Deserialize json with duplicate keys using Jackson

I am trying to parse a json payload with duplicate keys to a Map<String, List<String>> using Jackson

Consider the following payload

{
"foo" : "val1",
"foo" : "val2",
"bar" : "val3"
}

I want to convert this to a Map<String, List<String>> type.

ex:

+--------------+----------------------+
| Key (String) | Value (List<String>) |
+--------------+----------------------+
| foo          | [val1, val2]         |
| bar          | [val3]               |
+--------------+----------------------+

What would be the best way to handle this with Jackson? I'm hoping that there's a way in Jackson where I can register a custom deserialization logic for Map<String, List<String>> type. (Note that I have no control over the json payload sent in the request)

Appreciate any help on this.

Thanks!

Check if it works for you.

Using @JsonAnySetter to deserialize unmapped JSON properties. @JsonAnySetter annotation can be used to define "any setter" mutator.

Using @JsonAnyGetter Annotation to serialize any arbitrary properties. @JsonAnyGetter can be used on a method which returns a Map.

POJO Definition

Test.java

package oct2020.json;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.fasterxml.jackson.annotation.JsonAnyGetter;
import com.fasterxml.jackson.annotation.JsonAnySetter;

public class Test {
	
	private Map<String, List<String>> keyValuesMap = new HashMap<String, List<String>>();

	/**
	 * @return the keyValuesMap
	 */
	@JsonAnyGetter
	public Map<String, List<String>> getKeyValuesMap() {
		return keyValuesMap;
	}

	/**
	 * @param keyValuesMap
	 *            the keyValuesMap to set
	 */
	public void setKeyValuesMap(Map<String, List<String>> keyValuesMap) {
		this.keyValuesMap = keyValuesMap;
	}

	@JsonAnySetter
	public void duplicateKeyValues(String key, String value) {
		List<String> values = null;
		if (!keyValuesMap.containsKey(key)) {
			values = new ArrayList<String>();
		} else {
			values = keyValuesMap.get(key);
		}
		values.add(value);
		keyValuesMap.put(key, values);
	}
}

Converting the json to desired format.

JSONConverter .java

package oct2020.json;

import java.io.IOException;
import java.util.List;
import java.util.Map;

import com.fasterxml.jackson.databind.ObjectMapper;

public class JSONConverter {
	public static void main(String[] args) throws IOException {
		ObjectMapper mapper = new ObjectMapper();
		String json = "{\n\"foo\" : \"val1\",\n\"foo\" : \"val2\",\n\"bar\" : \"val3\"\n,"
				+ "\n\"bar\" : \"val3\"\n,\n\"bar\" : \"val3\"\n,\n\"bar\" : \"val3\"\n,"
				+ "\n\"bar\" : \"val3\"\n,\n\"bar\" : \"val3\"\n,\n\"bar\" : \"val3\"\n,"
				+ "\n\"bar\" : \"val3\"\n,\n\"bar\" : \"val3\"\n,\n\"bar\" : \"val3\"}";
		Test test = mapper.readValue(json, Test.class);
		Map<String, List<String>> keyValuesMap = test.getKeyValuesMap();
		System.out.println(mapper.writerWithDefaultPrettyPrinter()
				.writeValueAsString(keyValuesMap));
	}
}

Output:

{
  "bar" : [ "val3", "val3", "val3", "val3", "val3", "val3", "val3", "val3", "val3", "val3" ],
  "foo" : [ "val1", "val2" ]
}

please put this property JsonTypeInfo.As.EXISTING_PROPERTY in the @JsonTypeInfo annotation. It might work.