Declarative XML-based Caching

If annotations are not an option (perhaps due to having no access to the sources or no external code), you can use XML for declarative caching. So, instead of annotating the methods for caching, you can specify the target method and the caching directives externally (similar to the declarative transaction management advice). The example from the previous section can be translated into the following example:

<!-- the service we want to make cacheable -->
<bean id="bookService" class="x.y.service.DefaultBookService"/>

<!-- cache definitions -->
<cache:advice id="cacheAdvice" cache-manager="cacheManager">
	<cache:caching cache="books">
		<cache:cacheable method="findBook" key="#isbn"/>
		<cache:cache-evict method="loadBooks" all-entries="true"/>
	</cache:caching>
</cache:advice>

<!-- apply the cacheable behavior to all BookService interfaces -->
<aop:config>
	<aop:advisor advice-ref="cacheAdvice" pointcut="execution(* x.y.BookService.*(..))"/>
</aop:config>

<!-- cache manager definition omitted -->

In the preceding configuration, the bookService is made cacheable. The caching semantics to apply are encapsulated in the cache:advice definition, which causes the findBooks method to be used for putting data into the cache and the loadBooks method for evicting data. Both definitions work against the books cache.

The aop:config definition applies the cache advice to the appropriate points in the program by using the AspectJ pointcut expression (more information is available in Aspect Oriented Programming with Spring). In the preceding example, all methods from the BookService are considered and the cache advice is applied to them.

The declarative XML caching supports all of the annotation-based model, so moving between the two should be fairly easy. Furthermore, both can be used inside the same application. The XML-based approach does not touch the target code. However, it is inherently more verbose. When dealing with classes that have overloaded methods that are targeted for caching, identifying the proper methods does take an extra effort, since the method argument is not a good discriminator. In these cases, you can use the AspectJ pointcut to cherry pick the target methods and apply the appropriate caching functionality. However, through XML, it is easier to apply package or group or interface-wide caching (again, due to the AspectJ pointcut) and to create template-like definitions (as we did in the preceding example by defining the target cache through the cache:definitions cache attribute).