问题
Several Activitys in my app display images in a ListView where each row of the ListView contains an ImageView.
An example of this would be a search screen where the user searches, gets results, and a picture of each result is shown.
I'm trying to weigh the cost/benefits of implementing a global LruCache vs having each Activity contain its own local LruCache.
Here are my two main problems. Both revolve around the fact that my app is quite large, meaning there are quite a few screens which show these images. Also, my app has the popular side menu way of navigating. Because of this, I could open the menu, tap Activity B, open the menu, tap Activity A, open the menu... etc. and create an Activity stack of ABABABABABABABAB indefinitely.
Global
Won't Activitys with ImageViews using Bitmaps from a global LruCache contain references to these Bitmaps? Suppose the user navigates away from this Activity by clicking some Button. That Activity is now on the Activity stack and still holds references to those Bitmaps. If the LruCache pops a Bitmap off, can that Bitmap really be reclaimed when an ImageView in some Activity on the stack holds a reference to it?
I had previously created my own custom cache. If I called recycle() on a Bitmap and then the user hit the back button to go back to some Activity on the stack that contained an ImageView set to that Bitmap, the app would crash. This is why I believe ImageViews on Activitys on the stack still hold references to Bitmaps.
Local
As I mentioned earlier. My app is quite large, and side menu style of navigation allows the user to create rather large Activity stacks. This would create a lot of LruCaches. And, since you have to declare the size of the LruCache when you initialize it, there wouldn't seem to be any good way of picking a size.
Thoughts? Suggestions?
At this point I think I have to do global, but I don't know how to solve the Activity stack reference problem. I can't imagine this isn't a problem many apps haven't run into. I don't know why I'm not finding information about it.
回答1:
I'm trying to weigh the cost/benefits of implementing a global LruCache vs having each Activity contain its own local LruCache.
Global LruCache is the way to move forward, since the same set of bitmaps might be referred in different activity instances. The LruCache can be defined part of Application. If the activity stack can host multiple instances of the same activity (like ABABABAB..), then creating a LruCache locally in that activity will be a bad idea. Very soon Out Of Memory situtation will be reached, as LruCache in each activity instance reserves the defined amount of memory in Dalvik VM. Assume, application memory is 32Mb and you decide LruCache size as 4Mb i.e. 1/8th. Now when we create nearly 7 instances of Activity A, then memory consumption will go to 7*4=28Mb, that itself might trigger OOM.
Won't Activitys with ImageViews using Bitmaps from a global LruCache contain references to these Bitmaps?
Yes ImageView will also have a strong reference to the bitmap. If the reference is maintained in LruCache, then the reference count will be 2 at that moment.
If the LruCache pops a Bitmap off, can that Bitmap really be reclaimed when an ImageView in some Activity on the stack holds a reference to it?
No the bitmap memory can't be reclaimed, as still some ImageView is have a strong reference to it.
At this point I think I have to do global, but I don't know how to solve the Activity stack reference problem.
LruCache main role is holding strong reference to the bitmap which are more frequently used. So that if there is no strong reference held by any ImageView, the bitmap is prevented from being garbage collected.
Also remember, for Android 2.3.3 and lower versions, you need to implement reference counting mechanism, in order to recycle the bitmaps.
来源：https://stackoverflow.com/questions/25628734/implementing-a-global-lrucache-vs-many-local-lrucaches