RFR: 8258776: ThreadLocal#initialValue() Javadoc is unaware of ThreadLocal#withInitial() [v2]

[Jaikiran Pai]

On Thu, 5 Jan 2023 13:24:08 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

>> Can I please get a review of this doc only change which addresses the javadoc issue noted in https://bugs.openjdk.org/browse/JDK-8258776? 
>> 
>> As noted in that issue, the `ThreadLocal.initialValue()` API javadoc suggests subclassing `ThreadLocal` and overriding the `initialValue()` method instead of recommending the `withInitial` method that was added in Java 8.
>> 
>> The commit here updates the javadoc of `initialValue()` method to note that `withInitial` method is available for use if the programmer wants to provide an initial value. The updated javadoc continues to note that the `ThreadLocal` can be subclassed and the method overriden as the other way of providing an initial value. This change now uses the `@implSpec` construct to state this default behaviour of the `initialValue()` method.
>> 
>> Additionally, the `get()` method's javadoc has also been updated to account for the semantics of `initialValue()`. In fact, in its current form, before the changes in this PR, the `get()` method states:
>> 
>>> If the current thread does not support thread locals then
>>> this method returns its {@link #initialValue} (or {@code null}
>>> if the {@code initialValue} method is not overridden).
>> 
>> which isn't accurate, since the `get()` will return a non-null value if the ThreadLocal was constructed through `ThreadLocal.withInitial`. Here's a trivial code which contradicts this current javadoc:
>> 
>> 
>> public class Test {
>>   public static void main(final String[] args) throws Exception {
>>     Thread.ofPlatform().name("hello").allowSetThreadLocals(false).start(() -> {
>> 	var t = ThreadLocal.withInitial(() -> 2);
>> 	System.out.println("Thread local value is " + t.get());
>>     });
>>   }
>> }
>> 
>> Running this with `java --enable-preview --source 21 Test.java` returns:
>> 
>> Thread local value is 2
>> 
>> 
>> The updated javadoc of `get()` in this PR now matches the behaviour of this method. I believe this will need a CSR, which I'll open once we have an agreement on these changes.
>> 
>> Furthermore, the class level javadoc of `ThreadLocal` has an example of providing an initial value which uses the subclassing strategy. I haven't updated that part. Should we update that to show the `withInitialValue` usage instead (and maybe convert it to a snippet)?
>
> Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Review suggestion

The CSR has been approved. Thank you Alan for the review.

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PR: https://git.openjdk.org/jdk/pull/11846