fix bug for issue #3569 (#3574)

core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterAdapter.java

@@ -687,7 +687,11 @@ public class ObjectWriterAdapter<T>
                 }
                 if (valueWriter instanceof ObjectWriterAdapter) {
                     ObjectWriterAdapter objectWriterAdapter = (ObjectWriterAdapter) valueWriter;
-                    fieldValue = objectWriterAdapter.toJSONObject(fieldValue);
+                    if (!objectWriterAdapter.getFieldWriters().isEmpty()) {
+                        fieldValue = objectWriterAdapter.toJSONObject(fieldValue);
+                    } else {
+                        fieldValue = JSON.toJSON(fieldValue);
+                    }
                 }
             }
             jsonObject.put(fieldWriter.fieldName, fieldValue);

core/src/test/java/com/alibaba/fastjson2/issues_3500/Issue3569.java

@@ -0,0 +1,33 @@
+package com.alibaba.fastjson2.issues_3500;
+
+import com.alibaba.fastjson2.JSONObject;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue3569 {
+    @Test
+    public void test() {
+        TestClass<String> testClass1 = new TestClass<>("hello");
+        JSONObject jsonObject = JSONObject.from(testClass1);
+        assertEquals(jsonObject.toString(), "{\"generic\":\"hello\"}");
+
+        TestClass<TestData> testClass2 = new TestClass<>(new TestData("hello"));
+        jsonObject = JSONObject.from(testClass2);
+        assertEquals(jsonObject.toString(), "{\"generic\":{\"testData\":\"hello\"}}");
+    }
+
+    @Data
+    @AllArgsConstructor
+    public static class TestClass<T> {
+        private T generic;
+    }
+
+    @Data
+    @AllArgsConstructor
+    public static class TestData {
+        private String testData;
+    }
+}