Spring Use SingletonSupplier to create singletons

SingletonSupplier is introduced since 5.1, we could reuse this to create singleton such as DefaultConversionService

for example

    private static SingletonSupplier<DefaultConversionService> singletonSupplier = SingletonSupplier
            .of(DefaultConversionService::new);

    public static DefaultConversionService getSharedInstance() {
        return singletonSupplier.obtain();
    }

Comment From: quaff

Superseded by https://github.com/spring-projects/spring-framework/pull/24303