tutorial on using Cleaner-based finalization

Peter Levart peter.levart at gmail.com
Thu May 4 06:16:26 UTC 2017

Hi Rick,

Allow me to join the discussion...

On 05/04/2017 04:21 AM, Rick Hillegas wrote:
> Thanks, Roger. That is a helpful example.
> Derby is a component which can be quiesced and unloaded on 
> resource-constrained platforms, then re-loaded when resources free up. 
> Unloading means garbage collecting all the classloaders created by the 
> component so that, at the end, not a single scrap of Derby code 
> remains loaded. Has the Cleaner pattern been tested under these kinds 
> of conditions? Can you point me at a Cleaner-based library which 
> successfully unloads and reloads itself without leaking classloaders?

Can you point me to a place in Derby code where you are currently using 
finalize() method for your purpose and I'll try to show you how to 
convert this to Cleaner API...

I'm surprised that you actually need finalize() in Derby. Isn't this 
pure Java code? Usually finalize() is needed in situations where there's 
some non-Java resource which has to be cleaned-up when all the 
references to some Java object that represents it are gone. I'm curious 
what you need to clean in a pure Java library.

Regards, Peter

> Thanks,
> -Rick
> On 5/3/17 9:04 AM, Roger Riggs wrote:
>> Hi Rick,
>> The general nature of changes to use the Cleaner involve factoring 
>> out the cleanup
>> code from the object being cleaned, though in many cases it does not 
>> require restructuring.
>> For example, if an object holds a reference to a network socket that 
>> needs to be closed
>> when the object is collected the socket.close() can called by the 
>> cleaner:
>>    Cleaner cleaner = ...;
>>    final Socket socket = ...;
>>    Object obj = ...;
>>    cleaner.register(obj, () -> {
>>    try {
>>    socket.close();
>>         } catch (IOException ioe) { // ignore}
>>    });
>> Creating a cleaner starts a thread that does the work so you'll want 
>> to decide
>> how to share it across the uses or to use than one.
>> Using lambdas for the cleaner functions is very lightweight but be 
>> careful to avoid
>> the using bound variables in the lambda body because they implicitly 
>> retain
>> a reference to the enclosing instance which will prevent the instance 
>> from becoming unreferences.
>> If there are more specific cases of interest let me know,
>> Regards, Roger
>> On 5/2/2017 10:08 PM, Rick Hillegas wrote:
>>> When I compile Apache Derby using JDK 9 build 167, I see several 
>>> instances of the following warning:
>>>    warning: [deprecation] finalize() in Object has been deprecated
>>> The javadoc for java.lang.Object.finalize() suggests that affected 
>>> classes should migrate their finalization to a coding pattern based 
>>> on the newly introduced java.lang.ref.Cleaner class. I am hesitant 
>>> to try my hand at this without more guidance. Can you point me at a 
>>> tutorial or list of best practices for implementing Cleaner-based 
>>> finalization?
>>> Thanks,
>>> -Rick

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