What is a nonmemory resource?

Question

I am reading "Effective Java".

In the discussion about finalize, he says

C++ destructors are also used to reclaim other nonmemory resources. In Java, the try finally block is generally used for this purpose.

What are nonmemory resources?

Is a database connection a nonmemory resource? Doesn't the object for holding the database connection occupy some memory?

Answer 1

Database connections, network connections, file handles, mutexes, etc. Something which needs to be released (not just garbage-collected) when you're finished with it.

Yes, these objects typically occupy some memory, but the critical point is that they also have (possibly exclusive) access to some resource in addition to memory.

Answer 2

Is a database connection a non memory resource?

Yes, that's one of the most common examples. Others are file handles, native GUI objects (e.g. Swing or AWT windows) and sockets.

Doesn't the Object for holding the database connection occupy some memory?

Yes, but the point is that the non-memory part of the resource needs to be released as well and is typically much scarcer than the comparatively small amount of memory the object uses. Typically, such objects have a finalize() method that releases the non-memory resource, but the problem is that this finalizers will only run when the objects are garbage collected.

Since the objects are small, there may be plenty of available heap memory so that the garbage collector runs rarely. And in between runs of the garbage collector, the non-memory resources are not released and you may run out of them.

This may even cause problems with only a single object: for example, if you want to move a file between filesystems by opening it, opening the target file, copying the data and then deleting the original file, the delete will fail if the file is still opened - and it is almost certain to be if you only set the reference to the input stream to null and don't call close() explicitly, because it's very unlikely that the garbage collector would have run at exactly the right point between the object becoming eligible for garbage collection and the call to delete()