Improve tag performance + concurrency tests (#1165)

jcstress-tests/src/jcstress/java/net/minestom/server/tag/TagPathRehashTest.java

@@ -0,0 +1,50 @@
+package net.minestom.server.tag;
+
+import org.openjdk.jcstress.annotations.*;
+import org.openjdk.jcstress.infra.results.LL_Result;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.openjdk.jcstress.annotations.Expect.ACCEPTABLE;
+
+@JCStressTest
+@Outcome(id = "1, 198", expect = ACCEPTABLE)
+@Outcome(id = "1, 99", expect = ACCEPTABLE)
+@Outcome(id = "2, 198", expect = ACCEPTABLE)
+@Outcome(id = "2, 99", expect = ACCEPTABLE)
+@State
+public class TagPathRehashTest {
+    private static final int MAX_SIZE = 500;
+    private static final List<Tag<Integer>> TAGS;
+
+    static {
+        List<Tag<Integer>> tags = new ArrayList<>();
+        for (int i = 0; i < MAX_SIZE; i++) {
+            tags.add(Tag.Integer("key" + i).path("path"));
+        }
+        TAGS = List.copyOf(tags);
+    }
+
+    private final TagHandler handler = TagHandler.newHandler();
+
+    @Actor
+    public void actor1() {
+        for (int i = 0; i < MAX_SIZE; i++) {
+            handler.setTag(TAGS.get(i), i);
+        }
+    }
+
+    @Actor
+    public void actor2() {
+        for (int i = 0; i < MAX_SIZE; i++) {
+            handler.setTag(TAGS.get(i), i * 2);
+        }
+    }
+
+    @Arbiter
+    public void arbiter(LL_Result r) {
+        r.r1 = handler.getTag(TAGS.get(1));
+        r.r2 = handler.getTag(TAGS.get(99));
+    }
+}

jcstress-tests/src/jcstress/java/net/minestom/server/tag/TagRehashTest.java

@@ -0,0 +1,50 @@
+package net.minestom.server.tag;
+
+import org.openjdk.jcstress.annotations.*;
+import org.openjdk.jcstress.infra.results.LL_Result;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.openjdk.jcstress.annotations.Expect.ACCEPTABLE;
+
+@JCStressTest
+@Outcome(id = "1, 198", expect = ACCEPTABLE)
+@Outcome(id = "1, 99", expect = ACCEPTABLE)
+@Outcome(id = "2, 198", expect = ACCEPTABLE)
+@Outcome(id = "2, 99", expect = ACCEPTABLE)
+@State
+public class TagRehashTest {
+    private static final int MAX_SIZE = 500;
+    private static final List<Tag<Integer>> TAGS;
+
+    static {
+        List<Tag<Integer>> tags = new ArrayList<>();
+        for (int i = 0; i < MAX_SIZE; i++) {
+            tags.add(Tag.Integer("key" + i));
+        }
+        TAGS = List.copyOf(tags);
+    }
+
+    private final TagHandler handler = TagHandler.newHandler();
+
+    @Actor
+    public void actor1() {
+        for (int i = 0; i < MAX_SIZE; i++) {
+            handler.setTag(TAGS.get(i), i);
+        }
+    }
+
+    @Actor
+    public void actor2() {
+        for (int i = 0; i < MAX_SIZE; i++) {
+            handler.setTag(TAGS.get(i), i * 2);
+        }
+    }
+
+    @Arbiter
+    public void arbiter(LL_Result r) {
+        r.r1 = handler.getTag(TAGS.get(1));
+        r.r2 = handler.getTag(TAGS.get(99));
+    }
+}

jcstress-tests/src/jcstress/java/net/minestom/server/tag/TagUpdatePathRehashTest.java

@@ -0,0 +1,57 @@
+package net.minestom.server.tag;
+
+import org.openjdk.jcstress.annotations.*;
+import org.openjdk.jcstress.infra.results.L_Result;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.openjdk.jcstress.annotations.Expect.ACCEPTABLE;
+
+@JCStressTest
+@Outcome(id = "2000", expect = ACCEPTABLE)
+@State
+public class TagUpdatePathRehashTest {
+    private static final Tag<Integer> TAG = Tag.Integer("key").path("path").defaultValue(0);
+
+    private static final int MAX_SIZE = 500;
+    private static final List<Tag<Integer>> TAGS;
+
+    static {
+        List<Tag<Integer>> tags = new ArrayList<>();
+        for (int i = 0; i < MAX_SIZE; i++) {
+            tags.add(Tag.Integer("key" + i).path("path"));
+        }
+        TAGS = List.copyOf(tags);
+    }
+
+    private final TagHandler handler = TagHandler.newHandler();
+
+    @Actor
+    public void actor1() {
+        for (int i = 0; i < 1000; i++) {
+            handler.updateTag(TAG, integer -> integer + 1);
+        }
+    }
+
+    @Actor
+    public void actor2() {
+        for (int i = 0; i < 1000; i++) {
+            handler.updateTag(TAG, integer -> integer + 1);
+        }
+    }
+
+    @Actor
+    public void actor3() {
+        // May be able to disturb actor1/2
+        for (int i = 0; i < MAX_SIZE; i++) {
+            handler.setTag(TAGS.get(i), i);
+        }
+    }
+
+    @Arbiter
+    public void arbiter(L_Result r) {
+        r.r1 = handler.getTag(TAG);
+    }
+}
+

jcstress-tests/src/jcstress/java/net/minestom/server/tag/TagUpdateTest.java

@@ -1,36 +1,33 @@
 package net.minestom.server.tag;
 
-import org.openjdk.jcstress.annotations.Actor;
+import org.openjdk.jcstress.annotations.*;
-import org.openjdk.jcstress.annotations.JCStressTest;
-import org.openjdk.jcstress.annotations.Outcome;
-import org.openjdk.jcstress.annotations.State;
 import org.openjdk.jcstress.infra.results.L_Result;
 
 import static org.openjdk.jcstress.annotations.Expect.ACCEPTABLE;
 
