如何使用PowerMockito模拟Object.Mapper

我正在尝试使用PowerMockito涵盖My JsonService实用工具类，但是我遇到了异常，并且试图从很长时间内解决此问题，但是仍然无法解决问题，请有人帮我一下。

JsonService

public final class JsonService {
    public static <T> T getObjectFromJson(final Object jsonString, final Class<T> valueType) {

            // log.debug("Start of JsonService.getObjectFromJson() method.. {}");

            T object = null;
            if (jsonString != null) {
                try {
                    object = MAPPER.readValue(jsonString.toString(), valueType);
                    // log.debug(DEBUG_LOG_STR, object);
                } catch (IOException io) {
                    log.error(ERROR_LOG_STR + " in method getObjectFromJson(). Exception Message={}, Exception Stack ={}",
                            io.getMessage(), io);
                    throw new ServiceException(ErrorMessages.JSON_SERIALIZATION_ERROR, io.getCause());
                }
            }
            // log.debug("End of JsonService.getObjectFromJson() method.. {}");
            return object;
        }
}

JsonServiceTest

@PrepareForTest({ JsonService.class })
public class JsonServiceTest {
    @Rule
    public PowerMockRule rule = new PowerMockRule();
    static {
        PowerMockAgent.initializeIfNeeded();
    }

    @Test
    public void getObjectFromJsonTest() throws Exception {
        PowerMockito.mockStatic(JsonService.class);
        ObjectMapper mapper = new ObjectMapper();
        JsonNode actualObj = mapper.readTree(jsongString().toString());
        PowerMockito.when(JsonService.getObjectFromJson(jsongString(),JsonNode.class)).thenReturn(actualObj);

    }


    private Object jsongString() {
        String jsonResponse = "{\"content\":{\"status\":\"Success\",\"redirect\":false,\"redirectToModifyFlow\":false,\"response\":"
                + "{\"personalInfo\":{\"status\":\"Success\"}},\"expressCheckoutFlow\":false}}";
        return jsonResponse.toString();
    }

}