Servlet Gson().toJson infinite loop

Question

I am having some trouble in the servlet wherein everytime i change the option in a dropdown menu, a different value will be passed to the servlet and then it results into an infinite loop. When I am not changing the option (no change in value) in the dropdown, there is no error.

Here is mycode:

My Javascript:

<script>

function loadStaff(){
//dropdown
var positionDropDown = document.getElementById("positionsDropdown");
//value of the drop down
var positionID = positionDropDown.options[positionDropDown.selectedIndex].value;

    $.getJSON('loadStaff?positionID=' + positionID, function(data) {
            -- no populate code yet
});
}
</script>

My AjaxServlet:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

      String userPath = request.getServletPath();

    if (userPath.equals("/loadStaff")) {

        String positionID = request.getParameter("positionID");
        Position position = positionFacade.find(Integer.parseInt(positionID));
        Collection staffCollection = position.getStaffCollection();
        List<Staff> staffList = new ArrayList(staffCollection);

        String staffListJson = new Gson().toJson(staffList);
        response.setContentType("application/json");
        response.setCharacterEncoding("UTF-8");
        response.getWriter().write(staffListJson);

    }

}

Upon debuggig. The error comes at line:

String staffListJson = new Gson().toJson(staffList);

Output error:

> INFO:   WebModule[null] ServletContext.log(): The server side
> component of the HTTP Monitor has detected a
> java.lang.StackOverflowError. This happens when there is an infinite
> loop in the web module. Correct the cause of the infinite loop before
> running the web module again.
> 
> INFO:   The server side component of the HTTP Monitor has detected a
> java.lang.StackOverflowError. This happens when there is an infinite
> loop in the web module. Correct the cause of the infinite loop before
> running the web module again. WARNING:  
> StandardWrapperValve[AjaxServlet]: Servlet.service() for servlet
> AjaxServlet threw exception java.lang.StackOverflowError

> WARNING:   StandardWrapperValve[AjaxServlet]: Servlet.service() for
> servlet AjaxServlet threw exception java.lang.StackOverflowError  at
> sun.util.calendar.ZoneInfo.getOffsets(ZoneInfo.java:248)  at
> java.util.GregorianCalendar.computeFields(GregorianCalendar.java:2276)
>   at
> java.util.GregorianCalendar.computeFields(GregorianCalendar.java:2248)
>   at java.util.Calendar.setTimeInMillis(Calendar.java:1140)   at
> java.util.Calendar.setTime(Calendar.java:1106)    at
> java.text.SimpleDateFormat.format(SimpleDateFormat.java:955)  at
> java.text.SimpleDateFormat.format(SimpleDateFormat.java:948)  at
> java.text.DateFormat.format(DateFormat.java:336)  at
> com.google.gson.internal.bind.DateTypeAdapter.write(DateTypeAdapter.java:90)
>   at
> com.google.gson.internal.bind.DateTypeAdapter.write(DateTypeAdapter.java:41)
>   at
> com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
>   at com.google.gson.Gson$FutureTypeAdapter.write(Gson.java:892)  at
> com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
>   at
> com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
>   at com.google.gson.Gson$FutureTypeAdapter.write(Gson.java:892)  at
> com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
>   at
> com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
>   at
> com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
>   at com.google.gson.Gson$FutureTypeAdapter.write(Gson.java:892)

I also noticed that this traces are just repeating output of stacktrace;

