如何用 Jackson 解析嵌套数组？

Question

我曾经成功解析过以下 .json 文件：

[
    {
        "latitude": 49.419459253939316,
        "longitude": 8.676411621072491
    },
    {
        "latitude": 49.41946061080915,
        "longitude": 8.676411644939083
    },
    {
        "latitude": 49.420365910782735,
        "longitude": 8.676438042403413
    }
]

以下Jackson 脚本输出一个点List 。

private static <T> List<T> parseFile(final String fileName, 
                                     Class<T> contentType) {
    // ...
    InputStream inputStream = // Open file
    ObjectMapper objectMapper = new ObjectMapper();
    try {
        TypeFactory typeFactory = objectMapper.getTypeFactory();
        CollectionType collectionType = typeFactory
            .constructCollectionType(List.class, contentType);
        return objectMapper.readValue(inputStream, collectionType);
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;
}

现在数据集变得更加复杂。 点List变成点List List 。 我是这样构建的 - 如果这不正确，请纠正我。

[
    [
        {
            "latitude": 49.419459253939316,
            "longitude": 8.676411621072491
        },
        {
            "latitude": 49.41946061080915,
            "longitude": 8.676411644939083
        },
        {
            "latitude": 49.420365910782735,
            "longitude": 8.676438042403413
        }
    ],
    [
        {
            "latitude": 49.40460334213399,
            "longitude": 8.670034018853409
        },
        {
            "latitude": 49.404608057285145,
            "longitude": 8.670028775634165
        },
        {
            "latitude": 49.40506145685422,
            "longitude": 8.66955817506422
        }
    ]
]

我准备了以下 POJO 来存储数据：

public class GeoPoint {
    public double latitude;
    public double longitude;
}

...

public class ThreePoints {
    public List<GeoPoint> points;  
}

我必须如何更改上面的 Jackson 解析器才能处理嵌套数组？ Jackson 能否将数据解析为嵌套的类结构，例如ThreePoints.class ？

Answer 1

您可以简单地创建其他集合类型。 见下面的代码：

TypeFactory typeFactory = mapper.getTypeFactory();
CollectionType listType = typeFactory.constructCollectionType(List.class, contentType);
CollectionType listListType = typeFactory.constructCollectionType(List.class, listType);
List<List<GeoPoint>> readValue = mapper.readValue(json, rootCollectionType);
// convert to ThreePoints

编辑
不幸的是，您无法告诉Jackson将您的JSON转换为您的POJO类，因为它们不适合彼此。 Jackson也不包含可用于将JSON映射到POJO类的注释。 你必须手动完成。 使用我的代码，您可以为ThreePoints类编写自定义反序列化器，您可以在其中使用内部ObjectMapper和我上面的代码。 我认为，您可以轻松地将List<List<GeoPoint>> readValue转换为ThreePoints类。 另一种选择 - 您可以在JsonUtil类中编写一个简单的函数。 如果您真的无法更改此JSON ，则必须手动进行。

Answer 2

您可以编写一个简单的自定义反序列化器

要将其反序列化到您的班级：

public class GeoPoint {
    public double latitude;
    public double longitude;
}

public class ThreePoints {
    public List<GeoPoint> points;
}

编写自定义解串器：

class ThreePointsDeserializer extends StdDeserializer<ThreePoints> {

    protected ThreePointsDeserializer() {
        super(ThreePoints.class);
    }


    @Override
    public ThreePoints deserialize(JsonParser parser, DeserializationContext ctx)
            throws IOException, JsonProcessingException {
        ThreePoints result = new ThreePoints();
        GeoPoint[] points = parser.getCodec().readValue(parser, GeoPoint[].class);
        result.points = Arrays.asList(points);
        return result;
    }
}

使用该解串器：

SimpleModule module = new SimpleModule();
module.addDeserializer(ThreePoints.class, new ThreePointsDeserializer());

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);

TypeFactory tf = mapper.getTypeFactory();
CollectionType collectionType = tf.constructCollectionType(List.class, ThreePoints.class);

List<ThreePoints> result = mapper.readValue(YOUR_DATA, collectionType);

Answer 3

您需要将值读取为矩阵，首先您需要使用名为 Place 的 Pojo 映射这些值。

public class Place {

    double latitude;
    double longitude;

    public double getLatitude() {
        return latitude;
    }

    public void setLatitude(double latitude) {
        this.latitude = latitude;
    }

    public double getLongitude() {
        return longitude;
    }

    public void setLongitude(double longitude) {
        this.longitude = longitude;
    }
}       

第二步，你需要将 JsonNode 映射成一个矩阵。

import java.io.IOException;
import java.util.List;
import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.map.type.CollectionType;
import org.codehaus.jackson.map.type.TypeFactory;


public class Main {
   static String jsonString =  "[" +
"    [" +
"        {" +
"            \"latitude\": 49.419459253939316," +
"            \"longitude\": 8.676411621072491" +
"        }," +
"        {" +
"            \"latitude\": 49.41946061080915," +
"            \"longitude\": 8.676411644939083" +
"        }," +
"        {" +
"            \"latitude\": 49.420365910782735," +
"            \"longitude\": 8.676438042403413" +
"        }" +
"    ]," +
"    [" +
"        {" +
"            \"latitude\": 49.40460334213399," +
"            \"longitude\": 8.670034018853409" +
"        }," +
"        {" +
"            \"latitude\": 49.404608057285145," +
"            \"longitude\": 8.670028775634165" +
"        }," +
"        {" +
"            \"latitude\": 49.40506145685422," +
"            \"longitude\": 8.66955817506422" +
"        }" +
"    ]" +
"]";

    public static void main(String...args) {
        final ObjectMapper om = new ObjectMapper();
        try {
           final Place[][] dtos = om.readValue(new ObjectMapper()
               .readValue(jsonString, JsonNode.class), Place[][].class);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

干杯。

Answer 4

在我看来，最优雅的解决方案是使用 TypeReference

ObjectMapper mapper = new ObjectMapper();
TypeReference<List<List<GeoPoint>>> typeRef = new TypeReference<List<List<GeoPoint>>>() {};
List<List<GeoPoint>> locations = mapper.readValue(jsonAsString, typeRef);