Reintroduce synthesized annotation attribute value caching

[sbrannen]

Prior to the introduction of the MergedAnnotation in Spring Framework 5.2, our SynthesizedAnnotationInvocationHandler utilized a cache for annotation attribute values; whereas, the new SynthesizedMergedAnnotationInvocationHandler has no such caching in place.

Issues such as #24961 indicate a regression in performance caused by the lack of such an attribute value cache. For example, the required attribute in @RequestParam is looked up using the internal meta-model in the MergedAnnotation API twice per request for each @RequestParam in a given controller handler method.

Reintroducing the attribute value cache would avoid the unnecessary performance overhead associated with multiple lookups of the same attribute not only for use cases like RequestParam.required but also for equals() and toString() invocations on a synthesized annotation. Note that invocations of hashCode() also incur additional overhead for attribute value lookups but only once, since the resulting hash code is cached after the first computation.

[sbrannen]

Empirical Evidence

This is by no means a proper benchmark, but given the following modified test method from MergedAnnotationsTests, the approximate average runtime is:

• 700 ms without caching
• 400 ms with caching

@Test
void synthesizeWithTransitiveImplicitAliasesForAliasPair() throws Exception {
	TransitiveImplicitAliasesForAliasPairTestConfiguration config =
			TransitiveImplicitAliasesForAliasPairTestConfigurationClass.class.getAnnotation(
					TransitiveImplicitAliasesForAliasPairTestConfiguration.class);
	TransitiveImplicitAliasesForAliasPairTestConfiguration synthesized = MergedAnnotation.from(
			config).synthesize();
	assertThat(synthesized).isInstanceOf(SynthesizedAnnotation.class);

	IntStream.rangeClosed(1, 1_000_000).forEach(count -> {
		assertThat(synthesized.xml()).isEqualTo("test.xml");
		assertThat(synthesized.xml()).isEqualTo("test.xml");
		assertThat(synthesized.groovy()).isEqualTo("test.xml");
		assertThat(synthesized.groovy()).isEqualTo("test.xml");
		synthesized.toString();
		synthesized.hashCode();
		synthesized.equals(config);
	});
}

Although the caching introduces the use of a map to store the computed attribute values, we feel that the increase in performance is significant enough to justify the memory trade-off.

[sbrannen]

Reopening to fully address toString() for synthesized annotations as well.