RFR: 8358318: JFR: Tighten up PlatformTracer initialization

[Chen Liang]

On Mon, 2 Jun 2025 11:19:52 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Started as SonarCloud complaint that MethodSetting.apply initializes a static field. But I think there is a larger problem here, which I completely missed during [JDK-8352738](https://bugs.openjdk.org/browse/JDK-8352738) review: the `PlatformTracer` initialization can technically run several times, AFAICS. This PR tightens up the initialization. It also allows us to call `ensureInitialized` from other places, if we ever find a need.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `jdk_jfr`

src/jdk.jfr/share/classes/jdk/jfr/internal/tracing/PlatformTracer.java line 49:

> 47:  */
> 48: public final class PlatformTracer {
> 49:     private static volatile boolean INITIALIZED;

Can we make this a static final field in a holder class, so we call `PlatformTracer::initialize` from that holder class's `<clinit>`, and after initialization, the check to `INITIALIZED` field can be constant-folded instead of being a volatile read?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25584#discussion_r2121506115