How to cause soft references to be cleared in Java?

If you really wanted to, you can call clear() on your SoftReference to clear it.

That said, if the JVM is throwing an OutOfMemoryError and your SoftReference has not been cleared yet, then this means that you must have a hard reference to the object somewhere else. To do otherwise would invalidate the contract of SoftReference. Otherwise, you are never guaranteed that the SoftReference is cleared: as long as there is still memory available, the JVM does not need to clear any SoftReferences. On the other hand, it is allowed to clear them next time it does a GC cycle, even if it doesn't need to.

Also, you can consider looking into WeakReferences since the VM tends to be more aggressive in clear them. Technically, the VM isn't ever required to clear a WeakReference, but it is supposed to clean them up next time it does a GC cycle if the object would otherwise be considered dead. If your are trying to test what happens when your cache is cleared, using WeakReferences should help your entries go away faster.

Also, remember that both of these are dependent on the JVM doing a GC cycle. Unfortunately, there is no way to guarantee that one of those ever happens. Even if you call System.gc(), the garbage collector may decide that it is doing just peachy and choose to do nothing.

If you have a cache which is a Map of SoftReferences and you want them cleared you can just clear() the map and they will all be cleaned up (including their references)