Question

My App engine/ GWT project is spitting out a nasty little pile of stack trace whenever it attempts to return from my login method. I am using GAE version 1.5.0 and GWT version 2.3.0 .

It’s a facebook app, so what I’ve got is this:

• The player navigates to the app page.
• They click a button, and are redirected to the OAuth authentication page
• They are then redirected back to the app, with the authentication token in the query string
• I break the query string apart to get the UID, and then use that as the primary key for my Player entity (RPC to app engine backend)
• I retrieve the Player entity instance from the datastore, and turn it into a serializable type to return to the client
• Epic fail.

When I spit out the exception in a JSAlert, I get a big nasty pile of stack trace (I already was thoughtful enough to compile using “pretty” instead of “obfuscated”).

My login function looks like this:

@Override
public ClientPlayer login(String uid) {
    PersistenceManager pm=PMF.get().getPersistenceManager();
    log.warning(Player.class.getName());
    log.warning(uid);
    Key k=KeyFactory.createKey(Player.class.getSimpleName(), uid);
    Player p;
    List<List<Integer>> stats;
    try{
        p=pm.getObjectById(Player.class, k);
    } catch (JDOObjectNotFoundException e){
        p=new Player(uid);
        p.setKey(k);
        pm.makePersistent(p);
    } finally {
        pm.close();
    }
    stats=p.getStats();
    return new ClientPlayer(p.getUID(),p.getPerm(), p.getDecks(),stats.get(0), stats.get(1), stats.get(2));
}

Unfortunately, due to NDA, I can’t link to the app, but here’s the output:

    Failure to log in because of: 
    com.google.gwt.core.client.JavaScriptException: (TypeError): Cannot call method 'nullMethod' of null
     arguments: nullMethod,
     type: non_object_property_call
     stack: TypeError: Cannot call method 'nullMethod' of null
        at Object.ClientPlayer_1 (http://*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:993:89)
        at Object.ClientPlayer_0 (http://*com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:984:18)
        at Array.instantiate_1 [as 0] (http://*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:1031:10)
        at $instantiate_0 (http://*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:10660:34)
        at $instantiate (http://*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:1948:10)
        at $readObject (http://*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:10148:95)
        at Object.read_8 [as read] (http://*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:10608:10)
        at $onResponseReceived (http://*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:10352:247)
        at $fireOnResponseReceived (http://*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:5002:5)
        at Object.onReadyStateChange (http:/*.com/com.MES.Tap2/A37A2E2E9A65DB1BAAE2BFA42572F7F8.cache.html:5222:5)

Answer 1

The issue was in the use of the IsSerializable interface, or rather my poor understanding of it.

When you create an IsSerialiazable object, it requires a no-argument constructor. I was passing null values from that constructor to the main constructor, so when methods were called on them, null pointer exceptions occurred. This was dumb of me, but hey, it was a learning experience.

In my particular case, it went a little like this…

public class ClientObject implements IsSerializable {
    private Object field1;
    private Object field2;
    private String field3;

    public ClientObject(){
        this(null, null);
    }
    public ClientObject(Object arg1, Object arg2){
        field1=arg1;
        field2=arg2;
        field3=arg1.toString()+arg2.toString(); 
        //Error on above line, though not obviously mentioned in the message
    }
}

What should have been done was…

public ClientObject(){
    this(new Object(), new Object());
}

Hope this helps someone.