使用Jackson自定义JSON反序列化

Question

我正在使用Flickr API.调用flickr.test.login方法时,默认的JSON结果是:

{
    "user": {
        "id": "21207597@N07",
        "username": {
            "_content": "jamalfanaian"
        }
    },
    "stat": "ok"
}

我想将此响应解析为Java对象:

public class FlickrAccount {
    private String id;
    private String username;
    // ... getter & setter ...
}

应该像这样映射JSON属性:

"user" -> "id" ==> FlickrAccount.id
"user" -> "username" -> "_content" ==> FlickrAccount.username

不幸的是,我无法使用Annotations找到一个漂亮,优雅的方法.到目前为止,我的方法是将JSON字符串读入a Map<String, Object>并从那里获取值.

Map<String, Object> value = new ObjectMapper().readValue(response.getStream(),
        new TypeReference<HashMap<String, Object>>() {
        });
@SuppressWarnings( "unchecked" )
Map<String, Object> user = (Map<String, Object>) value.get("user");
String id = (String) user.get("id");
@SuppressWarnings( "unchecked" )
String username = (String) ((Map<String, Object>) user.get("username")).get("_content");
FlickrAccount account = new FlickrAccount();
account.setId(id);
account.setUsername(username);

但我认为,这是迄今为止最不优雅的方式.有没有简单的方法,使用注释还是自定义反序列化器？

这对我来说非常明显,但当然它不起作用:

public class FlickrAccount {
    @JsonProperty( "user.id" ) private String id;
    @JsonProperty( "user.username._content" ) private String username;
    // ... getter and setter ...
}

Answer 1

您可以为此类编写自定义反序列化程序.它可能看起来像这样:

class FlickrAccountJsonDeserializer extends JsonDeserializer<FlickrAccount> {

    @Override
    public FlickrAccount deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        Root root = jp.readValueAs(Root.class);

        FlickrAccount account = new FlickrAccount();
        if (root != null && root.user != null) {
            account.setId(root.user.id);
            if (root.user.username != null) {
                account.setUsername(root.user.username.content);
            }
        }

        return account;
    }

    private static class Root {

        public User user;
        public String stat;
    }

    private static class User {

        public String id;
        public UserName username;
    }

    private static class UserName {

        @JsonProperty("_content")
        public String content;
    }
}

之后,您必须为您的类定义反序列化器.你可以这样做:

@JsonDeserialize(using = FlickrAccountJsonDeserializer.class)
class FlickrAccount {
    ...
}

Answer 2

因为我不想实现自定义类(Username)只是为了映射用户名,所以我更优雅一点,但仍然是非常丑陋的方法:

ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.readTree(in);
JsonNode user = node.get("user");
FlickrAccount account = new FlickrAccount();
account.setId(user.get("id").asText());
account.setUsername(user.get("username").get("_content").asText());

它仍然没有我希望的那么优雅,但至少我摆脱了所有丑陋的演员.此解决方案的另一个优点是,我的域类(FlickrAccount)不会被任何Jackson注释污染.

基于@MichałZiober的回答,我决定使用 - 在我看来 - 最直接的解决方案.@JsonDeserialize在自定义反序列化器中使用注释:

@JsonDeserialize( using = FlickrAccountDeserializer.class )
public class FlickrAccount {
    ...
}

但是反序列化器不使用任何内部类,只是JsonNode如上所述:

class FlickrAccountDeserializer extends JsonDeserializer<FlickrAccount> {
    @Override
    public FlickrAccount deserialize(JsonParser jp, DeserializationContext ctxt) throws 
            IOException, JsonProcessingException {
        FlickrAccount account = new FlickrAccount();
        JsonNode node = jp.readValueAsTree();
        JsonNode user = node.get("user");
        account.setId(user.get("id").asText());
        account.setUsername(user.get("username").get("_content").asText());
        return account;
    }
}

Answer 3

您还可以使用 SimpleModule。

    SimpleModule module = new SimpleModule();
    module.setDeserializerModifier(new BeanDeserializerModifier() {
    @Override public JsonDeserializer<?> modifyDeserializer(
        DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
        if (beanDesc.getBeanClass() == YourClass.class) {
            return new YourClassDeserializer(deserializer);
        }

        return deserializer;
    }});

    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.registerModule(module);
    objectMapper.readValue(json, classType);

Answer 4

我是这样做的：

public class FlickrAccount {
  private String id;
  @JsonDeserialize(converter = ContentConverter.class)
  private String username;
  
  private static class ContentConverter extends StdConverter<Map<String, String>, String> {
    @Override
    public String convert(Map<String, String> content) {
      return content.get("_content"));
    }
  }
}