从JSON反序列化复杂的地图结构

Question

I have a JSON structure like this:

[
    {
        "topLevelOne": {
            "property1": false,

        },
        "topLevelTwo": [
            {
                "property2": false,
                "property3": false,
                "property4": false,
            },
            {
                "property2": false,
                "property3": false,
                "property4": false,
            }
        ]
    },

    {
        "topLevelOne": {
            "property1": false,

        },
        "topLevelTwo": [
            {
                "property2": false,
                "property3": false,

            },
            {
                "property2": false,
                "property3": false,
            },
        ]
    }
]

I am trying to convert this structure with Jackson into Map<CustomObject, List<CustomObject>> where topLevelOne corrsponds to Key , and the value is topLevelTwo ( List<CustomObject> )

So I tried this:

ObjectMapper JSONMapper = new ObjectMapper();
Map<CustomObject, List<CustomObject>> map = JSONMapper.readValue(json, new TypeReference<Map<CustomObject, List<CustomObject>>>(){});

However I'm getting this error message: com.fasterxml.jackson.databind.JsonMappingException: Can not find a (Map) Key deserializer for type [simple type, class CustomObject ]

Could anyone point out what I'm doing wrong here? Maybe a code example how this can be achieved?

Answer 1

The json isn't valid because you have mixed types at the same level in your structure. I think you might accomplish what you want with a Map of Maps. I'm not sure about your custom Objects, so I'll use Strings in case it helps.

If you define your json like this:

[{
        "topLevelOne": [{
            "property1": false
        }],
        "topLevelTwo": [{
                "property2": false,
                "property3": false,
                "property4": false
            },
            {
                "property2": false,
                "property3": false,
                "property4": false
            }
        ]
    },

    {
        "topLevelOne": [{
            "property1": false

        }],
        "topLevelTwo": [{
                "property2": false,
                "property3": false

            },
            {
                "property2": false,
                "property3": false
            }
        ]
    }
]

you could read it with something like this:

Map<String, Map<String,String>> map = mapper.readValue(json, new TypeReference <Map<String, Map<String,String>>>(){});

Answer 2

The built in Jackson Map deserialiser uses field names as keys and maps them to the related field values.

Your json is structured differently—it's a list of entries. The simplest way to deserialise your map would be to read the list and build the map afterwards.

You'll need a helper object to model the entries:

class CustomEntry {
    private CustomKey topLevelOne;
    private List<CustomObject> topLevelTwo;

    @JsonCreator
    public CustomEntry(@JsonProperty("topLevelOne") CustomKey topLevelOne, @JsonProperty("topLevelTwo") List<CustomObject> topLevelTwo) {
        this.topLevelOne = topLevelOne;
        this.topLevelTwo = topLevelTwo;
    }

    public CustomKey getTopLevelOne() {
        return topLevelOne;
    }

    public List<CustomObject> getTopLevelTwo() {
        return topLevelTwo;
    }
}

Then,

String json = ...
ObjectMapper mapper = new ObjectMapper();
// Read the list of map entries
List<CustomEntry> entries = mapper.readValue(json, new TypeReference<List<CustomEntry>>(){});
// Build the map
Map<CustomKey, List<CustomObject>> map = entries.stream().collect(Collectors.toMap(e -> e.topLevelOne, e -> e.topLevelTwo));

You could create a custom deserialiser to read the map directly from the list, but I'm not sure if it's worth the extra effort.