tutorial on using Cleaner-based finalization

[Peter Levart]

Thinking of this for some more time...

Although this is a nice exercise in converting finalize() to Cleaner 
API, I strongly suspect that ClientConnection.finalize() is doing 
unnecessary things. What it does is it:

- prints some trace message to logWriter
- closes logWriter (whatever it is)
- closes raw socket input and output streams
- closes the socket
- notifies listeners

I believe all this is unnecessary (apart from notifying listeners 
perhaps?) as those objects already have their own cleanup mechanism when 
they are left behind. I believe there would be no resource leaks if 
ClientSocket.finalize() was simply removed. Before doing the conversion 
from finalize() to Cleaner API one should always ask himself whether 
finalize() method is actually needed. All 3rd party code (with notable 
exceptions which use JNI) are based on Java SE APIs and these APIs 
already take care of resources held by objects that are left behind. 
There's usually no need to do the same in the higher layers of 3rd party 
code.

What do you think?

Regards, Peter


On 05/06/2017 10:01 PM, Peter Levart wrote:
> Hi Rick and others,
>
> On 05/04/2017 06:48 PM, Lance Andersen wrote:
>> Here are a few examples I believe:
>> http://svn.apache.org/repos/asf/db/derby/code/trunk/java/client/org/apache/derby/client/am/ClientConnection.java
>> http://svn.apache.org/repos/asf/db/derby/code/trunk/java/client/org/apache/derby/client/ClientPooledConnection.java
>> http://svn.apache.org/repos/asf/db/derby/code/trunk/java/engine/org/apache/derby/impl/jdbc/EmbedPreparedStatement.java
>
> I took a bite at the 1st one (ClientConnection). Here's the result:
>
> http://cr.openjdk.java.net/~plevart/misc/Cleaner/derby/ClientConnection_finalize2cleaner.patch
>
> I haven't tested this, but I believe it should work. It was quite a 
> challenge, because of the way current ClientConnection code is 
> structured. I tried to make the patch not incompatibly change public 
> API of ClientConnection and related classes and I almost succeeded. 
> The problematic part was the protected boolean 
> ClientConnection.closed_ flag. If there is any sublclass of 
> ClientConnection (apart from NetConnection which is derby code) that 
> modifies this field, you are out of luck as changing (not only 
> reading) this field directly my have an undesirable consequence (or it 
> may not, since the only thing that changing these field to false would 
> do is it would redundantly force performing the cleanup action. If the 
> cleanup is idempotent, then all is OK).
>
> Further complication with ClientConnection is that it maintains a 
> split state - some of it resides in ClientConnection and subclasses 
> (such as NetConnection) and some of it in embedded object of Agent 
> class and subclasses (such as NetAgent). Both - some of this state 
> from connection object and some from the agent state are needed to 
> perform the cleanup that is currently executed from the connection 
> finalize() method. When using Cleaner API, we have to capture this 
> state from both places (or more since each class has a hierarchy) and 
> then arrange for cleanup action to use this state. Captured state can 
> not reference the tracked object (ClientConnection in this case) 
> either directly or indirectly since then it will never be GCed. When 
> cleanup action is run, the tracked object is already unreachable - 
> this is the main difference from finalize() where there is a phase in 
> object's life-cycle where it is still reachable, albeit guaranteed 
> only from the thread executing finalize() method. We can not capture 
> the Agent object either, since it maintains a reference back to the 
> ClientConnection object. All this is further complicated by the fact 
> that captured state is mutable and we have to arrange for it to be 
> mutated in both places. If the mutable state is captured by reference 
> and the instance containing it never changes during the lifetime of 
> the tracked container object, then it is easy - we just capture the 
> object after the tracked container instance is constructed. If the 
> captured state includes mutable fields directly in the tracked 
> container object, then we must arrange for them to be synchronously 
> mutated in two places. Such fields are:
>
> - ClientConnection.open_ (replicated in 
> ClientConnection.CleanupAction.open)
> - Agent.logWriter_ (replicated in Agent.CleanupAction.logWriter)
> - NetAgent.rawSocketInputStream_ (replicated in 
> NetAgent.NetCleanupAction.rawSocketInputStream)
> - NetAgent.rawSocketOutputStream_ (replicated in 
> NetAgent.NetCleanupAction.rawSocketOutputStream)
>
> Fortunately all of this state is encapsulated with protected field 
> ClientConnection.open_ being an exception.
>
> Note that Cleaner API also allows for cleanup action to be triggered 
> explicitly, which then de-registers it. This is one of its advantages 
> over finalize() where you can not deregister an object when it is 
> already explicitly closed for example. finalize() will always be 
> called even if closed explicitly. If you create lots of finalizable 
> objects (such as connections, statements, etc...) and promptly close() 
> them, they still wait for finalization and use resources (heap, CPU 
> when GC searches for them, ReferenceHandler enqueues them, and finally 
> finalize() method which is executed after the fact). Explicit 
> triggering and de-registration of the cleanup action is performed in 
> the ClientConnection.closeResourcesX() (called from public close() and 
> closeResources()) after the connection has already being marked as 
> closed. Cleanup action will be a no-op at this point, but it will also 
> be de-registered. This is important to not bother GC with reference 
> processing when it is not needed any more. In situations whre cleanup 
> action logic is the same as explicit closing logic (in the case of 
> ClientConnection it is not), close() method could just invoke 
> cleanable.clean() and delegate the meat of processing to the cleanup 
> action.
>
> Hope this non trivial example helps illustrate what is needed when 
> converting finalize() to Cleanup API.
>
> Regards, Peter
>