 @JCStressTest
-@Outcome(id = "null", expect = ACCEPTABLE)
+@Outcome(id = "2000", expect = ACCEPTABLE)
-@Outcome(id = "3", expect = ACCEPTABLE)
-@Outcome(id = "4", expect = ACCEPTABLE)
-@Outcome(id = "10", expect = ACCEPTABLE)
-@Outcome(id = "11", expect = ACCEPTABLE)
 @State
 public class TagUpdateTest {
-    private static final Tag<Integer> TAG = Tag.Integer("key");
+    private static final Tag<Integer> TAG = Tag.Integer("key").defaultValue(0);
 
     private final TagHandler handler = TagHandler.newHandler();
 
     @Actor
     public void actor1() {
-        handler.updateAndGetTag(TAG, integer -> integer == null ? 3 : integer + 1);
+        for (int i = 0; i < 1000; i++) {
+            handler.updateAndGetTag(TAG, integer -> integer + 1);
+        }
     }
 
     @Actor
     public void actor2() {
-        handler.updateAndGetTag(TAG, integer -> integer == null ? 10 : integer + 1);
+        for (int i = 0; i < 1000; i++) {
+            handler.updateAndGetTag(TAG, integer -> integer + 1);
+        }
     }
 
-    @Actor
+    @Arbiter
     public void arbiter(L_Result r) {
         r.r1 = handler.getTag(TAG);
     }

src/main/java/net/minestom/server/item/ItemMeta.java

@@ -153,7 +153,7 @@ public sealed interface ItemMeta extends TagReadable, Writeable
 
         @Contract("_ -> this")
         default @NotNull Builder canPlaceOn(@NotNull Set<@NotNull Block> blocks) {
-            return set(ItemTags.CAN_PLACE_ON, List.copyOf(blocks));
+            return set(ItemTags.CAN_PLACE_ON, blocks.stream().map(block -> Block.fromNamespaceId(block.name())).toList());
         }
 
         @Contract("_ -> this")
@@ -163,7 +163,7 @@ public sealed interface ItemMeta extends TagReadable, Writeable
 
         @Contract("_ -> this")
         default @NotNull Builder canDestroy(@NotNull Set<@NotNull Block> blocks) {
-            return set(ItemTags.CAN_DESTROY, List.copyOf(blocks));
+            return set(ItemTags.CAN_DESTROY, blocks.stream().map(block -> Block.fromNamespaceId(block.name())).toList());
         }
 
         @Contract("_ -> this")

src/main/java/net/minestom/server/tag/Serializers.java

@@ -12,8 +12,6 @@ import java.util.function.Function;
  * Basic serializers for {@link Tag tags}.
  */
 final class Serializers {
-    static final Entry<TagHandlerImpl, NBTCompound> PATH = new Entry<>(NBTType.TAG_Compound, TagHandlerImpl::fromCompound, TagHandlerImpl::asCompound);
-
     static final Entry<Byte, NBTByte> BYTE = new Entry<>(NBTType.TAG_Byte, NBTByte::getValue, NBT::Byte);
     static final Entry<Boolean, NBTByte> BOOLEAN = new Entry<>(NBTType.TAG_Byte, NBTByte::asBoolean, NBT::Boolean);
     static final Entry<Short, NBTShort> SHORT = new Entry<>(NBTType.TAG_Short, NBTShort::getValue, NBT::Short);
@@ -44,7 +42,11 @@ final class Serializers {
                 });
     }
 
-    record Entry<T, N extends NBT>(NBTType<N> nbtType, Function<N, T> reader, Function<T, N> writer) {
+    record Entry<T, N extends NBT>(NBTType<N> nbtType, Function<N, T> reader, Function<T, N> writer, boolean isPath) {
+        Entry(NBTType<N> nbtType, Function<N, T> reader, Function<T, N> writer) {
+            this(nbtType, reader, writer, false);
+        }
+
         T read(N nbt) {
             return reader.apply(nbt);
         }

src/main/java/net/minestom/server/tag/StaticIntMap.java

@@ -0,0 +1,87 @@
+package net.minestom.server.tag;
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Range;
+
+import java.util.Arrays;
+import java.util.function.Consumer;
+
+sealed interface StaticIntMap<T> permits StaticIntMap.Array {
+
+    T get(@Range(from = 0, to = Integer.MAX_VALUE) int key);
+
+    void forValues(@NotNull Consumer<T> consumer);
+
+    @NotNull StaticIntMap<T> copy();
+
+    // Methods potentially causing re-hashing
+
+    void put(@Range(from = 0, to = Integer.MAX_VALUE) int key, T value);
+
+    void remove(@Range(from = 0, to = Integer.MAX_VALUE) int key);
+
+    void updateContent(@NotNull StaticIntMap<T> content);
+
+    final class Array<T> implements StaticIntMap<T> {
+        private static final Object[] EMPTY_ARRAY = new Object[0];
+
+        private T[] array;
+
+        public Array(T[] array) {
+            this.array = array;
+        }
+
+        public Array() {
+            //noinspection unchecked
+            this.array = (T[]) EMPTY_ARRAY;
+        }
+
+        @Override
+        public T get(int key) {
+            final T[] array = this.array;
+            return key < array.length ? array[key] : null;
+        }
+
+        @Override
+        public void forValues(@NotNull Consumer<T> consumer) {
+            final T[] array = this.array;
+            for (T value : array) {
+                if (value != null) consumer.accept(value);
+            }
+        }
+
+        @Override
+        public @NotNull StaticIntMap<T> copy() {
+            return new Array<>(array.clone());
+        }
+
+        @Override
+        public void put(int key, T value) {
+            T[] array = this.array;
+            if (key >= array.length) {
+                array = updateArray(Arrays.copyOf(array, key * 2 + 1));
+            }
+            array[key] = value;
+        }
+
+        @Override
+        public void updateContent(@NotNull StaticIntMap<T> content) {
+            if (content instanceof StaticIntMap.Array<T> arrayMap) {
+                updateArray(arrayMap.array.clone());
+            } else {
+                throw new IllegalArgumentException("Invalid content type: " + content.getClass());
+            }
+        }
+
+        @Override
+        public void remove(int key) {
+            T[] array = this.array;
+            if (key < array.length) array[key] = null;
+        }
+
+        T[] updateArray(T[] result) {
+            this.array = result;
+            return result;
+        }
+    }
+}

src/main/java/net/minestom/server/tag/Tag.java

@@ -210,6 +210,11 @@ public class Tag<T> {
         return supplier != null ? supplier.get() : null;
     }
 
