Jackson error: Cannot deserialize instance of `java.lang.String` out of START_ARRAY token

Question

Hey I have also problem here is my Json

[
{
    "aimid": "12345"
},
{
    "aimid": "333674"
},
{
    "aimid": [
        "4568999",
        "6789345"
    ]
}]

and This is my Pojo class:-

@JsonProperty("aimid")
private String aimid;


public String getAimid() {
    return aimid;
}

public void setAimid(String aimid) {
    this.aimid = aimid;
}

I want to store aimid in pojo . When i am writing like above in my application i am getting error.

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.lang.String` out of START_ARRAY token.

From my understanding i am getting error because of Array element so anyone can suggest me how i can capture both thing if it is coming as String or It is coming as a Array String

Answer 1

The challenge is that in some cases "aimid" is a string value but in another case it is an array.

If you have control over the structure of the JSON then update the structure so that each element of the root array has ONE of the following structures:

String { "aimid": "333674" } OR array { "aimid": [ "4568999", "6789345" ] }

If you do not have control of the structure of the data you will need to parse it yourself and process it into your POJO.

Please see these 3 code examples that should illustrate how you can go about this approaches. :

public class MyPojo {

    private List<String> aimid;

    @JsonProperty("aimid")
    public List<String> getAimid() {
    return aimid;
    }

    @JsonProperty("aimid_array")
    public void setAimid(final List<String> aimid) {
    this.aimid = aimid;
    }

    @JsonProperty("aimid")
    public void setAimid(final String aimid) {
    this.aimid = Arrays.asList(aimid);
    }
}


import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.node.*;
import java.io.IOException;
import java.util.*;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.junit.Test;

    public class UnitTest {

    private static final Logger LOGGER = Logger.getLogger(UnitTest.class.getName());

    public UnitTest() {
    }

    @Test
    public void testOneAimId() throws IOException {
    final String json = "[\n"
        + "{\n"
        + "    \"aimid\": \"12345\"\n"
        + "},\n"
        + "{\n"
        + "    \"aimid\": \"333674\"\n"
        + "}]";
    final List<MyPojo> result = new ObjectMapper().readValue(json, new TypeReference<List<MyPojo>>() {
    });
    log(Level.SEVERE, LOGGER, "testOneAimId", result);
    }

    @Test
    public void testListAimIds() throws IOException {
    final String json = "[\n"
        + "{\n"
        + "    \"aimid_array\": [\n" // HERE WE HAVE CHANGED THE JSON PROP NAME
        + "        \"4568999\",\n"
        + "        \"6789345\"\n"
        + "    ]\n"
        + "}]";
    final List<MyPojo> result = new ObjectMapper().readValue(json, new TypeReference<List<MyPojo>>() {
    });
    log(Level.SEVERE, LOGGER, "testListAimIds", result);
    }

    @Test
    public void testMixed() throws IOException {
    final String json = "[\n"
        + "{\n"
        + "    \"aimid\": \"12345\"\n"
        + "},\n"
        + "{\n"
        + "    \"aimid\": \"333674\"\n"
        + "},\n"
        + "{\n"
        + "    \"aimid_array\": [\n" // HERE WE HAVE CHANGED THE JSON PROP NAME
        + "        \"4568999\",\n"
        + "        \"6789345\"\n"
        + "    ]\n"
        + "}]";
    final List<MyPojo> result = new ObjectMapper().readValue(json, new TypeReference<List<MyPojo>>() {
    });
    log(Level.SEVERE, LOGGER, "testMixed", result);
    }

    @Test
    public void testMixed2() throws IOException {
    final String json = "[\n"
        + "{\n"
        + "    \"aimid\": \"12345\"\n"
        + "},\n"
        + "{\n"
        + "    \"aimid\": \"333674\"\n"
        + "},\n"
        + "{\n"
        + "    \"aimid\": [\n"
        + "        \"4568999\",\n"
        + "        \"6789345\"\n"
        + "    ]\n"
        + "}]";

    final JsonNode result = new ObjectMapper().readValue(json, JsonNode.class);
    final ArrayList<String> arrayList = new ArrayList<>();

    result.forEach((final JsonNode jsonNode) -> {

        if (jsonNode.getNodeType() != JsonNodeType.OBJECT)
        throw new IllegalArgumentException(jsonNode.toString());

        final ObjectNode obj = (ObjectNode) jsonNode;
        obj.forEach(o -> {
        switch (o.getNodeType()) {
            case ARRAY:
            final ArrayNode array = (ArrayNode) o;
            array.forEach(t -> arrayList.add(t.asText()));
            break;
            case STRING:
            arrayList.add(o.asText());
            break;
            default:
            throw new IllegalArgumentException(o.toString());
        }
        });
    });

    final MyPojo myPojo = new MyPojo();
    myPojo.setAimid(arrayList);
    log(Level.SEVERE, LOGGER, "myPojo", myPojo);
    }

    private void log(final Level level, final Logger logger, final String title, final Object obj) {
    try {
        if (title != null)
        logger.log(level, title);
        final ObjectWriter writer = new ObjectMapper().writerWithDefaultPrettyPrinter();
        logger.log(level, obj == null ? "null" : writer.writeValueAsString(obj));
    } catch (final JsonProcessingException ex) {
        logger.log(Level.SEVERE, ex.getMessage(), ex);
    }
    }
}