Re: [android-developers] AIDL callbacks leak memory?

The underlying binder mechanisms keeps track of references across objects.  Native IBinder objects are reference counted; the framework holds a reference on them as long as remote processes hold references.  The JNI wrapper holds a reference on the Java object as long as the native binder object is alive.

From a high-level perspective, if you hand a reference to a java IBinder object across processes, a reference will be held on it (preventing it from being GCed) until the remote proxy on the other side is JCed.

On Thu, Jun 14, 2012 at 9:13 PM, G. Blake Meike <blake.meike@gmail.com> wrote:

I've been digging into Android IPC, recently and there's something puzzling me.

Suppose I define a Parcelable that, as one of its arguments, takes a callback listener which is also Parcelable.

Like this:

oneway interface MyService {
    void start(in MyRequest req, in MyResponseListener listener);
}

oneway interface MyResponseListener {
    void onResponse(in MyResponse response);
}

In my code, I'm going to do something like this:

listener = new MyResponseListener() { /*... */ }

boundService.start(req, listener);

I think this is pretty much how Cursors work. I see one of two possibilites here:

1) the call to start() doesn't create an additional reference to the listener object.  That means that, to prevent its being gc'ed, my code must keep a reference to the listener, somewhere, until it ether gets the callback or doesn't want the it anymore. (btw, what happens if the listener is gone when the callback happens?)

2) the call to start() creates a new, GC-visible reference to listener.  I can pass an anonymous class without fear that it will be GCed.  On the other hand, it and everything to which it holds references are leaked until that reference gets nulled.  When does that happen?

I've followed this down as far as a Parcel.flatten_binder, but I kind of lose it after that.

What is the right way to keep from leaking, for instance, implicit pointers to my Activity, when I use asynchronous callbacks with a bound Service?

Thanks
  -blake

G. Blake Meike
Marakana

The second edition of Programming Android is now on-line:
http://shop.oreilly.com/product/0636920023005.do

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Dianne Hackborn
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Note: please don't send private questions to me, as I don't have time to provide private support, and so won't reply to such e-mails.  All such questions should be posted on public forums, where I and others can see and answer them.

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