+    final T copyValue(@NotNull T value) {
+        final UnaryOperator<T> copier = copy;
+        return copier != null ? copier.apply(value) : value;
+    }
+
     @Override
     public boolean equals(Object o) {
         if (this == o) return true;

src/main/java/net/minestom/server/tag/TagHandlerImpl.java

@@ -1,8 +1,7 @@
 package net.minestom.server.tag;
 
-import it.unimi.dsi.fastutil.ints.Int2ObjectMap;
-import it.unimi.dsi.fastutil.ints.Int2ObjectOpenHashMap;
 import net.minestom.server.utils.PropertyUtils;
+import org.jetbrains.annotations.Contract;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.jetbrains.annotations.UnknownNullability;
@@ -13,205 +12,227 @@ import org.jglrxavpok.hephaistos.nbt.NBTType;
 import org.jglrxavpok.hephaistos.nbt.mutable.MutableNBTCompound;
 
 import java.lang.invoke.VarHandle;
-import java.util.function.Supplier;
 import java.util.function.UnaryOperator;
 
 final class TagHandlerImpl implements TagHandler {
     private static final boolean CACHE_ENABLE = PropertyUtils.getBoolean("minestom.tag-handler-cache", true);
+    static final Serializers.Entry<Node, NBTCompound> NODE_SERIALIZER = new Serializers.Entry<>(NBTType.TAG_Compound, entries -> fromCompound(entries).root, Node::compound, true);
 
-    private final TagHandlerImpl parent;
+    private final Node root;
-    private volatile SPMCMap entries;
+    private volatile Node copy;
-    private Cache cache;
 
-    TagHandlerImpl(TagHandlerImpl parent) {
+    TagHandlerImpl(Node root) {
-        this.parent = parent;
+        this.root = root;
-        this.entries = new SPMCMap(this);
-        this.cache = null;
     }
 
     TagHandlerImpl() {
-        this(null);
+        this.root = new Node();
     }
 
     static TagHandlerImpl fromCompound(NBTCompoundLike compoundLike) {
         final NBTCompound compound = compoundLike.toCompound();
-        TagHandlerImpl handler = new TagHandlerImpl(null);
+        TagHandlerImpl handler = new TagHandlerImpl();
         TagNbtSeparator.separate(compound, entry -> handler.setTag(entry.tag(), entry.value()));
+        handler.root.compound = compound;
         return handler;
     }
 
     @Override
     public <T> @UnknownNullability T getTag(@NotNull Tag<T> tag) {
-        return read(entries, tag, this::asCompound);
+        VarHandle.fullFence();
+        return root.getTag(tag);
     }
 
     @Override
     public <T> void setTag(@NotNull Tag<T> tag, @Nullable T value) {
-        TagHandlerImpl local = this;
+        // Handle view tags
-        final Tag.PathEntry[] paths = tag.path;
+        if (tag.isView()) {
-        final boolean present = value != null;
-        final int tagIndex = tag.index;
-        final boolean isView = tag.isView();
             synchronized (this) {
-            if (paths != null) {
+                Node syncNode = traversePathWrite(root, tag, value != null);
-                if ((local = traversePathWrite(this, paths, present)) == null)
+                if (syncNode != null) {
-                    return; // Tried to remove an absent tag. Do nothing
+                    syncNode.updateContent(value != null ? (NBTCompound) tag.entry.write(value) : NBTCompound.EMPTY);
+                    syncNode.invalidate();
                 }
-            SPMCMap entries = local.entries;
-            if (present) {
-                if (!isView) {
-                    Entry previous = entries.get(tagIndex);
-                    if (previous != null && previous.tag().shareValue(tag)) {
-                        previous.updateValue(value);
-                    } else {
-                        entries.put(tagIndex, valueToEntry(local, tag, value));
             }
-                } else {
-                    local.updateContent((NBTCompound) tag.entry.write(value));
             return;
         }
+        // Normal tag
+        final int tagIndex = tag.index;
+        VarHandle.fullFence();
+        Node node = traversePathWrite(root, tag, value != null);
+        if (node == null)
+            return; // Tried to remove an absent tag. Do nothing
+        StaticIntMap<Entry<?>> entries = node.entries;
+        if (value != null) {
+            Entry previous = entries.get(tagIndex);
+            if (previous != null && previous.tag().shareValue(tag)) {
+                previous.updateValue(tag.copyValue(value));
             } else {
-                // Remove recursively
+                synchronized (this) {
-                if (!isView) {
+                    node = traversePathWrite(root, tag, true);
-                    if (entries.remove(tagIndex) == null) return;
+                    node.entries.put(tagIndex, valueToEntry(node, tag, value));
-                } else {
-                    entries.clear();
                 }
-                if (paths != null) {
-                    TagHandlerImpl tmp = local;
-                    int i = paths.length;
-                    do {
-                        if (!tmp.entries.isEmpty()) break;
-                        tmp = tmp.parent;
-                        tmp.entries.remove(paths[--i].index());
-                    } while (i > 0);
-                }
-            }
-            entries.invalidate();
-            assert !local.entries.rehashed;
-        }
-    }
-
-    private <T> Entry<?> valueToEntry(TagHandlerImpl parent, Tag<T> tag, @NotNull T value) {
-        if (value instanceof NBT nbt) {
-            if (nbt instanceof NBTCompound compound) {
-                var handler = new TagHandlerImpl(parent);
-                handler.updateContent(compound);
-                return new PathEntry(tag.getKey(), handler);
-            } else {
-                final var nbtEntry = TagNbtSeparator.separateSingle(tag.getKey(), nbt);
-                return new TagEntry<>(nbtEntry.tag(), nbtEntry.value());
             }
         } else {
-            final UnaryOperator<T> copy = tag.copy;
+            synchronized (this) {
-            if (copy != null) value = copy.apply(value);
+                node = traversePathWrite(root, tag, false);
-            return new TagEntry<>(tag, value);
+                if (node != null) node.entries.remove(tagIndex);
             }
         }
+        node.invalidate();
+    }
 
     @Override
-    public synchronized <T> void updateTag(@NotNull Tag<T> tag, @NotNull UnaryOperator<@UnknownNullability T> value) {
+    public <T> void updateTag(@NotNull Tag<T> tag, @NotNull UnaryOperator<@UnknownNullability T> value) {
-        setTag(tag, value.apply(getTag(tag)));
+        updateTag0(tag, value, false);
     }
 
