Question

So I have somewhat limited experience with serialization, Wicket, and multi thread projects so bear with me.

Essentially my web application class is instantiating a POJ (parentObject) which creates a starts a new timer and instantiates several POJs (childObjects) that also have timers in them. These childObjects are stored in a list in the parentObject class. Pages in my wicket application need to access parentObject, so I made it accessible as so:

public Object getParentObject
{
   return this.parentObject;
}

And it is retrieved in each page like so:

((MyApplication)Application.get()).getParentObject()

The problem currently is that the timertask for both the parentObject and childObjects are no longer being called every minute as they should be. My logs pick up the first start of the parentObject, but the logging message is never outputted again signalling that the run() method of parent Object’s timertask is not being executed every minute. The same holds true for the child Objects. It seems like the timers are only being executed once. Below is some pseudocode for what I have

public class childObject implements Serializable
{
    private transient NamedParameterJdbcTemplate njt;
    private transient Timer timer;

    public childObject(DataSource ds)
    {
        this.njt = new NamedParamterJdbcTemplate(ds);
    }

    public void start()
    {
        timer = new Timer();

        timer.schedule(new TimerTask(){

            public void run()
            {
                //do some stuff that is never happening
            }

        }, 0, 60000);
    }
}

public class ParentObject implements Serializable
{
    private DataSource ds;
    private List<ChildObject> childObjects;
    private transient Timer;

    public ParentObject(DataSource ds)
    {
        this.ds = ds;
        //add some stuff to childObjects

        timer = new Timer();

        timer.schedule(new TimerTask(){

            public void run()
            {
                for(some condition)
                {
                    //Do some stuff

                    if(/*condition is met*/)
                    {
                             //starts the child's timer to do stuff
                        childObjects.get(i).start();
                    }
                }
            }

        }, 0, 60000);
    }
}

public MyApplication extends WebApplication
{
    private ParentObject object;
    private DataSource ds;

    public void init()
    {
        super.init();

        ApplicationContext context = new ClassPathXmlApplicationContext("/applicationContext.xml");
        ds = (DataSource) context.getBean("dataSource");

        parentObject = new ParentObject(ds);
    }
}

Do I even need to make these objects Serializable? The objects themselves are never being attached to wicket components, although String, integer, Date sorts of variables that are members of their classes are.

Answer 1

Wicket is fundamentally single threaded (as are most good GUI frameworks due to the difficulty of getting multithreading right) and you should avoid instantiating tasks. (Marking the Timer as transient will mean it gets lost upon deserialization by the way, which might be the cause of your problems)

You should rearchitect your application to have a service layer which is accessed by Wicket components on demand, possibly using LoadableDetachableModels. The service layer can have tasks and so on, as it will be managed by Spring rather than Wicket.