JsonMappingException：没有为类型找到合适的构造函数——对于外部对象

Question

我有一个来自 spring 框架的名为 GeoJsonPoint 的对象，在我的集成测试中，jackson mapper 无法对其进行反序列化。此外，我不能添加一个虚拟构造函数，因为它是一个外部对象。所以我被困住了。这是我的主要实体；

@Document(collection = "foodTrucks")
@JsonSerialize(include = JsonSerialize.Inclusion.NON_EMPTY)
public class FoodTruckEntity {

    @Id
    private ObjectId id;

    private String applicant;
    private Status status;
    private String[] foodItems;
    private Double longitude;
    private Double latitude;
    private GeoJsonPoint geoJsonPoint;

    public FoodTruckEntity() {};

    // getters and setters
}

还有测试

@Test
public void test() {
    ClientConfig clientConfig = new DefaultClientConfig();
    clientConfig.getFeatures().put(JSONConfiguration.FEATURE_POJO_MAPPING, Boolean.TRUE);
    Client client = Client.create(clientConfig);

    String getNearFoodTrucksUrl = "http://localhost:8080/food-truck/near-locations/longitude/-122.398658184604/latitude/37.7901490737255/findAll";
    WebResource webResource = client.resource(getNearFoodTrucksUrl);
    ClientResponse response = webResource.get(ClientResponse.class);
    GeoResults<FoodTruckEntity> geoResults = webResource.get(new GenericType<GeoResults<FoodTruckEntity>>(){});

    if (response.getStatus() != 200) {
        throw new WebApplicationException();
    }
}

我得到的错误；

com.sun.jersey.api.client.ClientHandlerException: org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type [simple type, class org.springframework.data.geo.GeoResults<entity.FoodTruckEntity>]: can not instantiate from JSON object (need to add/enable type information?)
 at [Source: sun.net.www.protocol.http.HttpURLConnection$HttpInputStream@107ed6fc; line: 1, column: 2]
    at com.sun.jersey.api.client.ClientResponse.getEntity(ClientResponse.java:644)
    at com.sun.jersey.api.client.ClientResponse.getEntity(ClientResponse.java:604)

编辑：这是我拥有的球衣的依赖项

<!-- JERSEY JSON -->
<dependency>
    <groupId>com.sun.jersey</groupId>
    <artifactId>jersey-client</artifactId>
    <version>1.18.1</version>
</dependency>

<dependency>
    <groupId>com.sun.jersey</groupId>
    <artifactId>jersey-json</artifactId>
    <version>1.18.1</version>
</dependency>
<!-- JERSEY JSON -->

EDIT_2：作为字符串的响应如下所示，

{
   "averageDistance":{
      "value":0.0,
      "metric":"MILES",
      "normalizedValue":0.0,
      "unit":"mi"
   },
   "content":[
      {
         "content":{
            "id":{
               "timestamp":1429498845,
               "machineIdentifier":11487078,
               "processIdentifier":1432,
               "counter":9275496,
               "time":1429498845000,
               "date":1429498845000,
               "timeSecond":1429498845
            },
            "applicant":"Cupkates Bakery, LLC",
            "facilityType":"Truck",
            "status":"APPROVED",
            "foodItems":[
               "Cupcakes"
            ],
            "longitude":-122.398658184604,
            "latitude":37.7901490737255,
            "geoJsonPoint":{
               "x":-122.398658184604,
               "y":37.7901490737255,
               "type":"Point",
               "coordinates":[
                  -122.398658184604,
                  37.7901490737255
               ]
            }
         },
         "distance":{
            "value":0.0,
            "metric":"MILES",
            "normalizedValue":0.0,
            "unit":"mi"
         }
      }
   ]
}

Answer 1

因此，如果您查看 org.springframework.data.geo 中的所有类，您会注意到几乎所有类都没有无参数构造函数，ObjectMapper 的默认行为需要从其中反序列化 POJO JSON。

使用第三方 API 解决此问题的一种方法是使用 Jackson Mixins。如果您查看GeoModule，这是一个您可以在ObjectMapper 注册的模块，其中包括一些 Mixins

mapper.registerModule(new GeoModule());

