带有Traverson客户端和java.time.Instant的Spring HATEOAS

Question

当rest实体具有类型为"java.time.Instant“的属性时，将spring hateoas 1.0.3与traverson客户端一起使用会导致问题。我得到的错误是

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `java.time.Instant`

我发现traverson中RestTemplate内部使用的HttpMessageConverter只注册了Jackson2HalModule。

有没有办法在traverson中注册jackson-modules-java8模块？

或者，有没有一种方法可以在traverson之外在我的restTemplate中注册Jackson2HalModule？

Answer 1

在spring-hateoas 1.1.2.RELEASE中，下面的内容对我很有效

private static RestTemplate getRestTemplate() {
    List<HttpMessageConverter<?>> httpMessageConverters = SpringFactoriesLoader.loadFactories(TraversonDefaults.class,
            Traverson.class.getClassLoader()).get(0).getHttpMessageConverters(Collections.singletonList(MediaTypes.HAL_JSON));
    Optional<HttpMessageConverter<?>> first = httpMessageConverters.stream().filter(i -> i instanceof MappingJackson2HttpMessageConverter)
            .findFirst();

    if (first.isPresent()) {
        MappingJackson2HttpMessageConverter httpMessageConverter = (MappingJackson2HttpMessageConverter) first.get();
        httpMessageConverter.getObjectMapper().registerModule(new JavaTimeModule());
    }

    return new RestTemplateBuilder().messageConverters(httpMessageConverters).build();
}

然后使用它：

Traverson traverson = new Traverson(URI.create("http://localhost:8080"), MediaTypes.HAL_JSON);
    traverson.setRestOperations(getRestTemplate());

Answer 2

经过一番调查，我找到了一个适合我的解决方案。背景是traverson客户端使用包含Jackson2HalModule的注册MappingJackson2HttpMessageConverter。为了解决这个问题，我还必须注册JavaTimeModule。

我做了以下工作

RestTemplateBuilder genericBuilder = this.restTemplateBuilder
    .setConnectTimeout(Duration.ofMillis(configuration.getSecurityRestConnectTimeout()))
    .setReadTimeout(Duration.ofMillis(configuration.getSecurityRestReceiveTimeout()));

// my normal restTemplate
RestTemplateBuilder restTemplate = genericBuilder
                .defaultMessageConverters()
                .build();

// HAL specific restTemplate                
RestTemplateBuilder restTemplateHal = genericBuilder
                .messageConverters(getHalConverter(Arrays.asList(MediaTypes.HAL_JSON)))
                .build();

HalConverter是这样生成的(也注册JavaTimeModule)：

   private static HttpMessageConverter<?> getHalConverter(List<MediaType> halFlavours) {

        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new Jackson2HalModule());
        mapper.registerModule(new JavaTimeModule());
        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

        MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();

        converter.setObjectMapper(mapper);
        converter.setSupportedMediaTypes(halFlavours);

        return converter;
    }

然后，您可以使用Traverson设置刚刚生成的restTemplateHal

    Traverson traverson = new Traverson(uri, MediaTypes.HAL_JSON);
    traverson.setRestOperations(restTemplateHal);
    MyClass myclass = traverson.follow().toObject(MyClass.class);