fix(server): normalize typed DEFAULT_VALUE after JSON reload

hugegraph-server/hugegraph-core/src/main/java/org/apache/hugegraph/schema/PropertyKey.java

@@ -121,7 +121,23 @@ public void defineDefaultValue(Object value) {
 
     public Object defaultValue() {
         // TODO add a field default_value
-        return this.userdata().get(Userdata.DEFAULT_VALUE);
+        Object value = this.userdata().get(Userdata.DEFAULT_VALUE);
+        if (value == null) {
+            return null;
+        }
+
+        // Userdata is reloaded from JSON as a raw Map, so a typed default
+        // value (e.g. Date) comes back as a String. Normalize it to the
+        // runtime type expected by this property key's data type. Idempotent
+        // for values already of the expected type.
+        Object normalized = this.validValue(value);
+
+        // For SET cardinality, ensure we return a Set container and collapse duplicates
+        if (this.cardinality == Cardinality.SET && normalized instanceof Collection) {
+            return new LinkedHashSet<>((Collection<?>) normalized);
+        }
+
+        return normalized;
     }
 
     public boolean hasSameContent(PropertyKey other) {

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/core/VertexCoreTest.java

@@ -69,6 +69,7 @@
 import org.apache.hugegraph.type.define.WriteType;
 import org.apache.hugegraph.util.Blob;
 import org.apache.hugegraph.util.CollectionUtil;
+import org.apache.hugegraph.util.DateUtil;
 import org.apache.hugegraph.util.LongEncoding;
 import org.apache.tinkerpop.gremlin.process.traversal.P;
 import org.apache.tinkerpop.gremlin.process.traversal.dsl.graph.GraphTraversal;
@@ -826,6 +827,33 @@ public void testAddVertexWithDefaultPropertyValue() {
         Assert.assertFalse(vertex.values("age").hasNext());
     }
 
+    @Test
+    public void testAddVertexWithDateDefaultValue() {
+        SchemaManager schema = graph().schema();
+
+        Date joined = DateUtil.parse("2026-05-14 10:11:12.345");
+        schema.propertyKey("joinDate").asDate()
+              .userdata(Userdata.DEFAULT_VALUE, joined).create();
+        schema.vertexLabel("person")
+              .properties("joinDate")
+              .nullableKeys("joinDate").append();
+
+        // No 'joinDate' supplied
+        Vertex vertex = graph().addVertex(T.label, "person",
+                                          "name", "Baby", "city", "Shanghai");
+
+        this.commitTx();
+
+        // Reload from backend then query: the typed default must survive the
+        // JSON serialize/reload round-trip as a Date, not a String (#3028).
+        vertex = graph().vertex(vertex.id());
+        Object value = vertex.value("joinDate");
+        Assert.assertTrue("default 'joinDate' should be a Date, was " +
+                          (value == null ? "null" : value.getClass()),
+                          value instanceof Date);
+        Assert.assertEquals(joined, value);
+    }
+
     @Test
     public void testAddVertexWithNotExistsVertexPropKey() {
         HugeGraph graph = graph();

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/unit/core/SchemaElementTest.java

@@ -17,8 +17,11 @@
 
 package org.apache.hugegraph.unit.core;
 
+import java.util.Arrays;
+import java.util.Collection;
 import java.util.Date;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 
 import org.apache.hugegraph.backend.id.IdGenerator;
@@ -27,6 +30,8 @@
 import org.apache.hugegraph.schema.Userdata;
 import org.apache.hugegraph.schema.VertexLabel;
 import org.apache.hugegraph.testutil.Assert;
+import org.apache.hugegraph.type.define.Cardinality;
+import org.apache.hugegraph.type.define.DataType;
 import org.apache.hugegraph.unit.FakeObjects;
 import org.apache.hugegraph.util.DateUtil;
 import org.junit.Test;
@@ -199,6 +204,62 @@ public void testVertexLabelFromMapNormalizesCreateTimeString() {
                             createTime);
     }
 
