How to write a custom object serializer of entity classes with relationships?

Question

Java

In my web project, I use hiberante and jersey, which in turn comes with Jackson. To work with hiberante, I have 3 entity classes that represent the structure of database tables and have one-to-many and many-to-one relationships. The binding fields are marked with @OneToMany and @ManyToOne annotations . Objects of these classes must be serialized to json. To avoid errors during serialization, the binding fields were also annotated with @JsonManagedReference and @JsonBackReference annotations .
As a result of serialization of an object of the Shops class, the following line was obtained:

{
      "id": 2,
      "name": "Shop #2",
      "category": "auto",
      "distance": 120,
      "discounts": [
          {
              "id": 9,
              "title": "-50%",
              "startDate": 1425679200000,
              "endDate": 1425679200000
          }
      ]
 }

This did not suit me and I decided to write my own custom serializer. As a result, I wanted to see the following json:

{
      "id": 2,
      "name": "Shop #2",
      "category": "auto",
      "distance": 120,
      "locality_id": 10,
 }

To do this, I created 3 serializer classes for each of the entities and removed the @JsonManagedReference and @JsonBackReference annotations that were before and added @JsonSerialize with the serializer class.
Later, when I try to run, I get the following error:

com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[0])
at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:210)
at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:189)
at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:216)
at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:117)
at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:73)
at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:19)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:129)
at com.fasterxml.jackson.databind.ObjectWriter._configAndWriteValue(ObjectWriter.java:1052)

How can I fix this error?
Following are the entity classes and serializer classes.
Entity classes:
Localities

@XmlRootElement
@Entity
@JsonSerialize(using = LocalitiesSerializer.class)
@Table(name = "localities")
public class Localities {
    @Id
    @GeneratedValue
    @Column(name="id", unique=true, nullable=false)
    private Integer id;

    @Column(name="name",nullable=false, length=57)
    private String name;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "locality")
    private Set<Shops> shops = new HashSet<Shops>(0);

    //constructors, getters & setters
}

Shops

@XmlRootElement
@Entity
@JsonSerialize(using = ShopsSerializer.class)
@Table(name = "shops")
public class Shops {
    @Id
    @GeneratedValue
    @Column(name="id", unique=true, nullable=false)
    protected Integer id;

    @Column(name="name", length=45, nullable=false)
    protected String name;

    @Column(name="category", columnDefinition="ENUM('undefined','auto','children_prod','food','game','book','electronics','beuty_and_health','fashion','footwear','clothing','sports','homewere','pet_prod','services','gift_and_flowers')",
            nullable=false)
    @Enumerated(EnumType.STRING)
    protected Categories category;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "locality_id", nullable = false)
    protected Localities locality;

    @Transient
    private Double distance;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "shop")
    private Set<Discounts> discounts = new HashSet<Discounts>(0);

    //constructors, getters & setters    
}

Discount

@XmlRootElement
@Entity
@JsonSerialize(using = DiscountsSerializer.class)
@Table(name="discounts")
public class Discounts {

    @Id
    @GeneratedValue
    @Column(name="id")
    private Integer id;

    @Column(name="title")
    private String title;

    @Column(name="start_date")
    @Temporal(TemporalType.DATE) 
    private Calendar startDate;

    @Column(name="end_date")
    @Temporal(TemporalType.DATE) 
    private Calendar endDate;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "shop_id", nullable = false)
    private Shops shop;

    //constructors, getters & setters

}

Serializer classes:
LocalitiesSerializer

public class LocalitiesSerializer extends JsonSerializer<Localities> {
    @Override
    public void serialize(Localities locality, JsonGenerator jsonGen,
            SerializerProvider serProv) throws IOException,
            JsonProcessingException {
        jsonGen.writeStartObject();
        jsonGen.writeNumberField("id", locality.getId());
        jsonGen.writeStringField("name", locality.getName());
        jsonGen.writeEndObject();

    }
}

ShopsSerializer

public class ShopsSerializer extends JsonSerializer<Shops> {

    @Override
    public void serialize(Shops shop, JsonGenerator jsonGen,
            SerializerProvider serProv) throws IOException,
            JsonProcessingException {
        jsonGen.writeStartObject();
        jsonGen.writeNumberField("id", shop.getId());
        jsonGen.writeStringField("name", shop.getName());
        jsonGen.writeStringField("category", shop.getCategory().toString());
        jsonGen.writeNumberField("distance", shop.getDistance());
        jsonGen.writeNumberField("locality_id", shop.getLocality().getId());
        jsonGen.writeEndObject();

    }

}

DiscountsSerializer

public class DiscountsSerializer extends JsonSerializer<Discounts> {
    @Override
    public void serialize(Discounts discount, JsonGenerator jsonGen,
            SerializerProvider serProv) throws IOException,
            JsonProcessingException {

        jsonGen.writeStartObject();
        jsonGen.writeNumberField("id", discount.getId());
        jsonGen.writeStringField("title", discount.getTitle());
        jsonGen.writeStringField("start_date", discount.getStartDate().toString());
        jsonGen.writeStringField("end_date", discount.getEndDate().toString());
        jsonGen.writeNumberField("shop_id", discount.getShop().getId());
        jsonGen.writeEndObject();

    }
}

PS The @JsonIgnore annotation is not suitable in this case, since in the future I plan to remove the @JsonSerialize annotations and use serializer classes as follows:

ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addSerializer(Item.class, new ShopsSerializer());
mapper.registerModule(module);
String serialized = mapper.writeValueAsString(shop);

Thus, it becomes possible to use different serializers.