读取包含包含单个元素的列表的数据时，restTemplate.getForEntity给出异常

Question

I have a Detail object that has a property of object of type Product. Product has a property called xxx which is an arraylist. I do a GET on the URL with postman and the result looks like:

"Product": {
    "id": "2",
    "xxx": [
        "price": "50"
      },
      {
        "price": "60"
      }
    ]
  }

This result is good. But, in my Spring project, when I do a get Using RestTemplate as:

  restTemplate.getForEntity("someurl", Detail.class).getBody();

I get correct results when xxx list contains 2 or more element. However, when there is only element in this list, I get an error:

org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token; nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token

  How do I fix this issue that I am facing with my call to restTemplate.getForEntity as above?

Answer 1

I suspect that your encounter this error not when your list contains one item but rather when you have no list at all but a plain object, thus the parser complains about a misplaced START_OBJECT token. To remedy this and without being able to edit Swagger-generated domain classes, you could set the deserialization feature

ObjectMapper objectMapper = new ObjectMapper(); // maybe injected
objectMapper.configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);

directly on the ObjectMapper that you are using.

This object mapper can now be put into the RestTemplate config:

@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class JsonControllerIT {
    @LocalServerPort
    private int port;

    @Test
    public void jsonWithSerializationFeatureSet() {
        // given
        RestTemplate restTemplate = new RestTemplate();
        MappingJackson2HttpMessageConverter messageConverter = new MappingJackson2HttpMessageConverter();
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);
        messageConverter.setObjectMapper(objectMapper);
        restTemplate.getMessageConverters().removeIf(m -> m.getClass().getName().equals(MappingJackson2HttpMessageConverter.class.getName()));
        restTemplate.getMessageConverters().add(messageConverter);

        // when
        Detail detail = restTemplate.getForEntity("http://localhost:" + port + "/json", Detail.class).getBody();

        // then
        assertThat(detail.getSingleItemList().get(0)).isEqualTo(3);
    }
}

If you use the Spring-approach keep in mind that you also can inject all those beans, they are created here straight-forward just show them better together.

You can also check this running example:

• https://github.com/mle-enso/stackoverflow/blob/master/src/test/java/de/mle/stackoverflow/jackson/JsonControllerIT.java
• https://github.com/mle-enso/stackoverflow/blob/master/src/main/java/de/mle/stackoverflow/jackson/JsonController.java