fix: Handle custom ObjectReaders lacking createInstance in JSONObject…

core/src/main/java/com/alibaba/fastjson2/JSONObject.java

@@ -1292,7 +1292,12 @@ public <T> T to(Class<T> clazz, JSONReader.Feature... features) {
 
         ObjectReaderProvider provider = JSONFactory.getDefaultObjectReaderProvider();
         ObjectReader<T> objectReader = provider.getObjectReader(clazz, fieldBased);
-        return objectReader.createInstance(this, featuresValue);
+
+        try {
+            return objectReader.createInstance(this, featuresValue);
+        } catch (UnsupportedOperationException e) {
+            return JSON.parseObject(this.toJSONString(), clazz, features);
+        }
     }
 
     public void copyTo(Object object, JSONReader.Feature... features) {

core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3901.java

@@ -0,0 +1,41 @@
+package com.alibaba.fastjson2.issues_3900;
+
+import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.JSONObject;
+import com.alibaba.fastjson2.JSONReader;
+import com.alibaba.fastjson2.reader.ObjectReader;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Type;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue3901 {
+    @Test
+    public void test() {
+        JSONObject jsonObject = new JSONObject();
+        jsonObject.put("name", "James");
+
+        JSON.register(User.class, new UserReader());
+        User user = jsonObject.to(User.class);
+        assertEquals("ZhangSan", user.getName());
+    }
+
+    @Data
+    @NoArgsConstructor
+    @AllArgsConstructor
+    public static class User {
+        private String name;
+    }
+
+    public static class UserReader implements ObjectReader {
+        @Override
+        public Object readObject(JSONReader jsonReader, Type fieldType, Object fieldName, long features) {
+            jsonReader.readObject();
+            return new User("ZhangSan");
+        }
+    }
+}