fix_issus_3926

core/src/main/java/com/alibaba/fastjson2/reader/ObjectReaderCreator.java

@@ -2960,8 +2960,20 @@ public <T> FieldReader createFieldReaderMethod(
 
                     return new FieldReaderList(fieldName, fieldTypeResolved, fieldClassResolved, itemType, itemClass, ordinal, features, format, locale, null, jsonSchema, method, null, null);
                 }
+            } else {
+                Type itemType = BeanUtils.resolveCollectionItemType(fieldTypeResolved, fieldClass);
+                Class itemClass;
+                if (itemType == null) {
+                    itemType = Object.class;
+                    itemClass = Object.class;
+                } else {
+                    itemClass = TypeUtils.getMapping(itemType);
+                    if (itemClass == String.class) {
+                        return new FieldReaderList(fieldName, fieldTypeResolved != null ? fieldTypeResolved : fieldType, fieldClassResolved != null ? fieldClassResolved : fieldClass, String.class, String.class, ordinal, features, format, locale, null, jsonSchema, method, null, null);
+                    }
+                }
+                return new FieldReaderList(fieldName, fieldTypeResolved != null ? fieldTypeResolved : fieldType, fieldClassResolved != null ? fieldClassResolved : fieldClass, itemType, itemClass, ordinal, features, format, locale, null, jsonSchema, method, null, null);
             }
-            return new FieldReaderList(fieldName, fieldType, fieldClass, Object.class, Object.class, ordinal, features, format, locale, null, jsonSchema, method, null, null);
         }
 
         if (fieldClass == Date.class) {
@@ -3367,6 +3379,8 @@ public <T> FieldReader<T> createFieldReader(
                 if (actualTypeArguments.length > 0) {
                     itemType = actualTypeArguments[0];
                 }
+            } else {
+                itemType = BeanUtils.resolveCollectionItemType(fieldTypeResolved, fieldClass);
             }
             if (itemType == null) {
                 itemType = Object.class;

core/src/main/java/com/alibaba/fastjson2/util/BeanUtils.java

@@ -215,6 +215,28 @@ public static Field getDeclaredField(Class objectClass, String fieldName) {
         return fieldMap.get(fieldName);
     }
 
+    public static Type resolveCollectionItemType(Type fieldTypeResolved, Class<?> fieldClass) {
+        Type contextType = fieldTypeResolved != null ? fieldTypeResolved : fieldClass;
+
+        Type listType = getGenericSupertype(contextType, fieldClass, List.class);
+
+        if (listType == null || listType == List.class) {
+            listType = getGenericSupertype(contextType, fieldClass, Collection.class);
+        }
+
+        listType = BeanUtils.resolve(contextType, fieldClass, listType);
+
+        if (listType instanceof ParameterizedType) {
+            ParameterizedType parameterizedType = (ParameterizedType) listType;
+            Type[] actualTypeArguments = parameterizedType.getActualTypeArguments();
+
+            if (actualTypeArguments.length == 1) {
+                return actualTypeArguments[0];
+            }
+        }
+        return null;
+    }
+
     public static Method getSetter(Class objectClass, String methodName) {
         Method[] methods = new Method[1];
         setters(objectClass, e -> {

