36. Cache Abstraction

36.1 Introduction

Since version 3.1, Spring Framework provides support for transparently adding caching into an existing Spring application. Similar to the transaction support, the caching abstraction allows consistent use of various caching solutions with minimal impact on the code.

As from Spring 4.1, the cache abstraction has been significantly improved with the support of JSR-107 annotations and more customization options.

36.2 Understanding the cache abstraction

At its core, the abstraction applies caching to Java methods, reducing thus the number of executions based on the information available in the cache. That is, each time a targeted method is invoked, the abstraction will apply a caching behavior checking whether the method has been already executed for the given arguments. If it has, then the cached result is returned without having to execute the actual method; if it has not, then method is executed, the result cached and returned to the user so that, the next time the method is invoked, the cached result is returned. This way, expensive methods (whether CPU or IO bound) can be executed only once for a given set of parameters and the result reused without having to actually execute the method again. The caching logic is applied transparently without any interference to the invoker.

[Important]Important

Obviously this approach works only for methods that are guaranteed to return the same output (result) for a given input (or arguments) no matter how many times it is being executed.

Other cache-related operations are provided by the abstraction such as the ability to update the content of the cache or remove one of all entries. These are useful if the cache deals with data that can change during the course of the application.

Just like other services in the Spring Framework, the caching service is an abstraction (not a cache implementation) and requires the use of an actual storage to store the cache data - that is, the abstraction frees the developer from having to write the caching logic but does not provide the actual stores. This abstraction is materialized by the org.springframework.cache.Cache and org.springframework.cache.CacheManager interfaces.

There are a few implementations of that abstraction available out of the box: JDK java.util.concurrent.ConcurrentMap based caches, EhCache, Gemfire cache, Caffeine, Guava caches and JSR-107 compliant caches. See Section 36.7, “Plugging-in different back-end caches” for more information on plugging in other cache stores/providers.

[Important]Important

The caching abstraction has no special handling of multi-threaded and multi-process environments as such features are handled by the cache implementation. .

If you have a multi-process environment (i.e. an application deployed on several nodes), you will need to configure your cache provider accordingly. Depending on your use cases, a copy of the same data on several nodes may be enough but if you change the data during the course of the application, you may need to enable other propagation mechanisms.

Caching a particular item is a direct equivalent of the typical get-if-not-found-then- proceed-and-put-eventually code blocks found with programmatic cache interaction: no locks are applied and several threads may try to load the same item concurrently. The same applies to eviction: if several threads are trying to update or evict data concurrently, you may use stale data. Certain cache providers offer advanced features in that area, refer to the documentation of the cache provider that you are using for more details.

To use the cache abstraction, the developer needs to take care of two aspects:

  • caching declaration - identify the methods that need to be cached and their policy
  • cache configuration - the backing cache where the data is stored and read from

36.3 Declarative annotation-based caching

For caching declaration, the abstraction provides a set of Java annotations:

  • @Cacheable triggers cache population
  • @CacheEvict triggers cache eviction
  • @CachePut updates the cache without interfering with the method execution
  • @Caching regroups multiple cache operations to be applied on a method
  • @CacheConfig shares some common cache-related settings at class-level

Let us take a closer look at each annotation:

36.3.1 @Cacheable annotation

As the name implies, @Cacheable is used to demarcate methods that are cacheable - that is, methods for whom the result is stored into the cache so on subsequent invocations (with the same arguments), the value in the cache is returned without having to actually execute the method. In its simplest form, the annotation declaration requires the name of the cache associated with the annotated method:

@Cacheable("books")
public Book findBook(ISBN isbn) {...}

In the snippet above, the method findBook is associated with the cache named books. Each time the method is called, the cache is checked to see whether the invocation has been already executed and does not have to be repeated. While in most cases, only one cache is declared, the annotation allows multiple names to be specified so that more than one cache are being used. In this case, each of the caches will be checked before executing the method - if at least one cache is hit, then the associated value will be returned:

[Note]Note

All the other caches that do not contain the value will be updated as well even though the cached method was not actually executed.

@Cacheable({"books", "isbns"})
public Book findBook(ISBN isbn) {...}

Default Key Generation

Since caches are essentially key-value stores, each invocation of a cached method needs to be translated into a suitable key for cache access. Out of the box, the caching abstraction uses a simple KeyGenerator based on the following algorithm:

  • If no params are given, return SimpleKey.EMPTY.
  • If only one param is given, return that instance.
  • If more the one param is given, return a SimpleKey containing all parameters.

This approach works well for most use-cases; As long as parameters have natural keys and implement valid hashCode() and equals() methods. If that is not the case then the strategy needs to be changed.

To provide a different default key generator, one needs to implement the org.springframework.cache.interceptor.KeyGenerator interface.

