Customising Jackson to use new lines between root objects and also customise the serialization of a list. Useful when talking to Elasticsearch _msearch endpoint.

gistfile1.java

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.util.MinimalPrettyPrinter;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;

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

public class MgetDemo {

    public static void main(final String... args) throws Exception {
        final ObjectMapper mapper = new ObjectMapper();

        final Mget mget = new Mget();

        mget.add("inventory", "property", new ArrayList<>(Arrays.asList("1", "2", "3")));
        mget.add("inventory", "offer", new ArrayList<>(Arrays.asList("1", "2", "3")));

        mapper.writer(new MinimalPrettyPrinter("\n")).writeValue(System.out, mget);

    }

    @JsonSerialize(using = MgetSerializer.class)
    public static class Mget {

        public static class Item {

            @JsonProperty("_index")
            private final String index;

            @JsonProperty("_type")
            private final String type;

            @JsonProperty("_id")
            private final String id;

            public Item(final String index, final String type, final String id) {
                this.index = index;
                this.type = type;
                this.id = id;
            }

        }

        public final List<Item> items = new ArrayList<>();

        public void add(final String index, final String type, final Iterable<String> ids) {
            for (String id : ids) {
                add(new Item(index, type, id));
            }
        }

        public void add(final Item item) {
            items.add(item);
        }

    }

    public static class MgetSerializer extends JsonSerializer<Mget> {

        @Override
        public boolean isEmpty(final Mget value) {
            return value.items.isEmpty();
        }

        @Override
        public Class<Mget> handledType() {
            return Mget.class;
        }

        @Override
        public void serialize(final Mget value, final JsonGenerator jgen, final SerializerProvider provider) throws IOException, JsonProcessingException {
            for (Mget.Item item : value.items) {
                jgen.writeObject(item);
            }
        }

    }

}