fix: use soft/weak references when caching bukkit/P2 worlds

- Fixes #4435

Bukkit/src/main/java/com/plotsquared/bukkit/util/BukkitWorld.java

@@ -23,11 +23,15 @@ import com.plotsquared.core.location.World;
 import org.bukkit.Bukkit;
 import org.checkerframework.checker.nullness.qual.NonNull;
 
+import java.lang.ref.SoftReference;
+import java.lang.ref.WeakReference;
 import java.util.Map;
 
 public class BukkitWorld implements World<org.bukkit.World> {
 
-    private static final Map<String, BukkitWorld> worldMap = Maps.newHashMap();
+    // Upon world unload we should probably have the P2 BukkitWorld be GCed relatively eagerly, thus freeing the bukkit world.
+    //  We also want to avoid circumstances where a bukkit world has been GCed, but a P2 BukkitWorld has not
+    private static final Map<String, WeakReference<BukkitWorld>> worldMap = Maps.newHashMap();
     private static final boolean HAS_MIN_Y;
 
     static {
@@ -41,10 +45,11 @@ public class BukkitWorld implements World<org.bukkit.World> {
         HAS_MIN_Y = temp;
     }
 
-    private final org.bukkit.World world;
+    // We want to allow GC to remove bukkit worlds, but not too eagerly
+    private final SoftReference<org.bukkit.World> world;
 
     private BukkitWorld(final org.bukkit.World world) {
-        this.world = world;
+        this.world = new SoftReference<>(world);
     }
 
     /**
@@ -68,12 +73,13 @@ public class BukkitWorld implements World<org.bukkit.World> {
      * @return World instance
      */
     public static @NonNull BukkitWorld of(final org.bukkit.World world) {
-        BukkitWorld bukkitWorld = worldMap.get(world.getName());
-        if (bukkitWorld != null && bukkitWorld.getPlatformWorld().equals(world)) {
+        WeakReference<BukkitWorld> bukkitWorldRef = worldMap.get(world.getName());
+        BukkitWorld bukkitWorld;
+        if ((bukkitWorld = bukkitWorldRef.get()) != null && world.equals(bukkitWorld.world.get())) {
             return bukkitWorld;
         }
         bukkitWorld = new BukkitWorld(world);
-        worldMap.put(world.getName(), bukkitWorld);
+        worldMap.put(world.getName(), new WeakReference<>(bukkitWorld));
         return bukkitWorld;
     }
 
@@ -97,22 +103,26 @@ public class BukkitWorld implements World<org.bukkit.World> {
 
     @Override
     public org.bukkit.World getPlatformWorld() {
-        return this.world;
+        org.bukkit.World world = this.world.get();
+        if (world == null) {
+            throw new IllegalStateException("Bukkit platform world was unloaded from memory");
+        }
+        return world;
     }
 
     @Override
     public @NonNull String getName() {
-        return this.world.getName();
+        return this.getPlatformWorld().getName();
     }
 
     @Override
     public int getMinHeight() {
-        return getMinWorldHeight(world);
+        return getMinWorldHeight(getPlatformWorld());
     }
 
     @Override
     public int getMaxHeight() {
-        return getMaxWorldHeight(world) - 1;
+        return getMaxWorldHeight(getPlatformWorld()) - 1;
     }
 
     @Override