     @Override
-    public synchronized <T> @UnknownNullability T updateAndGetTag(@NotNull Tag<T> tag, @NotNull UnaryOperator<@UnknownNullability T> value) {
+    public <T> @UnknownNullability T updateAndGetTag(@NotNull Tag<T> tag, @NotNull UnaryOperator<@UnknownNullability T> value) {
-        final T next = value.apply(getTag(tag));
+        return updateTag0(tag, value, false);
-        setTag(tag, next);
-        return next;
     }
 
     @Override
-    public synchronized <T> @UnknownNullability T getAndUpdateTag(@NotNull Tag<T> tag, @NotNull UnaryOperator<@UnknownNullability T> value) {
+    public <T> @UnknownNullability T getAndUpdateTag(@NotNull Tag<T> tag, @NotNull UnaryOperator<@UnknownNullability T> value) {
-        final T prev = getTag(tag);
+        return updateTag0(tag, value, true);
-        setTag(tag, value.apply(prev));
+    }
-        return prev;
+
+    private synchronized <T> T updateTag0(@NotNull Tag<T> tag, @NotNull UnaryOperator<T> value, boolean returnPrevious) {
+        final int tagIndex = tag.index;
+        final Node node = traversePathWrite(root, tag, true);
+        StaticIntMap<Entry<?>> entries = node.entries;
+
+        final Entry previousEntry = entries.get(tagIndex);
+        final T previousValue = previousEntry != null ? (T) previousEntry.value() : tag.createDefault();
+        final T newValue = value.apply(previousValue);
+        if (newValue != null) entries.put(tagIndex, valueToEntry(node, tag, newValue));
+        else entries.remove(tagIndex);
+
+        node.invalidate();
+        return returnPrevious ? previousValue : newValue;
     }
 
     @Override
     public @NotNull TagReadable readableCopy() {
-        return updatedCache();
+        Node copy = this.copy;
-    }
+        if (copy == null) {
-
-    @Override
-    public @NotNull TagHandler copy() {
-        return fromCompound(asCompound());
-    }
-
-    @Override
-    public void updateContent(@NotNull NBTCompoundLike compound) {
-        final TagHandlerImpl converted = fromCompound(compound);
             synchronized (this) {
-            this.cache = converted.cache;
+                this.copy = copy = root.copy(null);
-            this.entries = new SPMCMap(this, converted.entries);
             }
         }
+        return copy;
+    }
+
+    @Override
+    public synchronized @NotNull TagHandler copy() {
+        return new TagHandlerImpl(root.copy(null));
+    }
+
+    @Override
+    public synchronized void updateContent(@NotNull NBTCompoundLike compound) {
+        this.root.updateContent(compound);
+    }
 
     @Override
     public @NotNull NBTCompound asCompound() {
-        return updatedCache().compound;
+        VarHandle.fullFence();
+        return root.compound();
     }
 
-    private static TagHandlerImpl traversePathWrite(TagHandlerImpl root, Tag.PathEntry[] paths,
+    private static Node traversePathRead(Node node, Tag<?> tag) {
+        final Tag.PathEntry[] paths = tag.path;
+        if (paths == null) return node;
+        for (var path : paths) {
+            final Entry<?> entry = node.entries.get(path.index());
+            if (entry == null || (node = entry.toNode()) == null)
+                return null;
+        }
+        return node;
+    }
+
+    @Contract("_, _, true -> !null")
+    private Node traversePathWrite(Node root, Tag<?> tag,
                                    boolean present) {
-        TagHandlerImpl local = root;
+        final Tag.PathEntry[] paths = tag.path;
+        if (paths == null) return root;
+        Node local = root;
         for (Tag.PathEntry path : paths) {
             final int pathIndex = path.index();
             final Entry<?> entry = local.entries.get(pathIndex);
-            if (entry instanceof PathEntry pathEntry) {
+            if (entry != null && entry.tag.entry.isPath()) {
                 // Existing path, continue navigating
-                assert pathEntry.value.parent == local : "Path parent is invalid: " + pathEntry.value.parent + " != " + local;
+                final Node tmp = (Node) entry.value;
-                local = pathEntry.value;
+                assert tmp.parent == local : "Path parent is invalid: " + tmp.parent + " != " + local;
+                local = tmp;
             } else {
                 if (!present) return null;
+                synchronized (this) {
+                    var synEntry = local.entries.get(pathIndex);
+                    if (synEntry != null && synEntry.tag.entry.isPath()) {
+                        // Existing path, continue navigating
+                        final Node tmp = (Node) synEntry.value;
+                        assert tmp.parent == local : "Path parent is invalid: " + tmp.parent + " != " + local;
+                        local = tmp;
+                        continue;
+                    }
+
                     // Empty path, create a new handler.
                     // Slow path is taken if the entry comes from a Structure tag, requiring conversion from NBT
-                TagHandlerImpl tmp = local;
+                    Node tmp = local;
-                local = new TagHandlerImpl(tmp);
+                    local = new Node(tmp);
-                if (entry != null && entry.nbt() instanceof NBTCompound compound) {
+                    if (synEntry != null && synEntry.nbt() instanceof NBTCompound compound) {
                         local.updateContent(compound);
                     }
-                tmp.entries.put(pathIndex, new PathEntry(path.name(), local));
+                    tmp.entries.put(pathIndex, Entry.makePathEntry(path.name(), local));
+                }
             }
         }
         return local;
     }
 
-    private synchronized Cache updatedCache() {
+    private <T> Entry<?> valueToEntry(Node parent, Tag<T> tag, @NotNull T value) {
-        Cache cache;
+        if (value instanceof NBT nbt) {
-        if (!CACHE_ENABLE || (cache = this.cache) == null) {
+            if (nbt instanceof NBTCompound compound) {
-            final SPMCMap entries = this.entries;
+                final TagHandlerImpl handler = fromCompound(compound);
-            if (!entries.isEmpty()) {
+                return Entry.makePathEntry(tag, new Node(parent, handler.root.entries));
-                MutableNBTCompound tmp = new MutableNBTCompound();
+            } else {
-                for (Entry<?> entry : entries.values()) {
+                final var nbtEntry = TagNbtSeparator.separateSingle(tag.getKey(), nbt);
-                    if (entry != null) tmp.put(entry.tag().getKey(), entry.nbt());
+                return new Entry<>(nbtEntry.tag(), nbtEntry.value());
             }
-                cache = new Cache(entries.clone(), tmp.toCompound());
+        } else {
-            } else cache = Cache.EMPTY;
+            return new Entry<>(tag, tag.copyValue(value));
-            this.cache = cache;
         }
-        return cache;
     }
 
