ObjectMapper fails to write a list of values as string

Question

I have a table commandeClient containing a list ligneCommandesClients with annotation json ignore for the list ligneCommandeCLients

    @Entity
public class CommandeClient implements Serializable {

    @Id
    @GeneratedValue
    private Long idCdeClient;

    private String codeCommande;
    @Temporal(TemporalType.TIME)
    private Date date;

    @OneToMany(mappedBy="commandeClient")
    private List<LigneCommandeClient>ligneCommandeClients;


    @JsonIgnore
    public List<LigneCommandeClient> getLigneCommandeClients() {
        return ligneCommandeClients;
    }

}

here is the entity LigneCommandeClient

@Entity
public class LigneCommandeClient implements Serializable {

    @Id
    @GeneratedValue
    private Long idLigneCommandeClient;

    private BigDecimal qty;

    private BigDecimal prixUnitaire;

    @ManyToOne
    @JoinColumn(name="idArticle")
    private Article article;

    @ManyToOne
    @JoinColumn(name="idCdeClient")
    private CommandeClient commandeClient;

in my jsp I added a jquery function to load the listCommandeClient in JSon format with an onclick event calling the following method

@Transient
    public String getLigneCommandeJSON() {

        if(!ligneCommandeClients.isEmpty()){

                ObjectMapper mapper = new ObjectMapper();
                try {
                    return mapper.writeValueAsString(ligneCommandeClients);
                } catch (JsonGenerationException e) {
                    e.printStackTrace();
                } catch (JsonMappingException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }

        }
        return "";
    }

however when executing the following instruction to transform the list to a json object

return mapper.writeValueAsString(ligneCommandeClients);

I keep repeating this error without a root cause (deleted du to the number of repeats of this error)

    at org.codehaus.jackson.map.ser.ContainerSerializers$CollectionSerializer.serializeContents(ContainerSerializers.java:360)
    at org.codehaus.jackson.map.ser.ContainerSerializers$CollectionSerializer.serializeContents(ContainerSerializers.java:312)
    at org.codehaus.jackson.map.ser.ContainerSerializers$AsArraySerializer.serialize(ContainerSerializers.java:111)
    at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:268)
    at org.codehaus.jackson.map.ser.BeanSerializer.serializeFields(BeanSerializer.java:146)
    at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:118)
    at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:268)
    at org.codehaus.jackson.map.ser.BeanSerializer.serializeFields(BeanSerializer.java:146)
    at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:118)
    at org.codehaus.jackson.map.ser.ContainerSerializers$CollectionSerializer.serializeContents(ContainerSerializers.java:360)
    at org.codehaus.jackson.map.ser.ContainerSerializers$CollectionSerializer.serializeContents(ContainerSerializers.java:312)
    at org.codehaus.jackson.map.ser.ContainerSerializers$AsArraySerializer.serialize(ContainerSerializers.java:111)
    at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:268)
    at org.codehaus.jackson.map.ser.BeanSerializer.serializeFields(BeanSerializer.java:146)
    at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:118)
    at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:268)
    at org.codehaus.jackson.map.ser.BeanSerializer.serializeFields(BeanSerializer.java:146)
    at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:118)
    at org.codehaus.jackson.map.ser.ContainerSerializers$CollectionSerializer.serializeContents(ContainerSerializers.java:360)
    at org.codehaus.jackson.map.ser.ContainerSerializers$CollectionSerializer.serializeContents(ContainerSerializers.java:312)

any idea how can I resolve this error thanks in advance