Custom deserializer for any list in Jackson

Question

I have a problem with wrong objects in lists. For instance I've a JSON model:

{
  "items": [
    {
      "id": 1,
      "name": "Item1"
    },
    {
      "id": 2,
      "name": "Item2"
    },
    {
      "id": [],
      "name": "Item3"
    }
  ]
}

and two POJO

data class BadList(val items: List<BadItem>)

data class BadItem(val id: Int, val name: String)

Of course, when the parser stumbles upon a third element I get the exception

com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.Integer out of START_ARRAY token
 at [Source: {"items":[{"id":1,"name":"Item1"},{"id":2,"name":"Item2"},{"id":[],"name":"Item3"}]}; line: 1, column: 19] (through reference chain: my.package.BadList["items"]->java.util.ArrayList[2]->my.package.BadItem["id"])

Who knows how to get around this? I want to skip that wrong item.

Answer 1

You can write a custom deserializer and implement deserialization logic in it, eg:

class ItemIdDeserialiser extends JsonDeserializer<Integer> {

    @Override
    public Integer deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        Object value = p.getCurrentValue();
        //Check if it's Integer
        if(value instanceof Integer){
            return (Integer) value;
        }
        return null; //Or return first element if it's a non empty list
    }
}

Once this is done, you can annotate the field with @JsonDeserialise to instruct jackson to use your class, eg:

class Item {

    @JsonDeserialize(using = ItemIdDeserialiser.class)
    private Integer id;
}

Update

If you just want to ignore the field in serialization/deserialization then you can annotate it with @JsonIgnore , eg

class Item {

        @JsonIgnore
        private Integer id;
    }

Or even better, remove id from pojo and add @JsonIgnoreProperties on the class, eg:

@JsonIgnoreProperties(ignoreUnknown = true)
class Item {

}

It will automatically ignore the properties which are present in json but not found in class.

Answer 2

You can use a "HidableSerializer" for this and check the data during serialization

1. Create a IHidable interface

The interface has a isHidden method which is called during serialization

package ch.hasselba.jackson.test;

public interface IHidable {
    public boolean isHidden();
}

2. Change your BadItem class

Add the interface and change the setter of id . When property id is deserialized, it is tested if it is an Integer. If not, the item is marked as bad.

package ch.hasselba.jackson.test;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties( {"hidden"} ) 
public class BadItem implements IHidable{
    private Integer id;
    public String name;
    private boolean isBadItem;
    public Integer getId(){
        return id;
    }
    public void setId(Object value){
        if( value instanceof Integer ){
            this.id = (Integer) value;
        }else{
            this.isBadItem = true;
        }
    }
    public boolean isHidden() {
        return isBadItem;
    }
}

3. Create a HidableSerializer

package ch.hasselba.jackson.test;

import java.io.IOException;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;


@SuppressWarnings("serial")
public class HidableSerializer<T> extends StdSerializer<T> {

    private JsonSerializer<T> defaultSerializer;

    protected HidableSerializer(Class<T> t) {
        super(t);
    }

    public JsonSerializer<T> getDefaultSerializer() {
        return defaultSerializer;
    }

    public void setDefaultSerializer(JsonSerializer<T> defaultSerializer) {
        this.defaultSerializer = defaultSerializer;
    }

    @Override
    public void serialize(T value, JsonGenerator jgen, SerializerProvider provider)
            throws IOException, JsonProcessingException {

        if( value instanceof IHidable ){
            IHidable hidableValue = (IHidable) value;
            if( hidableValue.isHidden() )
                return;
        }

        defaultSerializer.serialize(value, jgen, provider);
    }

}

4. Register the HidableSerializer and that's it

package ch.hasselba.jackson.test;

import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;

public class Demo {

    @SuppressWarnings("serial")
    public static void main(String[] args) {

        // register the HidableSerializer
        ObjectMapper mapper = new ObjectMapper();
        mapper.setSerializationInclusion(Include.NON_EMPTY);
        mapper.registerModule(new SimpleModule() {
            @Override
            public void setupModule(SetupContext context) {
                super.setupModule(context);
                context.addBeanSerializerModifier(new BeanSerializerModifier() {
                    @Override
                    public JsonSerializer<?> modifySerializer(
                      SerializationConfig config, BeanDescription desc, JsonSerializer<?> serializer) {
                        if (BadItem.class.isAssignableFrom(desc.getBeanClass())) {
                            HidableSerializer ser = new HidableSerializer(BadItem.class);
                            ser.setDefaultSerializer(serializer);
                            return ser;
                        }
                        return serializer;
                    }
                });
            }
        });

        String content = "{  \"items\": [    {      \"id\": 1,      \"name\": \"Item1\"    },    {      \"id\": 2,      \"name\": \"Item2\"    },    {      \"id\":[],      \"name\": \"Item3\"    }  ]}";

        // build the Object
        BadList test = null;
        try {
            test =  mapper.readValue(content, BadList.class);
        } catch (Exception e) {
            e.printStackTrace();
        }

        // and now convert it back to a String
        String data = null;
        try {
             data = mapper.writeValueAsString(test);
        } catch (Exception e) {
            e.printStackTrace();
        }

        // print the result
        System.out.println( data );
    }

}

When changing the id "[]" to an Integer value, the Item is displayed, otherwise it is empty.

The result:

{"items":[{"id":1,"name":"Item1"},{"id":2,"name":"Item2"}]}