I read the code sample/documentation about caching in the wiki page. I see that callback RemovalListener can be used to do tear down etc of evicted cached objects. My question is does the library make sure that the object is not being used by any other thread before calling the provided RemovalListener. Lets consider the code example from the docs:
CacheLoader<Key, DatabaseConnection> loader =
new CacheLoader<Key, DatabaseConnection> () {
public DatabaseConnection load(Key key) throws Exception {
return openConnection(key);
}
};
RemovalListener<Key, DatabaseConnection> removalListener =
new RemovalListener<Key, DatabaseConnection>() {
public void onRemoval(RemovalNotification<Key, DatabaseConnection> removal) {
DatabaseConnection conn = removal.getValue();
conn.close(); // tear down properly
}
};
return CacheBuilder.newBuilder()
.expireAfterWrite(2, TimeUnit.MINUTES)
.removalListener(removalListener)
.build(loader);
Here the cache is configured to evict elements 2 minutes after creation (I understand that it may not be exact two minutes because eviction would be piggybacked along with user read/write calls etc.) But whatever time be it, will the library check that there is no active reference present to the object being passed to the RemovalListener? Because I may have another thread who fetched the object from the cache long back but may be still using it. In that case I cannot call close() on it from RemovalListener.
Also the documentation of RemovalNotification says that: A notification of the removal of a single entry. The key and/or value may be null if they were already garbage collected.
So according to it conn could be null in the above example. How do we tear down the conn object properly in such case? Also the above code example in such case will throw NullPointerException.
The use case I am trying to address is:
- The cache element need to expire after two minutes of creation.
- The evicted object needs to be
closed, but only afte making sure no one is using them.