Weak Reference and Soft Reference

Question

Weak references allows GC to collect the references in next GC Cycle whereas Soft Reference will keep the reference until memory is full and before throwing out of memory error,it will remove soft references. Where we will be using these references? Which reference will be best for implementing caching? For eg:If I use soft reference for caching,then it will be cleared when memory is full. But lets suppose,I have fetched some database details and put that in memory and cached that detail in soft reference and now if i removed some key value from memory,it will still be there in cache.Do we need to use weak reference in this case?How the decision should be made.

Answer 1

Weak references allows GC to collect the references in next GC Cycle whereas Soft Reference will keep the reference until memory is full and before throwing out of memory error,it will remove soft references.

You are reading more into the docs than they actually say. All objects that are softly reachable (which precludes them being strongly reachable) will be cleaned up and released before the VM throws an OutOfMemoryError, but the VM is in no way required to preserve them past the point when they are initially determined to be softly reachable. The docs do not forbid that they be reclaimed in the same GC cycle in which they are found to be softly reachable.

Generally speaking, however, you can suppose that the GC will prefer to process phantom reachable and weakly reachable objects first.

Where we will be using these references? Which reference will be best for implementing caching?

The docs say this:

Soft references are for implementing memory-sensitive caches, weak references are for implementing canonicalizing mappings that do not prevent their keys (or values) from being reclaimed, and phantom references are for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.

You go on to ask:

For eg:If I use soft reference for caching,then it will be cleared when memory is full. But lets suppose,I have fetched some database details and put that in memory and cached that detail in soft reference and now if i removed some key value from memory,it will still be there in cache.Do we need to use weak reference in this case?How the decision should be made.

If you want to build a cache that can discard entries whose keys cease to be strongly reachable (which may mean that those entries can no longer be retrieved), that is squarely in the center of the intended purpose for weak references. The cache internally holds only a weak reference to the key of each entry, and it registers those references with a ReferenceQueue that lets it know when they should be discarded. This is precisely how WeakHashMap works.

If you want to build a cache that can respond to high memory demand by discarding entries, then that is the intended purpose of soft references (to the values); such a cache would work similarly to WeakHashMap, but with use of soft references to the values instead of weak references to the keys. The two could be combined, of course.

Note, by the way, that Reference objects become relevant to GC only when their referents cease to be strongly reachable. In particular, having a SoftReference to an object does not in itself guarantee that that object will ever be reclaimed, no matter what the demand is on memory. No object that is strongly reachable is ever eligible for finalization or reclamation.

Answer 2

Soft References are used for caching in most cases. You want to keep data in RAM as long as it possible, but it is better to purge cache than die with OOM.

Weak references can be used (for example) to keep extra info about your class. You have class User and you want to store some additional info, which should be deleted at the moment when user is deleted (you do not want to do it manually since it is bolierplate code). So, you use WeakHashMap using User as key, and when there is no reference to user, it is deleted from this map as well.

By the way: in languages with references counting weak references are used to prevent reference cycles, but java GC removes "islands of isolation", so this usage of weak reference is not for java.

Answer 3

Which reference will be best for implementing caching?

for caching use SoftReference class, the whole point of caching things is to keep it prepared for quick use - if memory is available. So when there is little memory ten its ok to flush your cache.

WeakReference are perfect for avoiding reference leaks, it happens when you have some static object or thread which keeps reference to object whose lifetime is shorter that this object/thread. I use a lot of WeakReference - s in android development, especially with AsyncTask's whose lifetime is often longer than of Activity-s lifetime which created them.

... Do we need to use weak reference in this case?

once you remove such entry from your cache collection it will be garbage collected, so no need for WeakReference.

I have never used SoftReferences - but thats because I mostly code under android platform, and acording to its docs http://developer.android.com/reference/java/lang/ref/SoftReference.html, they are useless for caching - at least under this platform.

Answer 4

If you are implementing your own cache, use a Soft Reference.

I once maintained a legacy system that used a cache of Weak References to store large objects that were very expensive to create. Almost every time a thread tried to fetch an object from that cache, it had already been GC'ed so the objects had to be expensively recreated a lot! It was practically like the cache wasn't there.

But lets suppose,I have fetched some database details and put that in memory and cached that detail in soft reference and now if i removed some key value from memory,it will still be there in cache. Do we need to use weak reference in this case?How the decision should be made.

I'm not sure I understand your question. When the original "hard" reference to those details is GC'ed it can still be in the cache of soft references. If you remove the item from the cache then there are no more references to the details at all so it will be GC'ed next time no matter what kind of reference used to point to it.

There are very few times when a WeakReference is useful. See Weak references - how useful are they? for some examples