android / java - how to be notified when there is no reference to an instance

Question

this question is for either android or java . suppose i have an instance of a class (even a thread) .

i wish that in order to make it fool proof against forgetting to dispose/close the instance (and avoid any possible memory leaks) , if there are no more references to this instance , it will automatically call a specific method , disposing/closing itself (in case of a thread , it will probably interrupt itself) .

is it possible to enforce such a thing? if needed , i don't mind that such a thing will occur only during GC .

Answer 1

if there are no more references to this instance , it will automatically call a specific method , disposing/closing itself (in case of a thread , it will probably interrupt itself)

finalize() does what you describe here. You very rarely see it used though, and there are some pitfalls when using it. As you cant control garbage collection, you cant be sure when finalize will be run, if ever! From the api:

the usual purpose of finalize, however, is to perform cleanup actions before the object is irrevocably discarded. For example, the finalize method for an object that represents an input/output connection might perform explicit I/O transactions to break the connection before the object is permanently discarded.

You cant enforce garbage collection. You can only suggest the JVM to do so using System.gc(), but it is not guaranteed that it will be done.

Answer 2

There is no way to access the references held by the VM. As Zavior suggested, the only way to know for sure that an object, or an "island" of objects is inaccessible, is to use the finalize method.

Please note that you will only get notified during GC runs. So it does not really help closing/disposing resources that are still referenced but should be closed. If you want to do that as well and do not want to use such constructs as try/catch/finally, you should write a manager class for the resources.

With any of the possibillities, including a manager class, you will not get a "bulletproof" way to consolidate your resources. Being careful is the best solution IMHO.

EDIT:

I have found this thread that may be useful.

Answer 3

I know this is very late but I hope it might help someone someday.

You can receive such events this by using the using the library that i am developing called gcRadar. It provides events when an object is orphaned and after the actual garbage collection of the object.

Any suggestions for improvements in the library are welcome.