-    private static <T> T read(Int2ObjectOpenHashMap<Entry<?>> entries, Tag<T> tag,
+    final class Node implements TagReadable {
-                              Supplier<NBTCompound> rootCompoundSupplier) {
+        final Node parent;
-        final Tag.PathEntry[] paths = tag.path;
+        final StaticIntMap<Entry<?>> entries;
-        TagHandlerImpl pathHandler = null;
+        NBTCompound compound;
-        if (paths != null) {
+
-            if ((pathHandler = traversePathRead(paths, entries)) == null)
+        public Node(Node parent, StaticIntMap<Entry<?>> entries) {
+            this.parent = parent;
+            this.entries = entries;
+        }
+
+        Node(Node parent) {
+            this(parent, new StaticIntMap.Array<>());
+        }
+
+        Node() {
+            this(null);
+        }
+
+        @Override
+        public <T> @UnknownNullability T getTag(@NotNull Tag<T> tag) {
+            final Node node = traversePathRead(this, tag);
+            if (node == null)
                 return tag.createDefault(); // Must be a path-able entry, but not present
-            entries = pathHandler.entries;
+            if (tag.isView()) return tag.read(node.compound());
-        }
 
-        if (tag.isView()) {
+            final StaticIntMap<Entry<?>> entries = node.entries;
-            return tag.read(pathHandler != null ?
+            final Entry<?> entry = entries.get(tag.index);
-                    pathHandler.asCompound() : rootCompoundSupplier.get());
+            if (entry == null)
-        }
+                return tag.createDefault(); // Not present
-
-        final Entry<?> entry;
-        if ((entry = entries.get(tag.index)) == null) {
-            return tag.createDefault();
-        }
             if (entry.tag().shareValue(tag)) {
                 // The tag used to write the entry is compatible with the one used to get
                 // return the value directly
@@ -225,143 +246,114 @@ final class TagHandlerImpl implements TagHandler {
             return type == null || type == nbt.getID() ? serializerEntry.read(nbt) : tag.createDefault();
         }
 
-    private static TagHandlerImpl traversePathRead(Tag.PathEntry[] paths,
+        void updateContent(@NotNull NBTCompoundLike compoundLike) {
-                                                   Int2ObjectOpenHashMap<Entry<?>> entries) {
+            final NBTCompound compound = compoundLike.toCompound();
-        assert paths != null && paths.length > 0;
+            final TagHandlerImpl converted = fromCompound(compound);
-        TagHandlerImpl result = null;
+            this.entries.updateContent(converted.root.entries);
-        for (var path : paths) {
+            this.compound = compound;
-            final Entry<?> entry;
+        }
-            if ((entry = entries.get(path.index())) == null)
+
-                return null;
+        NBTCompound compound() {
-            if (entry instanceof PathEntry pathEntry) {
+            NBTCompound compound;
-                result = pathEntry.value;
+            if (!CACHE_ENABLE || (compound = this.compound) == null) {
-            } else if (entry.nbt() instanceof NBTCompound compound) {
+                MutableNBTCompound tmp = new MutableNBTCompound();
-                // Slow path forcing a conversion of the structure to NBTCompound
+                this.entries.forValues(entry -> {
-                // TODO should the handler be cached inside the entry?
+                    final Tag tag = entry.tag();
-                result = fromCompound(compound);
+                    final NBT nbt = entry.nbt();
+                    if (!tag.entry.isPath() || !((NBTCompound) nbt).isEmpty()) {
+                        tmp.put(tag.getKey(), nbt);
+                    }
+                });
+                this.compound = compound = tmp.toCompound();
+            }
+            return compound;
+        }
+
+        @Contract("null -> !null")
+        Node copy(Node parent) {
+            MutableNBTCompound tmp = new MutableNBTCompound();
+            Node result = new Node(parent, new StaticIntMap.Array<>());
+            StaticIntMap<Entry<?>> entries = result.entries;
+            this.entries.forValues(entry -> {
+                Tag tag = entry.tag;
+                Object value = entry.value;
+                NBT nbt;
+                if (value instanceof Node node) {
+                    Node copy = node.copy(result);
+                    if (copy == null)
+                        return; // Empty node
+                    value = copy;
+                    nbt = copy.compound;
+                    assert nbt != null : "Node copy should also compute the compound";
                 } else {
-                // Entry is not path-able
+                    nbt = entry.nbt();
-                return null;
                 }
-            assert result != null;
+
-            entries = result.entries;
+                tmp.put(tag.getKey(), nbt);
-        }
+                entries.put(tag.index, valueToEntry(result, tag, value));
-        assert result != null;
+            });
+            if (tmp.isEmpty() && parent != null)
+                return null; // Empty child node
+            result.compound = tmp.toCompound();
             return result;
         }
 
-    private record Cache(Int2ObjectOpenHashMap<Entry<?>> entries, NBTCompound compound) implements TagReadable {
+        void invalidate() {
-        static final Cache EMPTY = new Cache(new Int2ObjectOpenHashMap<>(), NBTCompound.EMPTY);
+            Node tmp = this;
-
+            do tmp.compound = null;
-        @Override
+            while ((tmp = tmp.parent) != null);
-        public <T> @UnknownNullability T getTag(@NotNull Tag<T> tag) {
+            TagHandlerImpl.this.copy = null;
-            return read(entries, tag, () -> compound);
         }
     }
 
-    private sealed interface Entry<T>
+    private static final class Entry<T> {
-            permits TagEntry, PathEntry {
-        Tag<T> tag();
-
-        T value();
-
-        NBT nbt();
-
-        void updateValue(T value);
-    }
-
-    private static final class TagEntry<T> implements Entry<T> {
         private final Tag<T> tag;
-        volatile T value;
+        T value;
-        volatile NBT nbt;
+        NBT nbt;
 
-        TagEntry(Tag<T> tag, T value) {
+        Entry(Tag<T> tag, T value) {
             this.tag = tag;
             this.value = value;
         }
 