EDIT: Staff class

@Entity
public class Staff implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Integer id;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 45)
    @Column(name = "last_name")
    private String lastName;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 45)
    @Column(name = "first_name")
    private String firstName;
    @Size(max = 45)
    @Column(name = "middle_name")
    private String middleName;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 6)
    @Column(name = "gender")
    private String gender;
    @Basic(optional = false)
    @NotNull
    @Column(name = "date_of_birth")
    @Temporal(TemporalType.DATE)
    private Date dateOfBirth;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 45)
    @Column(name = "nationality")
    private String nationality;
    @Basic(optional = false)
    @NotNull
    @Column(name = "date_hired")
    @Temporal(TemporalType.TIMESTAMP)
    private Date dateHired;
    @Size(max = 20)
    @Column(name = "status")
    private String status;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "staff")
    private Collection<StaffApointments> staffApointmentsCollection;
    @OneToOne(cascade = CascadeType.ALL, mappedBy = "staff")
    private StaffContact staffContact;
    @JoinColumn(name = "account_id", referencedColumnName = "id")
    @ManyToOne(optional = false)
    private Account accountId;
    @JoinColumn(name = "position_id", referencedColumnName = "id")
    @ManyToOne(optional = false)
    private Position positionId;

    public Staff() {
    }

    public Staff(Integer id) {
        this.id = id;
    }

    public Staff(Integer id, String lastName, String firstName, String gender, Date dateOfBirth, String nationality, Date dateHired) {
        this.id = id;
        this.lastName = lastName;
        this.firstName = firstName;
        this.gender = gender;
        this.dateOfBirth = dateOfBirth;
        this.nationality = nationality;
        this.dateHired = dateHired;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getMiddleName() {
        return middleName;
    }

    public void setMiddleName(String middleName) {
        this.middleName = middleName;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public Date getDateOfBirth() {
        return dateOfBirth;
    }

    public void setDateOfBirth(Date dateOfBirth) {
        this.dateOfBirth = dateOfBirth;
    }

    public String getNationality() {
        return nationality;
    }

    public void setNationality(String nationality) {
        this.nationality = nationality;
    }

    public Date getDateHired() {
        return dateHired;
    }

    public void setDateHired(Date dateHired) {
        this.dateHired = dateHired;
    }

    public String getStatus() {
        return status;
    }

    public void setStatus(String status) {
        this.status = status;
    }

    @XmlTransient
    public Collection<StaffApointments> getStaffApointmentsCollection() {
        return staffApointmentsCollection;
    }

    public void setStaffApointmentsCollection(Collection<StaffApointments> staffApointmentsCollection) {
        this.staffApointmentsCollection = staffApointmentsCollection;
    }

    public StaffContact getStaffContact() {
        return staffContact;
    }

    public void setStaffContact(StaffContact staffContact) {
        this.staffContact = staffContact;
    }

    public Account getAccountId() {
        return accountId;
    }

    public void setAccountId(Account accountId) {
        this.accountId = accountId;
    }

    public Position getPositionId() {
        return positionId;
    }

    public void setPositionId(Position positionId) {
        this.positionId = positionId;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Staff)) {
            return false;
        }
        Staff other = (Staff) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "entity.Staff[ id=" + id + " ]";
    }

}

Here is the Position class as well:

@Entity
public class Position implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Integer id;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 45)
    @Column(name = "name")
    private String name;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "positionId")
    private Collection<Staff> staffCollection;

    public Position() {
    }

    public Position(Integer id) {
        this.id = id;
    }

    public Position(Integer id, String name) {
        this.id = id;
        this.name = name;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @XmlTransient
    public Collection<Staff> getStaffCollection() {
        return staffCollection;
    }

    public void setStaffCollection(Collection<Staff> staffCollection) {
        this.staffCollection = staffCollection;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Position)) {
            return false;
        }
        Position other = (Position) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "entity.Position[ id=" + id + " ]";
    }

}

EDIT 2:

I added @expose to the attributes in my staff class except for staffCollection. But I still have 1 problem. Everytime the 1st value in the dropdown is selected (value=1) it still gives the infinite loop error. Can anyone help me?

EDIT 3:

Fixed it! I added final GsonBuilder builder = new GsonBuilder(); builder.excludeFieldsWithoutExposeAnnotation(); as a whole. It now works

Answer 1

The problem is that every Staff object contains a Position object that contains a Collection of Staff objects, which each contain a Collection of Staff objects again, etc. GSON will continue walking this tree forever because it's never gonna stop.

To solve it you can take a look at the answers to this question.