Using LruCache: Is cache attatched to a LruCache instance?

Question

I might just be confused about how LruCache is supposed to work, but are does it not allow accessing objects from one instance that were saved on another instance? Surely this is not the case otherwise it kind of defeats the purpose of having cache.

Example:

class CacheInterface {

    private val lruCache: LruCache<String, Bitmap>

    init {
        val maxMemory = (Runtime.getRuntime().maxMemory() / 1024).toInt()
        // Use 1/8th of the available memory for this memory cache.
        val cacheSize = maxMemory / 8
        lruCache = object : LruCache<String, Bitmap>(cacheSize) {
            override fun sizeOf(key: String, value: Bitmap): Int {
                return value.byteCount / 1024
            }
        }
    }

    fun getBitmap(key: String): Bitmap? {
        return lruCache.get(key)
    }

    fun storeBitmap(key: String, bitmap: Bitmap) {
        lruCache.put(key, bitmap)
        Utils.log(lruCache.get(key))
    }

}

val bitmap = getBitmal()
val instance1 = CacheInterface()
instance1.storeBitmap("key1", bitmap)
log(instance1.getBitmap("key1")) //android.graphics.Bitmap@6854e91
log(CacheInterface().getBitmap("key1")) //null

As far as I understand, cache is stored until it's deleted by the user (manually or uninstalling the app), or cleared by the system when it exceeds the allowed space. What am I missing?

score 2 · Answer 1 · answered Apr 08 '19 at 02:23

2

An LruCache object just stores references to objects in memory. As soon as you lose the reference to the LruCache, the LruCache object and all of the objects within that cache are garbage collected. There's nothing stored to disk.

answered Apr 08 '19 at 02:23

ianhanniballake

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So how can you access your app's cache after the user kills your app, the process gets killed and any static references get garbage collected? – John Sardinha Apr 08 '19 at 09:28
>There's nothing stored to disk. The cache does not get cleared when I kill an app on my phone, or even reboot the system though – John Sardinha Apr 08 '19 at 09:43
@JohnSardinha: You may be having a problem with your tests, then. [Here is the source code](https://github.com/aosp-mirror/platform_frameworks_support/blob/androidx-collection-release/collection/src/main/java/androidx/collection/LruCache.java). You will find that there is no disk I/O in it. – CommonsWare Apr 08 '19 at 10:50
For cache meant to persist between processes, Is `LruCache` not the right interface to use? Is `LruCache` just to cache data meant to be accessed in the same "session"? – John Sardinha Apr 08 '19 at 10:54
1

@JohnSardinha - that's correct. It is just an in memory cache. – ianhanniballake Apr 09 '19 at 00:13

score 0 · Answer 2 · answered Apr 08 '19 at 23:00

Yes it is. I'll just share here what I was confused about in case anyone also is.

Initially because of this guide (Caching Bitmaps) that reccomends using LruCache, I was left under the impression that LruCache was an interface to access app's cache, but like @CommonsWare mentioned it has no I/O in it - it's just a utility class to hold memory using the LRU policy. To access your app's cache you need to use Context.getCacheDir(), good explanation here. In my case I ended up using a singleton of LruCache, since I already have a service running most of the time the app will not be killed every time it's closed.

score -1 · Answer 3 · answered Apr 08 '19 at 05:35

log(CacheInterface().getBitmap("key1")) //null

equals

val instance2 = CacheInterface()
log(instance2 .getBitmap("key1"))

instance1 != instance2

change to Singleton

object CacheInterface{
...
}

use

CacheInterface.storeBitmap("key1",bitmap)
CacheInterface.getBitmap("key1")

Using LruCache: Is cache attatched to a LruCache instance?

3 Answers3