Spring HATEOAS客户端忽略空链接-“；模板不能为null或空&引用；例外

我正在访问由第三方提供的HATEOAS API，出于某种原因，我们从他们那里收到的响应包含href的空值链接。这会引发异常。我无法更改响应，因为我无法控制此API。有办法解决这个问题吗

下面是JSON的示例：

{
    "_embedded": {
        "example": [{ ... }]
    }
    "_links": {
        "next": {
            "href": null
        },
        "prev": {
            "href": null
        },
        "self": {
            "href": "https://bag.basisregistraties.overheid.nl/api/v1/panden"
        }
    }
}

当我使用RestTemplate发出请求时，我得到一个IllegalArgumentException，消息为“Template不得为null或空！”

---

在花了一些时间查看Jackson2HalModule的代码后，我找到了这个问题的解决方案。我无法使用任何配置来告诉库忽略具有空值的链接。相反，我可以做的是重写deserialize（）方法，这样我就可以添加null检查并避免问题

为了做到这一点，我必须创建新的链接反序列化器类。它需要扩展Jackson2HalModule.HalLinkListDeserializer，然后我只需从库源代码中复制并粘贴代码，并进行所需的修改

public class CustomHalLinkListDeserializer extends Jackson2HalModule.HalLinkListDeserializer {

    @Override
    public List<Link> deserialize(JsonParser jp, DeserializationContext ctxt) 
                                      throws IOException, JsonProcessingException {

        List<Link> result = new ArrayList<Link>();
        String relation;

        // links is an object, so we parse till we find its end.
        while (!JsonToken.END_OBJECT.equals(jp.nextToken())) {

            if (!JsonToken.FIELD_NAME.equals(jp.getCurrentToken())) {
                throw new JsonParseException(jp, 
                              "Expected relation name", jp.getCurrentLocation());
            }

            // save the relation in case the link does not contain it
            relation = jp.getText();

            if (JsonToken.START_ARRAY.equals(jp.nextToken())) {
                while (!JsonToken.END_ARRAY.equals(jp.nextToken())) {
                    CustomLink link = jp.readValueAs(CustomLink.class);
                    String href = link.getHref() != null ? link.getHref() : "http://dummy";
                    result.add(new Link(href, relation));
                }
            } else {
                CustomLink link = jp.readValueAs(CustomLink.class);
                String href = link.getHref() != null ? link.getHref() : "http://dummy";
                result.add(new Link(href, relation));
            }
        }

        return result;

    }
}

    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    mapper.configure(MapperFeature.DEFAULT_VIEW_INCLUSION, false);
    Jackson2HalModule jackson2HalModule = new Jackson2HalModule();
    mapper.registerModule(jackson2HalModule);

    MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
    converter.setSupportedMediaTypes(MediaType.parseMediaTypes(Arrays.asList("application/json;charset=UTF-8", "application/hal+json")));
    converter.setObjectMapper(mapper);

    RestTemplateBuilder restTemplateBuilder = new RestTemplateBuilder();

    RestTemplate restTemplate = restTemplateBuilder.messageConverters(Collections.singletonList(converter)).build();

    HttpHeaders headers = new HttpHeaders();
    headers.set("Accept", "application/json, application/*+json");
    headers.setContentType(MediaType.APPLICATION_JSON_UTF8);

    HttpEntity<String> entity = new HttpEntity(entity.toString(), headers);

    ParameterizedTypeReference<Resources<Panden>> typeRefDevices = new ParameterizedTypeReference<>() {};
    ResponseEntity<Resources<Panden>> result = restTemplate.exchange(endpoint, HttpMethod.POST, entity, typeRefDevices);;

public class CustomHalLinkListDeserializer extends Jackson2HalModule.HalLinkListDeserializer {

    @Override
    public List<Link> deserialize(JsonParser jp, DeserializationContext ctxt) 
                                      throws IOException, JsonProcessingException {

        List<Link> result = new ArrayList<Link>();
        String relation;

        // links is an object, so we parse till we find its end.
        while (!JsonToken.END_OBJECT.equals(jp.nextToken())) {

            if (!JsonToken.FIELD_NAME.equals(jp.getCurrentToken())) {
                throw new JsonParseException(jp, 
                              "Expected relation name", jp.getCurrentLocation());
            }

            // save the relation in case the link does not contain it
            relation = jp.getText();

            if (JsonToken.START_ARRAY.equals(jp.nextToken())) {
                while (!JsonToken.END_ARRAY.equals(jp.nextToken())) {
                    CustomLink link = jp.readValueAs(CustomLink.class);
                    String href = link.getHref() != null ? link.getHref() : "http://dummy";
                    result.add(new Link(href, relation));
                }
            } else {
                CustomLink link = jp.readValueAs(CustomLink.class);
                String href = link.getHref() != null ? link.getHref() : "http://dummy";
                result.add(new Link(href, relation));
            }
        }

        return result;

    }
}

public abstract class CustomResourceSupportMixin extends ResourceSupport {

    @Override
    @XmlElement(name = "link")
    @JsonProperty("_links")
    @JsonInclude(JsonInclude.Include.NON_EMPTY)
    @JsonSerialize(using = Jackson2HalModule.HalLinkListSerializer.class)
    @JsonDeserialize(using = CustomHalLinkListDeserializer.class)
    public abstract List<Link> getLinks();

}

ObjectMapper mapper = new ObjectMapper();
Jackson2HalModule jackson2HalModule = new Jackson2HalModule();
jackson2HalModule.setMixInAnnotation(ResourceSupport.class,
                                         CustomResourceSupportMixin.class);
mapper.registerModule(jackson2HalModule);