY, UUID.class, uniqueId);
-
-        final Data data = storageFolder.getOrDefault(DATA_KEY, SerializableData.class, null);
-        setData(data);
     }
 
     @Override

src/main/java/net/minestom/server/reader/ChunkReader.java

@@ -17,6 +17,15 @@ public class ChunkReader {
 
     private static final BiomeManager BIOME_MANAGER = MinecraftServer.getBiomeManager();
 
+    /**
+     * Read a chunk from a byte array, the array should contain the whole chunk and only it
+     *
+     * @param b        the byte array containing the chunk
+     * @param instance the instance of the chunk
+     * @param chunkX   the chunk X
+     * @param chunkZ   the chunk Z
+     * @param callback the consumer called once the chunk has been read
+     */
     public static void readChunk(byte[] b, Instance instance, int chunkX, int chunkZ, Consumer<Chunk> callback) {
         BinaryReader binaryReader = new BinaryReader(b);
 
@@ -26,8 +35,10 @@ public class ChunkReader {
         ChunkBatch chunkBatch = null;
         try {
 
+            // Get if the chunk has data indexes (used for blocks data)
             final boolean hasIndex = binaryReader.readBoolean();
             if (hasIndex) {
+                // Get the data indexes which will be used to read all the individual data
                 typeToIndexMap = DataReader.readDataIndexes(binaryReader);
             }
 

src/main/java/net/minestom/server/storage/systems/FileStorageSystem.java

@@ -4,7 +4,8 @@ import net.minestom.server.storage.StorageOptions;
 import net.minestom.server.storage.StorageSystem;
 import org.rocksdb.*;
 
-import java.io.File;
+import java.nio.file.Files;
+import java.nio.file.Paths;
 
 /**
  * A storage system which is local using OS files system
@@ -20,7 +21,7 @@ public class FileStorageSystem implements StorageSystem {
 
     @Override
     public boolean exists(String folderPath) {
-        return new File(folderPath).exists();
+        return Files.isDirectory(Paths.get(folderPath));
     }
 
     @Override