Convert JSON record to LinkedHashMap<String,String> using Jackson API

Question

I have a JSON file(it contains an array of JSON objects.) I am trying to read it object by object. Each object I need to convert it to a LinkedHashMap<String,String> where both the key and value are strings. Note that even if the JSON objects contain a non-string value(Integer/Boolean), I want my LinkedHashMap to contain a string.

This is my JSON file (films.json):

[
  {
    "name": "Fight Club",
    "year": 1999,
  }
]

Now, this has 1 object. I want to convert it to a LinkedHashMap<String,String>.

So for the above example, my LinkedHashMap should contain(for the 1st JSON object) :

"name" : "Fight CLub"

"year" : "1999"

Notice how the year is String in the LinkedHashMap and not Integer.

This is what I tried.

Map<String, Object> myLinkedHashMap;

JsonParser jsonParser = new JsonFactory().createParser(new File("films.json"));
jsonParser = new JsonFactory().createParser(new File(filePath));
jsonParser.nextToken();

ObjectMapper  mapper = new ObjectMapper();
mapper.registerModule(new JavaTimeModule());
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);

while(jsonParser.nextToken() != JsonToken.END_ARRAY){
    myLinkedHashMap  = mapper.readValue(jsonParser, LinkedHashMap.class);
}

The variable myLinkedHashMap will contain a key/value pair for an object in my JSON file.

But the problem is that for 'year' of the JSON file, I am getting Integer in the LinkedHashMap as the JSON file also contains Integer.

Instead, I want the Integer as String in the LinkedHashMap.

Please help me get String in the LinkedHashMap instead of Integer.

Note: The solution should be generic to other data types also.

So if the JSON object contains boolean true, then my LinkedHashMap should contain "true".

why not call toString on value while you are using them, toString will return String — Ryuzaki L
– Ryuzaki L, Commented Nov 5, 2019 at 20:09
@Deadpool the code is tightly coupled and changes cant be made there — Zeus
– Zeus, Commented Nov 5, 2019 at 20:11

Michał Ziober · Accepted Answer · 2019-11-05 20:27:22Z

You can construct map type using TypeFactory and constructMapType method to tell exactly what do you need from readValue method. See below example:

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.type.MapType;
import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
import org.springframework.util.Assert;

import java.io.File;
import java.util.LinkedHashMap;

public class JsonMapApp {

    public static void main(String[] args) throws Exception {
        File jsonFile = new File("./resource/test.json").getAbsoluteFile();

        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new JavaTimeModule());
        mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);

        JsonParser jsonParser = mapper.getFactory().createParser(jsonFile);
        jsonParser.nextToken();

        MapType mapType = mapper.getTypeFactory().constructMapType(LinkedHashMap.class, String.class, String.class);
        while (jsonParser.nextToken() != JsonToken.END_ARRAY) {
            LinkedHashMap<String, String> map = mapper.readValue(jsonParser, mapType);
            map.forEach((k, v) -> {
                Assert.isInstanceOf(String.class, v);
                System.out.println(k + " -> " + v + " (" + v.getClass().getName() + ")");
            });
        }
    }
}

Above code prints:

name -> Fight Club (java.lang.String)
year -> 1999 (java.lang.String)

zain · Accepted Answer · 2019-11-05 20:33:35Z

0

Change

Map<String, Object> myLinkedHashMap;

to

 Map<String, String> myLinkedHashMap;

answered Nov 5, 2019 at 20:33

zain

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