core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3926.java

@@ -0,0 +1,171 @@
+package com.alibaba.fastjson2.issues_3900;
+
+import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.JSONReader;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class Issue3926 {
+    public static class MyDTO1 {
+        private MyList myList;
+
+        public MyList getMyList() {
+            return myList;
+        }
+
+        public void setMyList(MyList myList) {
+            this.myList = myList;
+        }
+    }
+
+    public static class MyList extends ArrayList<MyObject1> {
+    }
+
+    public static class MyObject1 {
+        private String name;
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+    }
+
+    @Test
+    public void testCustomList() {
+        String json = "{\"myList\":[{\"name\":\"张三\"}]}";
+        MyDTO1 myDTO = JSON.parseObject(json, MyDTO1.class);
+
+        assertEquals(1, myDTO.getMyList().size());
+
+        Object obj = myDTO.getMyList().get(0);
+        assertTrue(obj != null, "Element should be MyObject, but is " + obj.getClass().getName());
+
+        MyObject1 myObject = myDTO.getMyList().get(0);
+        assertEquals("张三", myObject.getName());
+    }
+
+    @Test
+    public void testCustomListMultiple() {
+        String json = "{\"myList\":[{\"name\":\"张三\"},{\"name\":\"李四\"}]}";
+        MyDTO1 myDTO = JSON.parseObject(json, MyDTO1.class);
+
+        assertEquals(2, myDTO.getMyList().size());
+
+        for (int i = 0; i < myDTO.getMyList().size(); i++) {
+            Object obj = myDTO.getMyList().get(i);
+            assertTrue(obj != null, "Element " + i + " should be MyObject, but is " + obj.getClass().getName());
+        }
+
+        MyObject1 myObject = myDTO.getMyList().get(0);
+        assertEquals("张三", myObject.getName());
+
+        MyObject1 myObject2 = myDTO.getMyList().get(1);
+        assertEquals("李四", myObject2.getName());
+    }
+
+    @Test
+    public void testFieldBasedList() {
+        String json = "{\"myList\":[{\"name\":\"张三\"}]}";
+        MyDTO1 myDTO = JSON.parseObject(json, MyDTO1.class, JSONReader.Feature.FieldBased);
+        assertEquals(1, myDTO.myList.size());
+        MyObject1 myObject = myDTO.myList.get(0);
+        assertEquals("张三", myObject.name);
+    }
+
+    @Test
+    public void testFieldBasedListMultiple() {
+        String json = "{\"myList\":[{\"name\":\"张三\"},{\"name\":\"李四\"}]}";
+        MyDTO1 myDTO = JSON.parseObject(json, MyDTO1.class, JSONReader.Feature.FieldBased);
+        assertEquals(2, myDTO.myList.size());
+        assertEquals("张三", myDTO.myList.get(0).name);
+        assertEquals("李四", myDTO.myList.get(1).name);
+    }
+
+    public static class MyDTO2 {
+        private MyList2 myList;
+
+        public MyList2 getMyList() {
+            return myList;
+        }
+
+        public void setMyList(MyList2 myList) {
+            this.myList = myList;
+        }
+    }
+
+    public interface MyListInterface extends java.util.List<MyObject2> {
+    }
+
+    public static class MyList2 extends ArrayList<MyObject2> implements MyListInterface {
+    }
+
+    public static class MyObject2 {
+        private String name;
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+    }
+
+    @Test
+    public void testCustomListWithInterface() {
+        String json = "{\"myList\":[{\"name\":\"张三\"}]}";
+        MyDTO2 myDTO = JSON.parseObject(json, MyDTO2.class);
+
+        assertEquals(1, myDTO.getMyList().size());
+
+        Object obj = myDTO.getMyList().get(0);
+        assertTrue(obj != null, "Element should be MyObject2, but is " + obj.getClass().getName());
+
+        MyObject2 myObject = myDTO.getMyList().get(0);
+        assertEquals("张三", myObject.getName());
+    }
+
+    @Test
+    public void testCustomListWithInterfaceMultiple() {
+        String json = "{\"myList\":[{\"name\":\"张三\"},{\"name\":\"李四\"}]}";
+        MyDTO2 myDTO = JSON.parseObject(json, MyDTO2.class);
+
+        assertEquals(2, myDTO.getMyList().size());
+
+        for (int i = 0; i < myDTO.getMyList().size(); i++) {
+            Object obj = myDTO.getMyList().get(i);
+            assertTrue(obj instanceof MyObject2, "Element " + i + " should be MyObject2, but is " + obj.getClass().getName());
+        }
+
+        MyObject2 myObject = myDTO.getMyList().get(0);
+        assertEquals("张三", myObject.getName());
+
+        MyObject2 myObject2 = myDTO.getMyList().get(1);
+        assertEquals("李四", myObject2.getName());
+    }
+
+    @Test
+    public void testFieldBasedInterfaceList() {
+        String json = "{\"myList\":[{\"name\":\"张三\"}]}";
+        MyDTO2 myDTO = JSON.parseObject(json, MyDTO2.class, JSONReader.Feature.FieldBased);
+        assertEquals(1, myDTO.myList.size());
+        MyObject2 myObject = myDTO.myList.get(0);
+        assertEquals("张三", myObject.name);
+    }
+
+    @Test
+    public void testFieldBasedInterfaceListMultiple() {
+        String json = "{\"myList\":[{\"name\":\"张三\"},{\"name\":\"李四\"}]}";
+        MyDTO2 myDTO = JSON.parseObject(json, MyDTO2.class, JSONReader.Feature.FieldBased);
+        assertEquals(2, myDTO.myList.size());
+        assertEquals("张三", myDTO.myList.get(0).name);
+        assertEquals("李四", myDTO.myList.get(1).name);
+    }
+}