yngling yngling - 1 month ago 5
Java Question

Why are array contents of a core datastore type not stored in app engine?

I have the following issue:

A java object contains two arrays of core datastore types (com.google.appengine.api.datastore.Text and java.util.Date), plus an int (for storing the current populated position in the arrays) and some other fields.

I believe the documentation states that arrays of core datatypes should be ok (cf. http://code.google.com/appengine/docs/java/datastore/jdo/dataclasses.html, under the "Class and Field Annotations").

The object is updated using a method named "updateAnswer". When this method is called, the object is indeed updated (the int is incremented and stored correctly), but the arrays never store anything but nulls.

I would greatly appreciate it if someone could point out where my mistake lies.

Here is the object (and its parent, for completeness):

public class TextualAnswer extends Answer {

private Text textAnswer;

private Date date;

private int pos;

private Text texts[];

private Date dates[];

public TextualAnswer(Key question, Key user, Date date) {
super(question, user, 0);
this.textAnswer = null;
this.date = date;
pos = 0;
texts = new Text[20];
dates = new Date[20];

public String getTextAnswer() {
return (textAnswer != null ? textAnswer.getValue() : null);

public Date getDate() {
return date;

public void updateAnswer(String textAnswer, Date date) {
if (texts.length == pos) { // expand?
Text ttemp[] = texts;
texts = new Text[pos * 2];
System.arraycopy(ttemp, 0, texts, 0, pos);

Date dtemp[] = dates;
dates = new Date[pos * 2];
System.arraycopy(dtemp, 0, dates, 0, pos);
texts[pos] = this.textAnswer;
dates[pos] = this.date;

this.textAnswer = (textAnswer != null ? new Text(textAnswer) : null);
this.date = date;

The parent:

@Inheritance(strategy = InheritanceStrategy.SUBCLASS_TABLE)
public abstract class Answer {
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;

private Key question;

private Key user;

private double score;

private boolean last;

private Text comment;

public Answer(Key question, Key user, double score) {
this.question = question;
this.user = user;
this.score = score;
last = false;
comment = null;

public Key getKey() {
return key;

public Key getQuestion() {
return question;

public Key getUser() {
return user;

public double getScore() {
return score;

public boolean isLast() {
return last;

public String getComment() {
return comment != null ? comment.getValue() : null;

public void setScore(double score) {
this.score = score;

public void setLast(boolean last) {
this.last = last;

public void setComment(String comment) {
this.comment = comment != null ? new Text(comment) : null;

A closing note. I realize I could use Lists etc instead, and if I do not figure this out that is indeed my backup plan. But, I would like to realize why this isn't working, so I'd love any suggestions that I switch to objects instead of arrays to be accompanied with an explanation as to why the arrays are not working ;) Thank you.

Ex animo, - Alexander Yngling


So, after some sleep it occurred to me... it's an array index. I'm using JDO. I'm an idiot ;)

I thought about deleting this question, but just in case someone else searches for this, here's the problem:


JDO cannot know that an array index has been updated, so you have to either set the field again, or use the following to tell JDO to update the database:

JDOHelper.makeDirty(obj, "fieldName");