Fix dimension not being sent for the first instance & force dimensions to be registered in the manager

src/main/java/net/minestom/server/entity/Player.java

@@ -679,11 +679,12 @@ public class Player extends LivingEntity implements CommandSender {
                 viewableChunk.removeViewer(this);
             }
 
-            // Send the new dimension
-            if (this.instance != null) {
+            // Send the new dimension if player isn't in any instance or if the dimension is different
+            {
                 final DimensionType instanceDimensionType = instance.getDimensionType();
-                if (dimensionType != instanceDimensionType)
+                if (this.instance == null || dimensionType != instanceDimensionType) {
                     sendDimension(instanceDimensionType);
+                }
             }
 
             // Load all the required chunks

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

@@ -112,6 +112,8 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param dimensionType the {@link DimensionType} of the instance
      */
     public Instance(@NotNull UUID uniqueId, @NotNull DimensionType dimensionType) {
+        Check.argCondition(!dimensionType.isRegistered(),
+                "The dimension " + dimensionType.getName() + " is not registered! Please use DimensionTypeManager#addDimension");
         this.uniqueId = uniqueId;
         this.dimensionType = dimensionType;
 

src/main/java/net/minestom/server/network/packet/server/play/JoinGamePacket.java

@@ -12,8 +12,8 @@ import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 public class JoinGamePacket implements ServerPacket {
 
     public int entityId;
-    public GameMode gameMode = GameMode.SURVIVAL;
-    public DimensionType dimensionType = DimensionType.OVERWORLD;
+    public GameMode gameMode;
+    public DimensionType dimensionType;
     public long hashedSeed;
     public int maxPlayers = 0; // Unused
     public int viewDistance;

src/main/java/net/minestom/server/world/DimensionType.java

@@ -1,6 +1,7 @@
 package net.minestom.server.world;
 
 import net.minestom.server.utils.NamespaceID;
+import org.jetbrains.annotations.NotNull;
 import org.jglrxavpok.hephaistos.nbt.NBTCompound;
 
 import java.util.Objects;
@@ -31,6 +32,8 @@ public class DimensionType {
 
     private final int id = idCounter.getAndIncrement();
 
+    protected volatile boolean registered;
+
     private final NamespaceID name;
     private final boolean natural;
     private final float ambientLight;
@@ -76,6 +79,7 @@ public class DimensionType {
         return new DimensionTypeBuilder();
     }
 
+    @NotNull
     public NBTCompound toIndexedNBT() {
         NBTCompound nbt = new NBTCompound();
         NBTCompound element = toNBT();
@@ -85,6 +89,7 @@ public class DimensionType {
         return nbt;
     }
 
+    @NotNull
     public NBTCompound toNBT() {
         NBTCompound nbt = new NBTCompound()
                 .setFloat("ambient_light", ambientLight)
@@ -114,6 +119,10 @@ public class DimensionType {
         return this.id;
     }
 
+    public boolean isRegistered() {
+        return registered;
+    }
+
     public NamespaceID getName() {
         return this.name;
     }
@@ -154,6 +163,10 @@ public class DimensionType {
         return this.bedSafe;
     }
 
+    public String getEffects() {
+        return effects;
+    }
+
     public boolean isPiglinSafe() {
         return this.piglinSafe;
     }

src/main/java/net/minestom/server/world/DimensionTypeManager.java

@@ -28,7 +28,8 @@ public final class DimensionTypeManager {
      * @param dimensionType the dimension to add
      */
     public void addDimension(@NotNull DimensionType dimensionType) {
-        dimensionTypes.add(dimensionType);
+        dimensionType.registered = true;
+        this.dimensionTypes.add(dimensionType);
     }
 
     /**
@@ -38,6 +39,7 @@ public final class DimensionTypeManager {
      * @return if the dimension type was removed, false if it was not present before
      */
     public boolean removeDimension(@NotNull DimensionType dimensionType) {
+        dimensionType.registered = false;
         return dimensionTypes.remove(dimensionType);
     }
 
@@ -51,6 +53,14 @@ public final class DimensionTypeManager {
         return Collections.unmodifiableList(dimensionTypes);
     }
 
+    /**
+     * Creates the {@link NBTCompound} containing all the registered dimensions.
+     * <p>
+     * Used when a player connects.
+     *
+     * @return an nbt compound containing the registered dimensions
+     */
+    @NotNull
     public NBTCompound toNBT() {
         NBTCompound dimensions = new NBTCompound();
         dimensions.setString("type", "minecraft:dimension_type");