+    @Test
+    public void testPropertyKeyFromMapNormalizesDateDefaultValue() {
+        String formatted = "2026-05-14 10:11:12.345";
+        Map<String, Object> userdata = new HashMap<>();
+        userdata.put(Userdata.DEFAULT_VALUE, formatted);
+
+        Map<String, Object> map = new HashMap<>();
+        map.put(PropertyKey.P.ID, 1);
+        map.put(PropertyKey.P.NAME, "birth");
+        map.put(PropertyKey.P.DATA_TYPE, DataType.DATE.string());
+        map.put(PropertyKey.P.CARDINALITY, Cardinality.SINGLE.string());
+        map.put(PropertyKey.P.USERDATA, userdata);
+
+        PropertyKey propertyKey = PropertyKey.fromMap(map,
+                                                      new FakeObjects().graph());
+
+        Object value = propertyKey.defaultValue();
+        Assert.assertTrue("DEFAULT_VALUE should be a Date, was " +
+                          (value == null ? "null" : value.getClass()),
+                          value instanceof Date);
+        Assert.assertEquals(DateUtil.parse(formatted), value);
+    }
+
+    @Test
+    public void testPropertyKeyFromMapNormalizesDateSetDefaultValue() {
+        String first = "2026-05-14 10:11:12.345";
+        String second = "2026-05-15 11:12:13.456";
+        Map<String, Object> userdata = new HashMap<>();
+        userdata.put(Userdata.DEFAULT_VALUE, Arrays.asList(first, second));
+
+        Map<String, Object> map = new HashMap<>();
+        map.put(PropertyKey.P.ID, 1);
+        map.put(PropertyKey.P.NAME, "tags");
+        map.put(PropertyKey.P.DATA_TYPE, DataType.DATE.string());
+        map.put(PropertyKey.P.CARDINALITY, Cardinality.SET.string());
+        map.put(PropertyKey.P.USERDATA, userdata);
+
+        PropertyKey propertyKey = PropertyKey.fromMap(map,
+                                                      new FakeObjects().graph());
+
+        Object value = propertyKey.defaultValue();
+        Assert.assertTrue("DEFAULT_VALUE should be a Collection, was " +
+                          (value == null ? "null" : value.getClass()),
+                          value instanceof Collection);
+        Collection<?> values = (Collection<?>) value;
+        Assert.assertEquals(2, values.size());
+        for (Object element : values) {
+            Assert.assertTrue("each element should be a Date, was " +
+                              (element == null ? "null" : element.getClass()),
+                              element instanceof Date);
+        }
+        List<Date> expected = Arrays.asList(DateUtil.parse(first),
+                                            DateUtil.parse(second));
+        Assert.assertTrue(values.containsAll(expected));
+    }
+
     @Test
     public void testBulkSetterRejectsNullUserdata() {
         SchemaElement schema = newSchema();

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/unit/serializer/BinarySerializerTest.java

@@ -29,6 +29,7 @@
 import org.apache.hugegraph.structure.HugeVertex;
 import org.apache.hugegraph.testutil.Assert;
 import org.apache.hugegraph.testutil.Whitebox;
+import org.apache.hugegraph.type.define.DataType;
 import org.apache.hugegraph.unit.BaseUnitTest;
 import org.apache.hugegraph.unit.FakeObjects;
 import org.apache.hugegraph.util.DateUtil;
@@ -132,6 +133,27 @@ public void testPropertyKeyUserdataCreateTimeRoundTripsAsDate() {
         Assert.assertEquals(created, value);
     }
 
+    @Test
+    public void testPropertyKeyDefaultValueRoundTripsAsDate() {
+        HugeConfig config = FakeObjects.newConfig();
+        BinarySerializer ser = new BinarySerializer(config);
+
+        FakeObjects objects = new FakeObjects();
+        PropertyKey original = objects.newPropertyKey(IdGenerator.of(1L),
+                                                      "name", DataType.DATE);
+        Date defaultValue = DateUtil.parse("2026-05-14 10:11:12.345");
+        original.userdata(Userdata.DEFAULT_VALUE, defaultValue);
+
+        BackendEntry entry = ser.writePropertyKey(original);
+        PropertyKey reloaded = ser.readPropertyKey(objects.graph(), entry);
+
+        Object value = reloaded.defaultValue();
+        Assert.assertTrue("DEFAULT_VALUE should be a Date after round-trip, " +
+                          "was " + (value == null ? "null" : value.getClass()),
+                          value instanceof Date);
+        Assert.assertEquals(defaultValue, value);
+    }
+
     @Test
     public void testEdgeForPartition() {
         BinarySerializer ser = new BinarySerializer(true, true, true);

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/unit/serializer/TextSerializerTest.java

