Introduce initial caching support

[sdeleuze]

We have basic configuration for caching in CachingHints, but EhCache is not supported, and enabling caching support in PetClinic JPA with the following configuration:

@Configuration
@EnableCaching
class CacheConfiguration {

	@Bean
	public JCacheManagerCustomizer petclinicCacheConfigurationCustomizer() {
		return cm -> {
			cm.createCache("vets", cacheConfiguration());
		};
	}

	private javax.cache.configuration.Configuration<Object, Object> cacheConfiguration() {
		return new MutableConfiguration<>().setStatisticsEnabled(true);
	}

}

Generates this error that seems to indicate the need for a processor:

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'vetController' defined in class path resource [org/springframework/samples/petclinic/vet/VetController.class]: Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'vetRepository': Post-processing of FactoryBean's singleton object failed; nested exception is com.oracle.svm.core.jdk.UnsupportedFeatureError: Proxy class defined by interfaces [interface org.springframework.samples.petclinic.vet.VetRepository, interface org.springframework.data.repository.Repository, interface org.springframework.transaction.interceptor.TransactionalProxy, interface org.springframework.aop.framework.Advised, interface org.springframework.core.DecoratingProxy, interface java.io.Serializable] not found. Generating proxy classes at runtime is not supported. Proxy classes need to be defined at image build time by specifying the list of interfaces that they implement. To define proxy classes use -H:DynamicProxyConfigurationFiles=<comma-separated-config-files> and -H:DynamicProxyConfigurationResources=<comma-separated-config-resources> options.

We should also document what kind of caching is supported (see the list in CacheType) and bring back caching support in the petclinic-jpa sample as configured in https://github.com/spring-projects/spring-petclinic.

[sdeleuze]

As a follow-up of #481, here is my proposal for a more efficient caching support.

First, once spring-projects/spring-boot#25321 is merged, we should configure CacheConfigurations to be initialized at build time to avoid using the cache configuration classes at runtime if they are not useful.

Second, we should update CacheConfigurationImportSelector substitution to add classpath check (based on manual conversion of @ConditionalOnClass to return only the relevant ones.

Third, we should update CacheHints to dynamically add the relevant hints based on a build-time execution of CacheConfigurationImportSelector.

[sdeleuze]

Tentatively scheduled for 0.9.0 since that's an important feature.

[snicoll]

@sdeleuze I can see this is blocked. Anything I can help you with?

[sdeleuze]

Thanks for the offer, I am progressing again on it after the discussion we had on #526.