如果您查看org.springframework.data.mongodb.core.geo，您将看到另一个模块GeoJsonModule，其中也包含一些Mixins。这个模块应该处理GeoJsonPoint。

但您的用例的主要问题是（如果您回头查看 GeoModule）是 GeoResult 或 GeoResults 没有 Mixin，您需要解析 JSON。

我制作了一个模块来处理GeoResult，但GeoResults 目前不起作用。

public class GeoModuleExt extends SimpleModule {

    public GeoModuleExt() {
        super("Mixins", new Version(1, 0, 0, null));
        setMixInAnnotation(GeoResult.class, GeoResultMixin.class);
        setMixInAnnotation(GeoResults.class, GeoResultsMixin.class);
    }
    
    static abstract class GeoResultMixin {
        GeoResultMixin(@JsonProperty("content") Object content, 
                       @JsonProperty("distance") Distance distance) {    
        }
    }
    
    static abstract class GeoResultsMixin {
        GeoResultsMixin(@JsonProperty("results")List<GeoResult> results) {
           
        }   
    }
}

你可以玩弄它。我现在没有时间研究它（因此是 half-@$$ 解决方案），但是当我有时间时，如果你还没有弄清楚，我会看看我能做什么。

作为测试，您可以单独使用ObjectMapper，以确保它首先工作

public class Test {

    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new GeoJsonModule());
        mapper.registerModule(new GeoModule());
        // Our custom module
        mapper.registerModule(new GeoModuleExt());

        // Create FoodTruckEntity - 
        GeoJsonPoint geoJsonPoint = new GeoJsonPoint(10, 10);
        FoodTruckEntity foodTruck = new FoodTruckEntity();
        // set all properties fro foodTruck

        // Create GeoResult
        GeoResult<FoodTruckEntity> geoResult
                = new GeoResult<FoodTruckEntity>(foodTruck, new Distance(10,
                                                 Metrics.KILOMETERS));
        
        // Serialize geoResult
        String geoResultString = mapper.writeValueAsString(geoResult);
        System.out.println(geoResultString);
        
        JavaType type = mapper.getTypeFactory().constructParametricType(
                GeoResult.class, FoodTruckEntity.class);
        
        // Deserialize geoResultString
        GeoResult<FoodTruckEntity> parsedGeoResult 
                = mapper.readValue(geoResultString, type);
        System.out.println(parsedGeoResult.getDistance());
        System.out.println(parsedGeoResult.getContent().getApplicant());
        
        // Up to this point everything is fine. It's the deserialization of
        // `GeoResults` thats a problem.
        
        /*
        List<GeoResult> results = new ArrayList<GeoResult>();
        results.add(geoResult);
        results.add(geoResult);
        GeoResults geoResults = new GeoResults(results);
        
        String resultsString = mapper.writeValueAsString(geoResults);
        System.out.println(resultsString);
        
        
        JavaType resultType = mapper.getTypeFactory().constructParametricType(
                                       GeoResults.class, FoodTruckEntity.class);
        
        GeoResults<FoodTruckEntity> parsedGeoResults 
                            = mapper.readValue(resultsString, resultType);
        for (GeoResult<FoodTruckEntity> gr: parsedGeoResults) {
            System.out.println(gr.getContent().getGeoJsonPoint());
        }*/
    }
}

当您开始测试时，您可以在 Jersey 中注册 ObjectMapper，例如

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new GeoJsonModule());
mapper.registerModule(new GeoModule());
// Our custom module
mapper.registerModule(new GeoModuleExt());

ClientConfig config = new DefaultClientConfig();
config.getSingletons().add(new JacksonJsonProvider(mapper));
Client client = Client.create(config);

---

更新

所以在玩了一些之后，我能够让它与这个 Mixin 一起为GeoResults 工作。只需更新上面的GeoModuleExt

static abstract class GeoResultsMixin {
    GeoResultsMixin(@JsonProperty("results") List<GeoResult> results, 
                    @JsonProperty("averageDistance") Distance averageDistance) {
    }
    
    @JsonProperty("results")
    abstract List<GeoResult> getContent(); 
}

上面的测试可以正常工作。尚未对 Jersey 进行测试，但如果它可以与 ObjectMapper 一起使用，那么 Jersey 应该不会有问题，只要我们将 Jackson 提供程序配置为使用映射器即可。