Using Java Jackson how to map a json that contains a list of maps to java pojo

Question

I have been struggling with this issue for a couple days now.

Let's say my json response is:

{
"ignoreMe": -1,
"ignoreMeToo": "Not interested in this",
"valuableInfo": [{
        "Info1": {
            "key1": 0.0537,
            "key2": 0.0759
        }
    }, {
        "Info2": {
            "key2": 0.0444,
            "key4": 0.2345
        }
    }
]}

From this response i want to be able to map to my custom pojo object, which should look like this;

@JsonIgnoreProperties(ignoreUnknown = true)
public class ResponseDTO{
   private List<Map<String, Double>> valuableInfo;

    public ResponseDTO(List<Map<String, Double>> valuableInfo) {
        this.valuableInfo = valuableInfo;
    }

    public List<Map<String, Double>> getvaluableInfo() {
        return valuableInfo;
    }

    public void setvaluableInfo(List<Map<String, Double>> valuableInfo) {
        this.valuableInfo = valuableInfo;
    }
}

How do i setup my mapper to perform this conversion using just;

mapper.readValue(jsonResponse, ResponseDTO.class)

You need to have a public constructor for the class with out parameter. — Kajal
– Kajal, Commented Jul 27, 2018 at 11:09
is "Info1" has list of elements? Means, "Info1" [ {} ] or its a just "Info1" {} — Shaunak Patel
– Shaunak Patel, Commented Jul 27, 2018 at 11:16

Lino · Accepted Answer · 2018-07-27 11:16:26Z

5

I guess your error is, that Jackson can't find a suitable no-arg constructor for ResponseDTO (because there is none). To overcome this you can work with the @JsonCreator and @JsonProperty annotations. See the changed constructor:

@JsonCreator
public ResponseDTO(@JsonProperty("valuableInfo") List<Map<String, Double>> valuableInfo){
    this.valuableInfo = valuableInfo;
}

The rest of the code can stay the same.

This just tells jackson that it should inject the property with name "valuableInfo" into that constructor.

edited Jul 27, 2018 at 11:16

answered Jul 27, 2018 at 11:11

Lino

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5 Comments

davidxxx Over a year ago

Nice answer. It is a pity that Java doesn't ease the retrieval of the parameter names by reflection (as it requires the debug option enabled). It would avoid the @JsonProperty("valuableInfo") declaration both verbose and not good for the maintenance.

Lino Over a year ago

@davidxxx It has been bothering me many times that it's not possible, but that's just how java is. It would probably be even better to not write such code yourself, but let it generate with Swagger or something, which would almost eliminate the case of maintenance problems.

zaf187 Over a year ago

Thank you for your suggestion, after making the changes you suggested, i get this error; "com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of java.lang.Double out of START_OBJECT token" i think it might have something to do with the Map<String, Double>

Lino Over a year ago

@waqask87 after looking at your question It is clear why that is. The valuableInfo contains a List of objects, with one property, which then contains a map. [{ "Info1":{"key1": 0.0537}}] So you'd need a List<Map<String, Map<String, Double>>> which makes me want to vomit. So instead, create new POJO instead using maps

zaf187 Over a year ago

Is there no way i could bypass this interim object? @Lino

Golam Mazid Sajib · Accepted Answer · 2018-07-27 19:47:31Z

0

Observe carefully your json. Here valuableInfo contains List of objects. And every object contains Map< String,Object >. Then you class need to be like:

@JsonIgnoreProperties(ignoreUnknown = true)
public class ResponseDTO{
private List<Map<String, Map<String, Double>>> valueableInfo;

@JsonCreator
public ResponseDTO(@JsonProperty("valuableInfo") List<Map<String, Map<String, Double>>> valuableInfo){
    this.valueableInfo = valuableInfo;
}

public List<Map<String, Map<String, Double>>> getValueableInfo() {
    return valueableInfo;
}

public void setValueableInfo(List<Map<String, Map<String, Double>>> valueableInfo) {
    this.valueableInfo = valueableInfo;
}
}

edited Jul 27, 2018 at 19:47

answered Jul 27, 2018 at 12:41

Golam Mazid Sajib

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2 Comments

zaf187 Over a year ago

Is there no way around that issue? i really just want a List<String, Map<String, Double>>

Golam Mazid Sajib Over a year ago

then need to change in your json

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