[android-developers] Re: Singletons in Android .... Final solution?

It is not surprising at all to me. This is expected behavior:

The process is killed and then there is nothing left. There is nothing to 'come back to'. 

Then Android restarts the process and instantiates a new Application object in that new process. Since the 'now' variable in your sample-code is based on the system-time, it's no wonder that it will have a different value the next time it is instantiated. 

If you  want consistent behavior in singletons where the life-cycle of objects is not under your control (e.g. activities and such), then make the accessors of these singleton self-contained. E.g:

public class MyApp extends Application {

  private static SomeObject someObject = null;

  

  public static getSomeObject(Context context)  {

    if (someObject == null) {

      someObject = ... ... ... // create/instantiate this object here....

    }

    return someObject;

  }

  ...

  ...

}

Then calling the class method MyApp.getSomeObject(ctxt) will always return a proper value, because this method is self contained; it is solely responsible for creating the singleton (and returning it to the caller). 

Of course, if someObject depends on a variable external value (such as the system's time), then the contents/value of someObject is depending on the first-time access of this method for this particular process.

}

On Thursday, May 3, 2012 7:24:23 PM UTC-4, blake wrote:

On May 3, 4:15 pm, Streets Of Boston <flyingdutc...@gmail.com> wrote:
> You are right that the enum has been initialized a second time.
> But the second time is a different process (26202 != 26607).

I know!  I know!  Different process, different class loaders,
(...oddly, might be the same thread)

> In other words, the first process (26202) did not see this enum being
> initialized twice. The process was killed and the new process had its enum
> initialized instead.

The point of this whole thread, I believe, is that it is not safe to
put stuff in to static fields of the application.  They will not be
there when you get back.  Among the things that it is not safe to do,
is to "initialize" the static field with some value and then assume
that the field will continue to have that value.  From the point of
view of your application/task the field has become uninitialized.

I completely agree that this might not be a bug.  It is outright
documented to happen in an Activity.  It is a little surprising, in
the activity, though...

Blake Meike
Marakana

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