@@ -26,6 +26,7 @@
 import org.apache.hugegraph.schema.PropertyKey;
 import org.apache.hugegraph.schema.Userdata;
 import org.apache.hugegraph.testutil.Assert;
+import org.apache.hugegraph.type.define.DataType;
 import org.apache.hugegraph.unit.BaseUnitTest;
 import org.apache.hugegraph.unit.FakeObjects;
 import org.apache.hugegraph.util.DateUtil;
@@ -53,4 +54,25 @@ public void testPropertyKeyUserdataCreateTimeRoundTripsAsDate() {
                           value instanceof Date);
         Assert.assertEquals(created, value);
     }
+
+    @Test
+    public void testPropertyKeyDefaultValueRoundTripsAsDate() {
+        HugeConfig config = FakeObjects.newConfig();
+        TextSerializer ser = new TextSerializer(config);
+
+        FakeObjects objects = new FakeObjects();
+        PropertyKey original = objects.newPropertyKey(IdGenerator.of(1L),
+                                                      "name", DataType.DATE);
+        Date defaultValue = DateUtil.parse("2026-05-14 10:11:12.345");
+        original.userdata(Userdata.DEFAULT_VALUE, defaultValue);
+
+        BackendEntry entry = ser.writePropertyKey(original);
+        PropertyKey reloaded = ser.readPropertyKey(objects.graph(), entry);
+
+        Object value = reloaded.defaultValue();
+        Assert.assertTrue("DEFAULT_VALUE should be a Date after round-trip, " +
+                          "was " + (value == null ? "null" : value.getClass()),
+                          value instanceof Date);
+        Assert.assertEquals(defaultValue, value);
+    }
 }

hugegraph-struct/src/main/java/org/apache/hugegraph/struct/schema/PropertyKey.java

@@ -126,7 +126,23 @@ public void defineDefaultValue(Object value) {
 
     public Object defaultValue() {
         // TODO add a field default_value
-        return this.userdata().get(Userdata.DEFAULT_VALUE);
+        Object value = this.userdata().get(Userdata.DEFAULT_VALUE);
+        if (value == null) {
+            return null;
+        }
+
+        // Userdata is reloaded from JSON as a raw Map, so a typed default
+        // value (e.g. Date) comes back as a String. Normalize it to the
+        // runtime type expected by this property key's data type. Idempotent
+        // for values already of the expected type.
+        Object normalized = this.validValue(value);
+
+        // For SET cardinality, ensure we return a Set container and collapse duplicates
+        if (this.cardinality == Cardinality.SET && normalized instanceof Collection) {
+            return new LinkedHashSet<>((Collection<?>) normalized);
+        }
+
+        return normalized;
     }
 
     public boolean hasSameContent(PropertyKey other) {

hugegraph-struct/src/test/java/org/apache/hugegraph/struct/schema/PropertyKeyTest.java

@@ -0,0 +1,70 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hugegraph.struct.schema;
+
+import java.util.Arrays;
+import java.util.Date;
+import java.util.Set;
+
+import org.apache.hugegraph.id.IdGenerator;
+import org.apache.hugegraph.type.define.Cardinality;
+import org.apache.hugegraph.type.define.DataType;
+import org.apache.hugegraph.util.DateUtil;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class PropertyKeyTest {
+
+    @Test
+    public void testDefaultValueNormalizedToDate() {
+        // Userdata reloaded from JSON keeps ~default_value as a String;
+        // defaultValue() must normalize it to the data type's runtime type
+        // (#3028).
+        String formatted = "2026-05-14 10:11:12.345";
+        PropertyKey propertyKey = new PropertyKey(null, IdGenerator.of(1),
+                                                  "joinDate");
+        propertyKey.dataType(DataType.DATE);
+        propertyKey.userdata(Userdata.DEFAULT_VALUE, formatted);
+
+        Object value = propertyKey.defaultValue();
+        Assert.assertTrue("DEFAULT_VALUE should be a Date, was " +
+                          (value == null ? "null" : value.getClass()),
+                          value instanceof Date);
+        Assert.assertEquals(DateUtil.parse(formatted), value);
+    }
+
+    @Test
+    public void testSetDefaultValueCollapsesDuplicatesAndReturnsSet() {
+        String formatted = "2026-05-14 10:11:12.345";
+        PropertyKey propertyKey = new PropertyKey(null, IdGenerator.of(1),
+                                                  "joinDate");
+        propertyKey.dataType(DataType.DATE);
+        propertyKey.cardinality(Cardinality.SET);
+        propertyKey.userdata(Userdata.DEFAULT_VALUE,
+                             Arrays.asList(formatted, formatted));
+
+        Object value = propertyKey.defaultValue();
+        Assert.assertTrue("DEFAULT_VALUE should be a Set, was " +
+                          (value == null ? "null" : value.getClass()),
+                          value instanceof Set);
+
+        Set<?> values = (Set<?>) value;
+        Assert.assertEquals(1, values.size());
+        Assert.assertTrue(values.contains(DateUtil.parse(formatted)));
+    }
+}