Parsing a .json file (java). trying to get the item from the file, the result is null

Question

Need some help with java+json. My question is about the .json file. When parsing a file, I get null value. There may be a problem in the file itself, or i'm using the wrong parsing path. I have a class PhotocameraDTO:

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonProperty;

@JsonIgnoreProperties(ignoreUnknown = true)
public class PhotocameraDTO {

@JsonProperty("id")
private int id;

@JsonProperty("name")
private String name;

@JsonProperty("type")
private String type;

@JsonProperty("sensor")
private String sensor;

@JsonProperty("lenses")
private String lenses;

@JsonProperty("display")
private String display;

public int getId() {
    return id;
}

public String getName() {
    return name;
}

public String getType() {
    return type;
}

public String getSensor() {
    return sensor;
}

public String getLenses() {
    return lenses;
}

public String getDisplay() {
    return display;
}

@Override
public String toString() {
    return ("PhotocameraDTO [id=" + id + ", name=" + name + ", type=" + type + ", sensor=" + sensor + ", lenses="
            + lenses + ", display=" + display + "]");
}

And .json file (made a screenshot, since the code does not load as valid): http://prntscr.com/ls3fgn

And my class with main:

package task2;

import java.io.FileInputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.type.TypeFactory;

public class PhotocamerasTypes {

public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {
    FileInputStream fis = new FileInputStream("src/main/resources/photocameras.json");

    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);
    TypeFactory typeFactory = TypeFactory.defaultInstance();
    List<PhotocameraDTO> photocameras = objectMapper.readValue(fis, typeFactory.constructCollectionType(ArrayList.class, PhotocameraDTO.class));

    System.out.println(photocameras);
}
}

Result: [PhotocameraDTO [id=0, name=null, type=null, sensor=null, lenses=null, display=null]]

And now i found the mistake, but i don't now how to fix it. My json file has a structure like: { [{....},{.....}] } My code works if json file has structure like: [{....},{.....}]

But I don't how to fix my code for the structure with "{" at the beginning of the file.

Unrelated, but I'd recommend using Gson instead. – Frontear Dec 07 '18 at 19:10 — Frontear, Dec 07 '18 at 19:10

Soufiane Sakhi · Accepted Answer · 2018-12-07T22:04:15.783

0

You can add the class:

public class ItemsDTO<T> {
    @JsonProperty("items")
    private List<T> items;

    public List<T> getItems() {
        return items;
    }

    public void setItems(List<T> items) {
        this.items = items;
    }
}

then modify the main method:

public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {
    FileInputStream fis = new FileInputStream("src/main/resources/photocameras.json");
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);
    ItemsDTO<PhotocameraDTO> itemsDto = objectMapper.readValue(fis, new TypeReference<ItemsDTO<PhotocameraDTO>>() {});
    List<PhotocameraDTO> photocameras = itemsDto.getItems();
    System.out.println(photocameras);
}

edited Dec 07 '18 at 22:04

answered Dec 07 '18 at 21:19

Soufiane Sakhi

809
10
13

Thanks for your answer! I tryed, but it has an exception: "Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `task2.PhotocamerasTypes$ItemsDTO` out of START_ARRAY token at [Source: (FileInputStream); line: 1, column: 1] " – YanaKit Dec 07 '18 at 22:20
If you declared the ItemsDTO class as an inner class, you should add the `static` keyword: `public static class ItemsDTO` – Soufiane Sakhi Dec 08 '18 at 10:06

Parsing a .json file (java). trying to get the item from the file, the result is null

1 Answers1