-        @Override
+        static Entry<?> makePathEntry(String path, Node node) {
+            return new Entry<>(Tag.tag(path, NODE_SERIALIZER), node);
+        }
+
+        static Entry<?> makePathEntry(Tag<?> tag, Node node) {
+            return makePathEntry(tag.getKey(), node);
+        }
+
         public Tag<T> tag() {
             return tag;
         }
 
-        @Override
         public T value() {
             return value;
         }
 
-        @Override
         public NBT nbt() {
+            if (tag.entry.isPath()) return ((Node) value).compound();
             NBT nbt = this.nbt;
             if (nbt == null) this.nbt = nbt = tag.entry.write(value);
             return nbt;
         }
 
-        @Override
         public void updateValue(T value) {
+            assert !tag.entry.isPath();
             this.value = value;
             this.nbt = null;
         }
-    }
 
-    private record PathEntry(Tag<TagHandlerImpl> tag,
+        Node toNode() {
-                             TagHandlerImpl value) implements Entry<TagHandlerImpl> {
+            if (tag.entry.isPath()) return (Node) value;
-        PathEntry(String key, TagHandlerImpl value) {
+            if (nbt() instanceof NBTCompound compound) {
-            this(Tag.tag(key, Serializers.PATH), value);
+                // Slow path forcing a conversion of the structure to NBTCompound
+                // TODO should the handler be cached inside the entry?
+                return fromCompound(compound).root;
             }
-
+            // Entry is not path-able
-        @Override
+            return null;
-        public NBTCompound nbt() {
-            return value.asCompound();
-        }
-
-        @Override
-        public void updateValue(TagHandlerImpl value) {
-            throw new UnsupportedOperationException("Cannot update a path entry");
-        }
-    }
-
-    static final class SPMCMap extends Int2ObjectOpenHashMap<Entry<?>> {
-        final TagHandlerImpl handler;
-        volatile boolean rehashed;
-
-        SPMCMap(TagHandlerImpl handler) {
-            super();
-            this.handler = handler;
-            assertState();
-        }
-
-        SPMCMap(TagHandlerImpl handler, Int2ObjectMap<TagHandlerImpl.Entry<?>> m) {
-            super(m.size(), DEFAULT_LOAD_FACTOR);
-            this.handler = handler;
-            assertState();
-            putAll(m);
-        }
-
-        @Override
-        protected void rehash(int newSize) {
-            assertState();
-            this.handler.entries = new SPMCMap(handler, this);
-            this.rehashed = true;
-        }
-
-        @Override
-        public SPMCMap clone() {
-            return (SPMCMap) super.clone();
-        }
-
-        void invalidate() {
-            if (!CACHE_ENABLE) return;
-            TagHandlerImpl tmp = handler;
-            do {
-                tmp.cache = null;
-            } while ((tmp = tmp.parent) != null);
-            VarHandle.fullFence();
-        }
-
-        private void assertState() {
-            assert !rehashed;
-            assert handler != null;
         }
     }
 }

src/main/java/net/minestom/server/tag/TagRecord.java

@@ -71,9 +71,9 @@ final class TagRecord {
     }
 
     static final class Serializer<T extends Record> implements TagSerializer<T> {
-        Constructor<T> constructor;
+        final Constructor<T> constructor;
-        Entry[] entries;
+        final Entry[] entries;
-        Serializers.Entry<T, NBTCompound> serializerEntry;
+        final Serializers.Entry<T, NBTCompound> serializerEntry;
 
         Serializer(Constructor<T> constructor, Entry[] entries) {
             this.constructor = constructor;

src/test/java/net/minestom/server/item/ItemBlockTest.java

@@ -0,0 +1,62 @@
+package net.minestom.server.item;
+
+import net.minestom.server.instance.block.Block;
+import org.junit.jupiter.api.Test;
+
+import static net.minestom.server.api.TestUtils.assertEqualsSNBT;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ItemBlockTest {
+
+    @Test
+    public void canPlace() {
+        var item = ItemStack.builder(Material.STONE)
+                .meta(builder -> builder.canPlaceOn(Block.STONE))
+                .build();
+        assertTrue(item.meta().getCanPlaceOn().contains(Block.STONE));
+    }
+
+    @Test
+    public void canPlaceNbt() {
+        var item = ItemStack.builder(Material.STONE)
+                .meta(builder -> builder.canPlaceOn(Block.STONE))
+                .build();
+        assertEqualsSNBT("""
+                {"CanPlaceOn":["minecraft:stone"]}
+                """, item.meta().toNBT());
+    }
+
+    @Test
+    public void canPlaceMismatchProperties() {
+        var item = ItemStack.builder(Material.STONE)
+                .meta(builder -> builder.canPlaceOn(Block.SANDSTONE_STAIRS.withProperty("facing", "south")))
+                .build();
+        assertTrue(item.meta().getCanPlaceOn().contains(Block.SANDSTONE_STAIRS));
+    }
+
+    @Test
+    public void canDestroy() {
+        var item = ItemStack.builder(Material.STONE)
+                .meta(builder -> builder.canDestroy(Block.STONE))
+                .build();
+        assertTrue(item.meta().getCanDestroy().contains(Block.STONE));
+    }
+
+    @Test
+    public void canDestroyNbt() {
+        var item = ItemStack.builder(Material.STONE)
+                .meta(builder -> builder.canDestroy(Block.STONE))
+                .build();
+        assertEqualsSNBT("""
+                {"CanDestroy":["minecraft:stone"]}
+                """, item.meta().toNBT());
+    }
+
+    @Test
+    public void canDestroyMismatchProperties() {
+        var item = ItemStack.builder(Material.STONE)
+                .meta(builder -> builder.canDestroy(Block.SANDSTONE_STAIRS.withProperty("facing", "south")))
+                .build();
+        assertTrue(item.meta().getCanDestroy().contains(Block.SANDSTONE_STAIRS));
+    }
+}

