Java finalize : How can I free non-GC resource even if there's mistake

Question

As I know, there's no guarantee that Object.finalize() is called always. but If there an important non-GC resource, and user didn't call close() accidentally, How can I free the resource?

PS. Does Object.Finalize() in .NET have the same problem? If so, how can I solve this problem in that case?

Answer 1

You should use try-finally or better try-with-resources

try(Resource res = /*aquire resource*/){
//do stuff
}catch(Exception e //or watever){
//handle exceptions
}

and how to do it before Java7 :

Resource res = null;
try{
res = //aquire resource
    //do stuff 
}catch(Exception e /*or whatever*/){
 //handle exceptions
}finally{
    if(null != res){
        res.close();
    }
}

In .Net you should use

using (Resource resource = /*aquire resource*/)
{
    //do stuff
}

The simple answer is that YOU CANNOT. Such behavior is a BUG and you should locate it and fix it.

Answer 2

.Net have the same problem, Finalize method is not reliable. So in .Net 2.0 to address this problem Safe Handles are introduced, which provides a reliable way to release the unmanaged resource.

All you have to do is inherit your class from SafeHandle or CriticalHandle. These both indeed derives from CritialFinializationObject which turns to be the key for reliablity.

Read more about safehandles here.

P.S:This is only partial answer, am a .Net guy so apologizes for no java part of answer.

Answer 3

finalize has the same problem in .NET (i.e. if the object is never garbage collected, then finalize will never be called)

In Java, the best practice to guarantee cleanup is:

final Resource resource = getResource();
try {
    // Do stuff
    ...
} finally {
    resource.dispose();
}

In .NET, the best practice to guarantee cleanup is:

using (Resource resource = GetResource())
{
    // Do stuff
    ...
}