// DynPage.java
@Data
public class DynPage {
    private List<DynArea> dets;
}
// DynArea.java
@Data
public class DynArea {
    private List<DynField> fields;
}
// DynField.java
@Data
public class DynField {
    private String fname;
    private Serializable value;
}

// FastjsonTest.java
@Slf4j
class FastjsonTest {
    @Test
    @SneakyThrows
    void dynPageToJSONString() {
        ClassPathResource resource = new ClassPathResource("test.json");
        try (InputStream is = resource.getInputStream()) {
            String json = IOUtils.toString(is, StandardCharsets.UTF_8);
            DynPage data = JSON.parseObject(json, DynPage.class);
            log.info("{}", data);
            Assertions.assertNotNull(data);

            String jsonv = JSON.toJSONString(data);
            log.info(jsonv);
            Assertions.assertNotNull(jsonv);
        }
    }
}