Answer to https://stackoverflow.com/questions/48882899/supply-long-pool-to-object-mapper

JacksonLongPool.java

import com.fasterxml.jackson.annotation.JsonAnySetter;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.KeyDeserializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.google.common.collect.Interner;
import com.google.common.collect.Interners;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class JacksonLongPool {

    public static void main(String[] args) throws IOException {
        final String map1 = "{\"1\": \"Hello\", \"10000000\": \"world!\"}";
        final String map2 = "{\"1\": \"You\", \"10000000\": \"rock!\"}";


        System.out.println("-- Standard deserialization --");
        standardDeserialization(map1, map2);
        System.out.println("-------------------------------------");

        System.out.println("-- AnySetter deserialization --");
        anySetterDeserialization(map1, map2);
        System.out.println("-------------------------------------");

        System.out.println("-- Custom KeyDeserializer deserialization --");
        customKeyDeserialization(map1, map2);
        System.out.println("-------------------------------------");

        System.out.println("-- Annotation-based KeyDeserializer deserialization --");
        annotationBasedCustomKeyDeserialization(map1, map2);
        System.out.println("-------------------------------------");
    }

    private static void standardDeserialization(String map1, String map2) throws IOException {
        final ObjectMapper mapper = new ObjectMapper();
        final TypeReference<Map<Long, String>> typeRef = new TypeReference<Map<Long, String>>() {};
        final Map<Long, String> deserializedMap1 = mapper.readValue(map1, typeRef);
        final Map<Long, String> deserializedMap2 = mapper.readValue(map2, typeRef);

        printMap(deserializedMap1);
        printMap(deserializedMap2);
    }

    static class CustomLongPoolingMap {
        private static final Interner<Long> LONG_POOL = Interners.newStrongInterner();
        private final Map<Long, String> map = new HashMap<>();

        @JsonAnySetter
        public void addEntry(String key, String value) {
            map.put(LONG_POOL.intern(Long.parseLong(key)), value);
        }

        public Map<Long, String> getMap() {
            return map;
        }
    }

    private static void anySetterDeserialization(String map1, String map2) throws IOException {
        final ObjectMapper mapper = new ObjectMapper();
        final Map<Long, String> deserializedMap1 = mapper.readValue(map1, CustomLongPoolingMap.class).getMap();
        final Map<Long, String> deserializedMap2 = mapper.readValue(map2, CustomLongPoolingMap.class).getMap();

        printMap(deserializedMap1);
        printMap(deserializedMap2);
    }

    public static class MyCustomKeyDeserializer extends KeyDeserializer {
        private static final Interner<Long> LONG_POOL = Interners.newStrongInterner();
        @Override
        public Long deserializeKey(String key, DeserializationContext ctxt) {
            return LONG_POOL.intern(Long.parseLong(key));
        }
    }

    private static void customKeyDeserialization(String map1, String map2) throws IOException {
        final SimpleModule module = new SimpleModule();
        module.addKeyDeserializer(Long.class, new MyCustomKeyDeserializer());
        final ObjectMapper mapper = new ObjectMapper().registerModule(module);
        final TypeReference<Map<Long, String>> typeRef = new TypeReference<Map<Long, String>>() {};
        final Map<Long, String> deserializedMap1 = mapper.readValue(map1, typeRef);
        final Map<Long, String> deserializedMap2 = mapper.readValue(map2, typeRef);

        printMap(deserializedMap1);
        printMap(deserializedMap2);
    }

    static class MapWrapper {
        @JsonDeserialize(keyUsing = MyCustomKeyDeserializer.class)
        private Map<Long, String> map1;
        @JsonDeserialize(keyUsing = MyCustomKeyDeserializer.class)
        private Map<Long, String> map2;
    }

    private static void annotationBasedCustomKeyDeserialization(String map1, String map2) throws IOException {
        final ObjectMapper mapper = new ObjectMapper();
        final String json = "{\"map1\": " + map1 + ", \"map2\": " + map2 + "}";
        final MapWrapper wrapper = mapper.readValue(json, MapWrapper.class);
        final Map<Long, String> deserializedMap1 = wrapper.map1;
        final Map<Long, String> deserializedMap2 = wrapper.map2;

        printMap(deserializedMap1);
        printMap(deserializedMap2);
    }

    private static void printMap(Map<Long, String> longStringMap) {
        longStringMap.forEach((Long k, String v) -> {
            System.out.printf("key object id %d \t %s -> %s %n", System.identityHashCode(k), k, v);
        });
    }
}