src/test/java/net/minestom/server/item/ItemDisplayTest.java

@@ -0,0 +1,55 @@
+package net.minestom.server.item;
+
+import net.kyori.adventure.text.Component;
+import net.kyori.adventure.text.serializer.gson.GsonComponentSerializer;
+import org.jglrxavpok.hephaistos.nbt.NBTString;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class ItemDisplayTest {
+
+    @Test
+    public void lore() {
+        var item = ItemStack.of(Material.DIAMOND_SWORD);
+        assertEquals(List.of(), item.getLore());
+        assertNull(item.meta().toNBT().get("display"));
+
+        {
+            var lore = List.of(Component.text("Hello"));
+            item = item.withLore(lore);
+            assertEquals(lore, item.getLore());
+            var loreNbt = item.meta().toNBT().getCompound("display").<NBTString>getList("Lore");
+            assertNotNull(loreNbt);
+            assertEquals(1, loreNbt.getSize());
+            assertEquals(lore, loreNbt.asListView().stream().map(line -> GsonComponentSerializer.gson().deserialize(line.getValue())).toList());
+        }
+
+        {
+            var lore = List.of(Component.text("Hello"), Component.text("World"));
+            item = item.withLore(lore);
+            assertEquals(lore, item.getLore());
+            var loreNbt = item.meta().toNBT().getCompound("display").<NBTString>getList("Lore");
+            assertNotNull(loreNbt);
+            assertEquals(2, loreNbt.getSize());
+            assertEquals(lore, loreNbt.asListView().stream().map(line -> GsonComponentSerializer.gson().deserialize(line.getValue())).toList());
+        }
+
+        {
+            var lore = Stream.of("string test").map(Component::text).toList();
+            item = item.withLore(lore);
+            assertEquals(lore, item.getLore());
+            var loreNbt = item.meta().toNBT().getCompound("display").<NBTString>getList("Lore");
+            assertNotNull(loreNbt);
+            assertEquals(1, loreNbt.getSize());
+            assertEquals(lore, loreNbt.asListView().stream().map(line -> GsonComponentSerializer.gson().deserialize(line.getValue())).toList());
+        }
+
+        // Ensure that lore can be properly removed without residual (display compound)
+        item = item.withLore(List.of());
+        assertNull(item.meta().toNBT().get("display"));
+    }
+}

src/test/java/net/minestom/server/item/ItemEnchantTest.java

@@ -0,0 +1,42 @@
+package net.minestom.server.item;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ItemEnchantTest {
+
+    @Test
+    public void enchant() {
+        var item = ItemStack.of(Material.DIAMOND_SWORD);
+        var enchantments = item.meta().getEnchantmentMap();
+        assertTrue(enchantments.isEmpty(), "items do not have enchantments by default");
+
+        item = item.withMeta(meta -> meta.enchantment(Enchantment.EFFICIENCY, (short) 10));
+        enchantments = item.meta().getEnchantmentMap();
+        assertEquals(enchantments.size(), 1);
+        assertEquals(enchantments.get(Enchantment.EFFICIENCY), (short) 10);
+
+        item = item.withMeta(meta -> meta.enchantment(Enchantment.INFINITY, (short) 5));
+        enchantments = item.meta().getEnchantmentMap();
+        assertEquals(enchantments.size(), 2);
+        assertEquals(enchantments.get(Enchantment.EFFICIENCY), (short) 10);
+        assertEquals(enchantments.get(Enchantment.INFINITY), (short) 5);
+
+        item = item.withMeta(meta -> meta.enchantments(Map.of()));
+        enchantments = item.meta().getEnchantmentMap();
+        assertTrue(enchantments.isEmpty());
+
+        // Ensure that enchantments can still be modified after being emptied
+        item = item.withMeta(meta -> meta.enchantment(Enchantment.EFFICIENCY, (short) 10));
+        enchantments = item.meta().getEnchantmentMap();
+        assertEquals(enchantments.get(Enchantment.EFFICIENCY), (short) 10);
+
+        item = item.withMeta(ItemMeta.Builder::clearEnchantment);
+        enchantments = item.meta().getEnchantmentMap();
+        assertTrue(enchantments.isEmpty());
+    }
+}

src/test/java/net/minestom/server/item/ItemTest.java

@@ -2,14 +2,8 @@ package net.minestom.server.item;
 
 import net.kyori.adventure.text.Component;
 import net.kyori.adventure.text.format.NamedTextColor;
-import net.kyori.adventure.text.serializer.gson.GsonComponentSerializer;
-import org.jglrxavpok.hephaistos.nbt.NBTString;
 import org.junit.jupiter.api.Test;
 
-import java.util.List;
-import java.util.Map;
-import java.util.stream.Stream;
-
 import static org.junit.jupiter.api.Assertions.*;
 
 public class ItemTest {
@@ -78,78 +72,6 @@ public class ItemTest {
         assertEquals(createItem(), item, "Items must be equal if created from the same meta nbt");
     }
 
-    @Test
-    public void testEnchant() {
-        var item = ItemStack.of(Material.DIAMOND_SWORD);
-        var enchantments = item.meta().getEnchantmentMap();
-        assertTrue(enchantments.isEmpty(), "items do not have enchantments by default");
-
-        item = item.withMeta(meta -> meta.enchantment(Enchantment.EFFICIENCY, (short) 10));
-        enchantments = item.meta().getEnchantmentMap();
-        assertEquals(enchantments.size(), 1);
-        assertEquals(enchantments.get(Enchantment.EFFICIENCY), (short) 10);
-
-        item = item.withMeta(meta -> meta.enchantment(Enchantment.INFINITY, (short) 5));
-        enchantments = item.meta().getEnchantmentMap();
-        assertEquals(enchantments.size(), 2);
-        assertEquals(enchantments.get(Enchantment.EFFICIENCY), (short) 10);
-        assertEquals(enchantments.get(Enchantment.INFINITY), (short) 5);
-
-        item = item.withMeta(meta -> meta.enchantments(Map.of()));
-        enchantments = item.meta().getEnchantmentMap();
-        assertTrue(enchantments.isEmpty());
-
-        // Ensure that enchantments can still be modified after being emptied
-        item = item.withMeta(meta -> meta.enchantment(Enchantment.EFFICIENCY, (short) 10));
-        enchantments = item.meta().getEnchantmentMap();
-        assertEquals(enchantments.get(Enchantment.EFFICIENCY), (short) 10);
-
-        item = item.withMeta(ItemMeta.Builder::clearEnchantment);
-        enchantments = item.meta().getEnchantmentMap();
-        assertTrue(enchantments.isEmpty());
-    }
-
-    @Test
-    public void testLore() {
-        var item = ItemStack.of(Material.DIAMOND_SWORD);
-        assertEquals(List.of(), item.getLore());
-        assertNull(item.meta().toNBT().get("display"));
-
-        {
-            var lore = List.of(Component.text("Hello"));
-            item = item.withLore(lore);
-            assertEquals(lore, item.getLore());
-            var loreNbt = item.meta().toNBT().getCompound("display").<NBTString>getList("Lore");
-            assertNotNull(loreNbt);
-            assertEquals(1, loreNbt.getSize());
-            assertEquals(lore, loreNbt.asListView().stream().map(line -> GsonComponentSerializer.gson().deserialize(line.getValue())).toList());
-        }
-
-        {
-            var lore = List.of(Component.text("Hello"), Component.text("World"));
-            item = item.withLore(lore);
-            assertEquals(lore, item.getLore());
-            var loreNbt = item.meta().toNBT().getCompound("display").<NBTString>getList("Lore");
-            assertNotNull(loreNbt);
-            assertEquals(2, loreNbt.getSize());
-            assertEquals(lore, loreNbt.asListView().stream().map(line -> GsonComponentSerializer.gson().deserialize(line.getValue())).toList());
-        }
-
-        {
-            var lore = Stream.of("string test").map(Component::text).toList();
-            item = item.withLore(lore);
-            assertEquals(lore, item.getLore());
-            var loreNbt = item.meta().toNBT().getCompound("display").<NBTString>getList("Lore");
-            assertNotNull(loreNbt);
-            assertEquals(1, loreNbt.getSize());
-            assertEquals(lore, loreNbt.asListView().stream().map(line -> GsonComponentSerializer.gson().deserialize(line.getValue())).toList());
-        }
-
-        // Ensure that lore can be properly removed without residual (display compound)
-        item = item.withLore(List.of());
-        assertNull(item.meta().toNBT().get("display"));
-    }
-
     @Test
     public void testBuilderReuse() {
         var builder = ItemStack.builder(Material.DIAMOND);

