As the author of the IBM Developerworks article I referenced mentions, that is not good enough. The Garbage Collector runs on a different thread, you have no control over when it will be called. It is quite possible that just after you test for null, the GC is called, and it is no longer valid. Not likely, but possible.
If instead you create a strong reference and use that, discarding it when done, then you have shut the door on a really nasty transient bug. Or perhaps the version of 'ref()' you found did this for you? What package did you find it it? On Jul 23, 5:21 am, DanH <[email protected]> wrote: > Yes, you must "guard" any use of the WeakReference by taking the ref() > of it, testing that for null, and then proceeding to use the result of > the ref() if not null. The size of the "guarded" sections is up to > the programmer -- if too large then the object will never get deleted, > if too small then the code gets chopped up. > > On Jul 23, 1:37 am, Indicator Veritatis <[email protected]> wrote: > > > You left out something very important: the code hidden under "// > > reload the image" must not assume that it is not itself interrupted by > > yet another call to the garbage collector. That is, instead of simply > > continuing to use the soft/weak reference, it should make a strong > > reference to the same object, allowing this latter reference to either > > go out of scope or be set to null when it is done. > > > But if we are making this strong reference, what was the point of > > using the weak/soft reference in the first place? Ah, that is the > > tricky thing about using them. Depending on when you can make and > > release the strong reference, they might not buy you much; they might > > not buy you anything at all. That is why they are not recommended for > > much outside of caches and normalized mappings. > > > On Jul 22, 11:07 am, Joseph Earl <[email protected]> wrote: > > > > Suppose you had a long list of images. As the user scrolled down you > > > load the images from the net, and then display them. > > > To avoid having to reload the images again if the user scrolls back > > > up, you put the images in a cache (probably something like a > > > Map<String, Drawable>) > > > > However because it is a long list you don't want to run into an out of > > > memory situation if the user scrolls very far down and lots of images > > > are put in the cache. > > > So instead of storing the Drawables directly in the map, you create a > > > Map<String, WeakReference<Type>> (although I would use SoftReference > > > for the purpose described here). > > > This means that if Android is going to encounter an out of memory > > > situation it will clear all of the Soft/Weak references (and thus > > > hopefully avoid running out of memory). You will have to load the > > > images again since your cache has been cleared, but this is far better > > > than your application running out of memory and crashing. > > > > So you do something like: > > > > // caching an image > > > Map<String, SoftReference> cache = new HashMap<String, > > > SoftReference<Drawable>>(); > > > cache.put("http://mysite.com/images/1.jpg";, new > > > SoftReference<Drawable>.put(myDrawable)); > > > > // retrieve an image > > > if (cache.containsKey(url)) { > > > // looks like we have this image cached > > > Drawable drawable = cache.get(url).get(); > > > if (drawable == null) { > > > // the softreference has been cleared by the GC, reload the > > > image > > > } else { > > > // softreference is still valid, got our image > > > } > > > > } > > > > Essentially a weak reference is a weaker reference than a soft > > > reference - the GC should free weak references to regain memory before > > > soft references. > > > > I think that's (mostly) correct, hope it helps. > > > > On Jul 22, 6:48 pm, GodsMoon <[email protected]> wrote: > > > > > Google just posted a new blog post > > > > onhttp://android-developers.blogspot.com/2010/07/multithreading-for-per.... > > > > I understand the AsyncTask and I'm even using one in a list with > > > > images already. > > > > > But I don't understand what a WeakReference is. I gather is is a > > > > garbage collector directive, but I thought I didn't need to manage > > > > garbage collection on Android. > > > > >http://developer.android.com/reference/java/lang/ref/WeakReference.html > > > > isn't as helpful as I was hoping it would be. > > -- You received this message because you are subscribed to the Google Groups "Android Developers" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/android-developers?hl=en
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