使用Jackson将Json子对象转换为HashMap

Question

I would like to convert the bpi field of the following json string:

{
    "bpi": {
        "2017-10-15": 5697.3917,
        "2017-10-16": 5754.2213,
        "2017-10-17": 5595.235,
        "2017-10-18": 5572.1988,
        "2017-10-19": 5699.5838,
        "2017-10-20": 5984.0863,
        "2017-10-21": 6013.2288,
        "2017-10-22": 5984.9563,
        "2017-10-23": 5895.2988,
        "2017-10-24": 5518.85,
        "2017-10-25": 5733.9038,
        "2017-10-26": 5888.145,
        "2017-10-27": 5767.68,
        "2017-10-28": 5732.825,
        "2017-10-29": 6140.5313,
        "2017-10-30": 6121.8,
        "2017-10-31": 6447.6675
    },
        "disclaimer": "This data was produced from the CoinDesk Bitcoin Price Index. BPI value data returned as USD.",
        "time": {
        "updated": "Nov 1, 2017 13:49:57 UTC",
        "updatedISO": "2017-11-01T13:49:57+00:00"
    }
}

into a hashmap - preferably Map<Date, Double> but Map<String, String> will do as well (then I would have to process it later on). I read somewhere how to convert json arrays to java arrays but since the desired array is wrapped in the bpi sub field I dont know how to access it. Could you please help me with that?

Thanks a lot

Answer 1

It's pretty straight forward with Jackson. First define a class to hold the values:

@JsonIgnoreProperties(ignoreUnknown = true)
public class Foo {

    private Map<LocalDate, Double> bpi;

    // Getters and setters ommited
}

Then read your JSON string:

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

Foo foo = mapper.readValue(json, Foo.class);

---

Alternatively you can use:

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

JsonNode node = mapper.readTree(json).get("bpi");

ObjectReader reader = mapper.readerFor(new TypeReference<Map<LocalDate, Double>>() {});
Map<LocalDate, Double> bpi = reader.readValue(node);

---

The LocalDate support comes from the jackson-datatype-jsr310 module.

Add the following dependency you project:

<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-jsr310</artifactId>
    <version>${jackson.version}</version>
</dependency>

And then register the JavaTimeModule in your ObjectMapper instance:

mapper.registerModule(new JavaTimeModule());

If you prefer to use Date instead of LocalDate , then you don't need any to add any dependency and you don't need to register any module either. Just use Date instead of LocalDate in the above examples. However I advise you to use LocalDate instead.