RFR: 7357: Double-Checked Locking in Agent Plugin code [v2]

[Guru Hb]

On Mon, 26 Jul 2021 13:48:12 GMT, Marcus Hirt <hirt at openjdk.org> wrote:

>> application/org.openjdk.jmc.console.agent/src/main/java/org/openjdk/jmc/console/agent/manager/model/PresetRepositoryFactory.java line 53:
>> 
>>> 51: 		if (singleton == null) {
>>> 52: 			synchronized (PresetRepositoryFactory.class) {
>>> 53: 				singleton = create();
>> 
>> Actually, singleton field need also to be `volatile`.
>> see https://shipilev.net/blog/2014/safe-public-construction/ for more info
>
> Indeed. I haven't checked, but couldn't all this be avoided by simply creating the singleton at the field declaration - does it really have to be lazily created?

We could have created a Singleton in field declaration if below points were true :  
1. If we had only "PresetRepository" as a main class with Singleton (Currently this is a factory).
2. Likelihood of "PresetRepository" object required all the time of JMC's lifecycle . 
+ "- does it really have to be lazily created?"
3. Singleton Object + init (PresetRepository.addLocalPresetTo(...) ) is doing a File Operation.

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PR: https://git.openjdk.java.net/jmc/pull/282