Question 220770 in Json Question posted in
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JSON to Java object deserialization exception

@Data
@NoArgsConstructor
@AllArgsConstructor
public class MatchingResponse {
    @JsonProperty(value = "Code")
    private String code;
    @JsonProperty(value = "Msg")
    private String message;
    @JsonProperty(value = "Id")
    private int id;
    @JsonProperty(value = "Date")
    private String date;
    @JsonProperty(value = "ClientIin")
    private String clientIin;
    @JsonProperty(value = "Similarity")
    private String similarity;
    @JsonProperty(value = "VendorName")
    private String vendorName;
}

This is my class. I`m trying to deserialize this JSON:

{
    "Code": "200",
    "Msg": "",
    "Id": 20148,
    "Date": "2023-08-01T15:21:13.9704572+06:00",
    "ClientIin": "888888888888",
    "Similarity": "0.007720462",
    "VendorName": "SomeName"
}

to my custom class with this code:

ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
byte[] byteResponse = httpResponse.getEntity().getContent().readAllBytes();
MatchingResponse response = mapper.readValue(byteResponse, MatchingResponse.class);

but because of JSON has empty variable "Msg", I`m getting exception:

Cannot construct instance of `MatchingResponse` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('{"Code":"200","Msg":"","Id":20148,"Date":"2023-08-01T15:18:03.883337+06:00","ClientIin":"888888888888","Similarity":"0.0738383234","VendorName":"SomeName"}')
 at [Source: (byte[])""{"Code":"200","Msg":"","Id":20147,"Date":"2023-08-01T15:18:03.883337+06:00","ClientIin":"830921301593","Similarity":"0.0738383234","VendorName":"VisionLabs"}""; line: 1, column: 1]

I tried creating JsonSetter:

@JsonSetter
    public void setMessage(String message) {
        if (message != null && message.isEmpty()) {
            this.message = null;
        } else {
            this.message = message;
        }
    }

like this or tried creating deserializers:

public class MatchingResponseDeserializer extends StdDeserializer<MatchingResponse> {
    public MatchingResponseDeserializer() {
        this(null);
    }

    public MatchingResponseDeserializer(Class<?> vc) {
        super(vc);
    }

    @Override
    public MatchingResponse deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JacksonException {
        JsonNode node = jsonParser.getCodec().readTree(jsonParser);
        String code = getOptionalString(node, "Code");
        String message = getOptionalString(node, "Msg");
        int id = getOptionalInt(node, "Id");
        String date = getOptionalString(node, "Date");
        String clientIin = getOptionalString(node, "ClientIin");
        String similarity = getOptionalString(node, "Similarity");
        String vendorName = getOptionalString(node, "VendorName");

        return new MatchingResponse(code, message, id, date, clientIin, similarity, vendorName);
    }

    private String getOptionalString(JsonNode node, String fieldName) {
        JsonNode valueNode = node.get(fieldName);
        return (valueNode != null) ? valueNode.asText() : null;
    }

    private int getOptionalInt(JsonNode node, String fieldName) {
        JsonNode valueNode = node.get(fieldName);
        return (valueNode != null) ? valueNode.asInt() : 0; // Provide a default value if needed
    }
}

Set this deserializer to the class:

@JsonDeserialize(using = MatchingResponseDeserializer.class)
public class MatchingResponse {

This deserializer:

public class EmptyStringDeserializer extends JsonDeserializer<String> {

    public EmptyStringDeserializer() {
        this(null);
    }

    public EmptyStringDeserializer(Class<?> vc) {
        super(vc);
    }
    @Override
    public String deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JacksonException {
        JsonNode node = jsonParser.readValueAsTree();
        if (node.asText().isEmpty()) {
            return null;
        }
        return node.toString();
    }
}

I set to variable:

@JsonProperty(value = "Msg")
@JsonDeserialize(using = EmptyStringDeserializer.class)
private String message;

but none of it worked.

I tried use Gson, but get exception:

java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 2 path $

So my question is, how can I fix this error? How to work with empty JSON variables?

Tags:

3

Answers


  1. 0

    You can try:

    ObjectMapper mapper = new ObjectMapper();
byte[] byteResponse = httpResponse.getEntity().getContent().readAllBytes();
String jsonString = new String(byteResponse); // parse byte array to String
JsonNode rootNode = mapper.readTree(jsonString); // parse String to JsonNode
MatchingResponse response = mapper.treeToValue(rootNode, MatchingResponse.class); // parse JsonNode to your object

    The mapper.readTree(jsonString) parses the string into a JsonNode, which is Jackson’s tree model for JSON.mapper.treeToValue(rootNode, MatchingResponse.class) then converts this tree into your object.

    Also, you don’t need to enable DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT. Jackson automatically converts empty strings in the JSON to empty strings in the object, not null.

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  2. 0

    Why don’t you try using json as jsonString and use

    MatchingResponse matchingResponse = objectMapper.readValue(jsonString, MatchingResponse.class);

    instead of byte[]

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  3. 0

    try removing @AllArgsConstructor, a small test without Lombok works fine…

    I guess there’s some issue with missing @JsonCreator annotation on the constructor generated by Lombok.

    package com.example.demo;

import com.fasterxml.jackson.annotation.JsonProperty;

public class MatchingResponse {
    @JsonProperty(value = "Code")
    private String code;
    @JsonProperty(value = "Msg")
    private String message;
    @JsonProperty(value = "Id")
    private int id;
    @JsonProperty(value = "Date")
    private String date;
    @JsonProperty(value = "ClientIin")
    private String clientIin;
    @JsonProperty(value = "Similarity")
    private String similarity;
    @JsonProperty(value = "VendorName")
    private String vendorName;

    public void setCode(String code) {
        this.code = code;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setDate(String date) {
        this.date = date;
    }

    public void setClientIin(String clientIin) {
        this.clientIin = clientIin;
    }

    public void setSimilarity(String similarity) {
        this.similarity = similarity;
    }

    public void setVendorName(String vendorName) {
        this.vendorName = vendorName;
    }

    public String getCode() {
        return code;
    }

    public String getMessage() {
        return message;
    }

    public int getId() {
        return id;
    }

    public String getDate() {
        return date;
    }

    public String getClientIin() {
        return clientIin;
    }

    public String getSimilarity() {
        return similarity;
    }

    public String getVendorName() {
        return vendorName;
    }
}



@Test
void deserialize() throws IOException {
    ObjectMapper mapper = new ObjectMapper();
    MatchingResponse matchingResponse = mapper.readValue(getClass().getResourceAsStream("/msg.json"), MatchingResponse.class);

    assertNotNull(matchingResponse);
}
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