java,spring,hibernate,spring-mvc,dto , Spring-MVC, Hibernate : Creating DTO objects from Domain objects

Spring-MVC, Hibernate : Creating DTO objects from Domain objects


Tag: java,spring,hibernate,spring-mvc,dto

I am working on a Spring-MVC application in which I am trying to search for List of GroupNotes in database. The mapping in my project is GroupCanvas has one-to-many mapping with GroupSection and GroupSection has one-to-many mapping with GroupNotes. Because of these mappings, I was getting LazyInitializationException. As suggested on SO, I should be converting the objects to a DTO objects for transfer. I checked on net, but couldnt find a suitable way to translate those.

I have just created a new List to avoid the error, but one field is still giving me an error. I would appreciate if anyone tells me either how to fix this error or convert the objects to a DTO objects so they can be transferred.

Controller code :

 @RequestMapping(value = "/findgroupnotes/{days}/{canvasid}")
    public @ResponseBody List<GroupNotes> findGroupNotesByDays(@PathVariable("days")int days, @PathVariable("canvasid")int canvasid){
        List<GroupNotes> groupNotesList = this.groupNotesService.findGroupNotesByDays(days,canvasid);

        List<GroupNotes> toSendList = new ArrayList<>();
        for(GroupNotes groupNotes : groupNotesList){
            GroupNotes toSendNotes = new GroupNotes();
        return toSendList;


GroupNotesDAOImpl :

    public List<GroupNotes> searchNotesByDays(int days, int mcanvasid) {
        Session session = this.sessionFactory.getCurrentSession();
        Calendar cal = Calendar.getInstance();
        cal.add(Calendar.DAY_OF_YEAR, -days);
        long daysAgo = cal.getTimeInMillis();
        Timestamp nowMinusDaysAsTimestamp = new Timestamp(daysAgo);
        GroupCanvas groupCanvas = (GroupCanvas) session.get(GroupCanvas.class,mcanvasid);
        Query query = session.createQuery("from GroupSection as n where n.currentcanvas.mcanvasid=:mcanvasid");
        query.setParameter("mcanvasid", mcanvasid);

        List<GroupSection> sectionList = query.list();
        List<GroupNotes> notesList = new ArrayList<GroupNotes>();
        for (GroupSection e : sectionList) {
            System.out.println("Section name is "+e.getMsectionname());
            GroupSection groupSection = (GroupSection) session.get(GroupSection.class,e.getMsectionid());
            Query query1 = session.createQuery("from GroupNotes as gn where gn.ownednotes.msectionid=:msectionid and gn.noteCreationTime >:limit");
            query1.setParameter("limit", nowMinusDaysAsTimestamp);
// I am getting the data below, but I get JSON errors.
        for(GroupNotes groupNotes : notesList){
            System.out.println("Group notes found are "+groupNotes.getMnotetext());
        return notesList;

GroupCanvas model :

@Table(name = "membercanvas")
public class GroupCanvas{
  @OneToMany(mappedBy = "currentcanvas",fetch=FetchType.LAZY, cascade = CascadeType.REMOVE)
    private Set<GroupSection> ownedsection = new HashSet<>();

    public Set<GroupSection> getOwnedsection() {
        return this.ownedsection;

    public void setOwnedsection(Set<GroupSection> ownedsection) {
        this.ownedsection = ownedsection;

GroupSection model :

@Table(name = "membersection")
public class GroupSection{

    @OneToMany(mappedBy = "ownednotes", fetch = FetchType.EAGER,cascade = CascadeType.REMOVE)
    private Set<GroupNotes> sectionsnotes = new HashSet<>();

    public Set<GroupNotes> getSectionsnotes(){
        return this.sectionsnotes;

    public void setSectionsnotes(Set<GroupNotes> sectionsnotes){

GroupNotes model :

public class GroupNotes{

    @GeneratedValue(strategy = GenerationType.SEQUENCE,generator = "mnote_gen")
    @SequenceGenerator(name = "mnote_gen",sequenceName = "mnote_seq")
    @org.hibernate.annotations.Index(name = "mnoticesidindex")
    private int mnoticesid;

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

    @Column(name = "mnoteheadline")
    private String mnotetag;

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

    @Column(name = "mnoteorder")
    private double mnoteorder;

    @Column(name = "attachmentcount")
    private int attachCount;

    @Column(name = "notedate")
    private String noteDate;

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

    @Column(name = "activeedit")
    private boolean activeEdit;

    @Column(name = "notedisabled")
    private boolean noteDisabled;

    @Column(name = "noteinactive")
    private boolean noteInActive;

    @Column(name = "notecreatoremail")
    private String noteCreatorEmail;

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

    @Column(name = "timestamp")
    private Timestamp noteCreationTime;

    private boolean notRead;

    private String tempNote;

    private String canvasUrl;

    @JoinColumn(name = "msectionid")
    private GroupSection ownednotes;

    public GroupSection getOwnednotes(){return this.ownednotes;}

    public void setOwnednotes(GroupSection ownednotes){this.ownednotes=ownednotes;}

    public int getOwnedSectionId(){
        return this.ownednotes.getMsectionid();

    @OneToMany(mappedBy = "mnotedata",fetch = FetchType.LAZY,cascade = CascadeType.REMOVE)
    private Set<GroupAttachments> mattachments = new HashSet<>();

    public Set<GroupAttachments> getMattachments() {
        return this.mattachments;

    public void setMattachments(Set<GroupAttachments> mattachments) {

        this.mattachments = mattachments;

    @OneToMany(mappedBy = "mhistory",fetch = FetchType.LAZY,cascade = CascadeType.REMOVE)
    private Set<GroupNoteHistory> groupNoteHistorySet = new HashSet<>();

    public Set<GroupNoteHistory> getGroupNoteHistorySet(){
        return this.groupNoteHistorySet;

    public void setGroupNoteHistorySet(Set<GroupNoteHistory> groupNoteHistorySet){
        this.groupNoteHistorySet = groupNoteHistorySet;

    @OneToMany(mappedBy = "unreadNotes",fetch = FetchType.LAZY,cascade = CascadeType.REMOVE)
    private Set<UnreadNotes> unreadNotesSet = new HashSet<>();

    public Set<UnreadNotes> getUnreadNotesSet(){
        return this.unreadNotesSet;

    public void setUnreadNotesSet(Set<UnreadNotes> unreadNotesSet){
        this.unreadNotesSet = unreadNotesSet;
//getters and setters ignored

Error log :

org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[0]->com.journaldev.spring.model.GroupNotes["ownedSectionId"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[0]->com.journaldev.spring.model.GroupNotes["ownedSectionId"])

Kindly let me know what to do, as I am stuck on that error since some time.


What I think that happens is Jackson tries to serialize all fields in the hierarchy based on getter methods. In some situation NullPointerException is thrown in the following method:

public int getOwnedSectionId(){
    return this.ownednotes.getMsectionid();

replace it with the following method:

public int getOwnedSectionId(){
    if(this.ownednotes != null)
        return this.ownednotes.getMsectionid();
    return 1;

I don't have an explanation why jackson tries to serialize it when is market with @JsonIgnore but you can give a try with my proposal


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