How to read a specific object from a Json file using Jackson

Question

Let's say I want to directly read the object fruits from a Json file and store it to the Fruit.class . How should I do it?

{
  "fruits": [
    { "name":"Apple", "price":"0.8"}
  ],
  "vegetables": [
    { "name":"Carrot", "price":"0.4"}
  ]
}

My model class

public class Fruit {
    private String name;
    private double price;

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public double getPrice() { return price; }
    public void setPrice(double price) { this.price= price; }
}

What I've tried:

String file = "src/main/resources/basket.json";
String json = new String(Files.readAllBytes(Paths.get(file)));
objectMapper.readValue(json, Fruit.class);

Answer 1

Your json must match with what you try to convert. In your case you need

public class FruitWrapper {

   private List<Fruits> fruits;
   private List<Fruits> vegetables;

   getters, setters...

   }

Then this will work for you

 FruitWrapper fruitWrapper =  objectMapper.readValue(json, FruitWrapper.class);

Answer 2

Make a class called food:

public class Food {
    private Fruit[] fruits;
    private Fruit[] vegetables;
    
    public Food() {
    }

    public Fruit[] getFruits() {
        return fruits;
    }

    public Fruit[] getVegetables() {
        return vegetables;
    }
}

and then use:

objectMapper.readValue(json, Food.class);

after that you can get the fruits and vegetables from your food instance

Answer 3

We can write a little helper method, to convert raw string representation of the nested JSON array, to the Java list:

public class EntryPoint {

    public static void main(String[] args) throws Exception {
        String json = new String(Files.readAllBytes(Paths.get("src/main/resources/basket.json")));

        //maybe better to use more generic type Product? as fruits and vegs have same structure
        List<Fruit> fruits = getNestedJSONArray(json, "fruits");
        List<Fruit> vegetables = getNestedJSONArray(json, "vegetables");

    }

    private static List<Fruit> getNestedJSONArray(String json, String key) throws Exception {
        List<Fruit> fruits = new ArrayList<>();
        new ObjectMapper().readTree(json)
                .get(key)
                .forEach(e -> {
                    Fruit tmpf = new Fruit();
                    tmpf.setName(e.get("name").textValue());
                    tmpf.setPrice(e.get("price").asDouble());
                    fruits.add(tmpf);
                });
        return fruits;
    }
}