Why are some some resources in Java not garbage collected and must be closed or autoclosed?

Question

If you are lucky some of these classes implement AutoClosable but sometimes you just have to be careful and inspect the existing methods to notice that there is a close, destroy or shutdown method (or what ever the author decided to name it).

This is a major source of resource leaks in Java.

I was discussing this with a colleague and wondered too: why can this not be automated in some way ?

In theory you can use finalize for such cases, but it is not recommended. So why is there no way to just use some of those closable resources and let the GC autoclose them when the instance is no longer reachable without having to remember to explicitely write some close handling code (like try ...) ?

Is this because the system may have been resource starved (File descriptors, ...) before the GC kicks in ?

NOTE: I use autoclose when possible and check my code for memory leaks using FindBugs (+ FB contrib), but still, I wonder ...

Also of interest (as discussed in the answers): deprecation of finalize.

Answer 1

The Garbage Collector's only job is to collect memory that is no longer used. Adding closing of resources will have negative effects on the performance of the Garbage Collector and is currently done by the Finalizer thread that is called by the Garbage Collector anyway in order to allow implementations to clear resources before being collected. It's worth noting that this mechanism is declared deprecated because it wasn't the best solution for this kind of thing from the start, but for the time being it's possible to implement your classes to clean themselves up before they are going to be collected.

The Finalizer (or the new mechanim in Java 9) might be extended to check if the class to be collected implements AutoClosable (an interface added with Java 1.7, so it's not that old anyway) and call it in addition to finalize. This would have a similar effect as you proposed without the need to change the behavior and the role of the Garbage Collector. Maybe that's already happening (haven't tested it myself, yet).