src/test/java/net/minestom/server/tag/TagAtomicTest.java

@@ -2,6 +2,7 @@ package net.minestom.server.tag;
 
 import org.junit.jupiter.api.Test;
 
+import static net.minestom.server.api.TestUtils.assertEqualsSNBT;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
 
@@ -23,6 +24,48 @@ public class TagAtomicTest {
         assertEquals(10, handler.getTag(tag));
     }
 
+    @Test
+    public void updateDefault() {
+        var tag = Tag.Integer("coin").defaultValue(25);
+        var handler = TagHandler.newHandler();
+        handler.updateTag(tag, integer -> {
+            assertEquals(25, integer);
+            return 5;
+        });
+        assertEquals(5, handler.getTag(tag));
+        handler.updateTag(tag, integer -> {
+            assertEquals(5, integer);
+            return 10;
+        });
+        assertEquals(10, handler.getTag(tag));
+    }
+
+    @Test
+    public void updateRemoval() {
+        var tag = Tag.Integer("coin");
+        var handler = TagHandler.newHandler();
+        handler.setTag(tag, 5);
+        handler.updateTag(tag, integer -> {
+            assertEquals(5, integer);
+            return null;
+        });
+        assertNull(handler.getTag(tag));
+        assertEqualsSNBT("{}", handler.asCompound());
+    }
+
+    @Test
+    public void updateRemovalPath() {
+        var tag = Tag.Integer("coin").path("path");
+        var handler = TagHandler.newHandler();
+        handler.setTag(tag, 5);
+        handler.updateTag(tag, integer -> {
+            assertEquals(5, integer);
+            return null;
+        });
+        assertNull(handler.getTag(tag));
+        assertEqualsSNBT("{}", handler.asCompound());
+    }
+
     @Test
     public void updateAndGet() {
         var tag = Tag.Integer("coin");

src/test/java/net/minestom/server/tag/TagHandlerReadableCopyTest.java

@@ -0,0 +1,63 @@
+package net.minestom.server.tag;
+
+import org.junit.jupiter.api.Test;
+
+import static net.minestom.server.api.TestUtils.assertEqualsSNBT;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+public class TagHandlerReadableCopyTest {
+
+    @Test
+    public void copyCache() {
+        var tag = Tag.String("key");
+        var handler = TagHandler.newHandler();
+        handler.setTag(tag, "test");
+
+        var copy = handler.readableCopy();
+        assertEquals(handler.getTag(tag), copy.getTag(tag));
+
+        handler.setTag(tag, "test2");
+        assertEquals("test2", handler.getTag(tag));
+        assertEquals("test", copy.getTag(tag));
+    }
+
+    @Test
+    public void copyCachePath() {
+        var tag = Tag.String("key").path("path");
+        var handler = TagHandler.newHandler();
+        handler.setTag(tag, "test");
+        assertEqualsSNBT("""
+                {"path":{"key":"test"}}
+                """, handler.asCompound());
+
+        var copy = handler.readableCopy();
+        handler.setTag(tag, "test2");
+        assertEquals("test2", handler.getTag(tag));
+        assertEquals("test", copy.getTag(tag));
+    }
+
+    @Test
+    public void copyCacheReuse() {
+        var handler = TagHandler.newHandler();
+        handler.setTag(Tag.String("key"), "test");
+        assertSame(handler.readableCopy(), handler.readableCopy());
+    }
+
+    @Test
+    public void copyRehashing() {
+        var tag = Tag.String("key");
+        var handler = TagHandler.newHandler();
+        handler.setTag(tag, "test");
+        var copy = handler.readableCopy();
+        for (int i = 0; i < 1000; i++) {
+            handler.setTag(Tag.Integer("copyRehashing" + i), i);
+        }
+        assertEquals("test", handler.getTag(tag));
+        assertEquals("test", copy.getTag(tag));
+
+        handler.setTag(tag, "test2");
+        assertEquals("test2", handler.getTag(tag));
+        assertEquals("test", copy.getTag(tag));
+    }
+}

src/test/java/net/minestom/server/tag/TagTest.java

@@ -62,6 +62,14 @@ public class TagTest {
         assertEquals(mutable.toCompound(), handler.asCompound(), "NBT is not the same");
     }
 
+    @Test
+    public void fromNbtCache() {
+        // Ensure that TagHandler#asCompound reuse the same compound used for construction
+        var compound = NBT.Compound(Map.of("key", NBT.Int(5)));
+        var handler = TagHandler.fromCompound(compound);
+        assertSame(compound, handler.asCompound(), "NBT is not the same");
+    }
+
     @Test
     public void defaultValue() {
         var nullable = Tag.String("key");