[Note]Note

The default key generation strategy changed with the release of Spring 4.0. Earlier versions of Spring used a key generation strategy that, for multiple key parameters, only considered the hashCode() of parameters and not equals(); this could cause unexpected key collisions (see SPR-10237 for background). The new 'SimpleKeyGenerator' uses a compound key for such scenarios.

If you want to keep using the previous key strategy, you can configure the deprecated org.springframework.cache.interceptor.DefaultKeyGenerator class or create a custom hash-based 'KeyGenerator' implementation.

Custom Key Generation Declaration

Since caching is generic, it is quite likely the target methods have various signatures that cannot be simply mapped on top of the cache structure. This tends to become obvious when the target method has multiple arguments out of which only some are suitable for caching (while the rest are used only by the method logic). For example:

@Cacheable("books")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

At first glance, while the two boolean arguments influence the way the book is found, they are no use for the cache. Further more what if only one of the two is important while the other is not?

For such cases, the @Cacheable annotation allows the user to specify how the key is generated through its key attribute. The developer can use SpEL to pick the arguments of interest (or their nested properties), perform operations or even invoke arbitrary methods without having to write any code or implement any interface. This is the recommended approach over the default generator since methods tend to be quite different in signatures as the code base grows; while the default strategy might work for some methods, it rarely does for all methods.

Below are some examples of various SpEL declarations - if you are not familiar with it, do yourself a favor and read Chapter 10, Spring Expression Language (SpEL):

@Cacheable(cacheNames="books", key="#isbn")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

@Cacheable(cacheNames="books", key="#isbn.rawNumber")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

@Cacheable(cacheNames="books", key="T(someType).hash(#isbn)")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

The snippets above show how easy it is to select a certain argument, one of its properties or even an arbitrary (static) method.

If the algorithm responsible to generate the key is too specific or if it needs to be shared, you may define a custom keyGenerator on the operation. To do this, specify the name of the KeyGenerator bean implementation to use:

@Cacheable(cacheNames="books", keyGenerator="myKeyGenerator")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)
[Note]Note

The key and keyGenerator parameters are mutually exclusive and an operation specifying both will result in an exception.

Default Cache Resolution

Out of the box, the caching abstraction uses a simple CacheResolver that retrieves the cache(s) defined at the operation level using the configured CacheManager.

To provide a different default cache resolver, one needs to implement the org.springframework.cache.interceptor.CacheResolver interface.

Custom cache resolution

The default cache resolution fits well for applications working with a single CacheManager and with no complex cache resolution requirements.

For applications working with several cache managers, it is possible to set the cacheManager to use per operation:

@Cacheable(cacheNames="books", cacheManager="anotherCacheManager")
public Book findBook(ISBN isbn) {...}

It is also possible to replace the CacheResolver entirely in a similar fashion as for key generation. The resolution is requested for every cache operation, giving a chance to the implementation to actually resolve the cache(s) to use based on runtime arguments:

@Cacheable(cacheResolver="runtimeCacheResolver")
public Book findBook(ISBN isbn) {...}
[Note]Note

Since Spring 4.1, the value attribute of the cache annotations are no longer mandatory since this particular information can be provided by the CacheResolver regardless of the content of the annotation.

Similarly to key and keyGenerator, the cacheManager and cacheResolver parameters are mutually exclusive and an operation specifying both will result in an exception as a custom CacheManager will be ignored by the CacheResolver implementation. This is probably not what you expect.

Synchronized caching

In a multi-threaded environment, certain operations might be concurrently invoked for the same argument (typically on startup). By default, the cache abstraction does not lock anything and the same value may be computed several times, defeating the purpose of caching.

For those particular cases, the sync attribute can be used to instruct the underlying cache provider to lock the cache entry while the value is being computed. As a result, only one thread will be busy computing the value while the others are blocked until the entry is updated in the cache.

@Cacheable(cacheNames="foos", sync="true")
public Foo executeExpensiveOperation(String id) {...}
[Note]Note

This is an optional feature and your favorite cache library may not support it. All CacheManager implementations provided by the core framework support it. Check the documentation of your cache provider for more details.

Conditional caching

Sometimes, a method might not be suitable for caching all the time (for example, it might depend on the given arguments). The cache annotations support such functionality through the condition parameter which takes a SpEL expression that is evaluated to either true or false. If true, the method is cached - if not, it behaves as if the method is not cached, that is executed every time no matter what values are in the cache or what arguments are used. A quick example - the following method will be cached only if the argument name has a length shorter than 32:

@Cacheable(cacheNames="book", condition="#name.length < 32")
public Book findBook(String name)

In addition the condition parameter, the unless parameter can be used to veto the adding of a value to the cache. Unlike condition, unless expressions are evaluated after the method has been called. Expanding on the previous example - perhaps we only want to cache paperback books:

@Cacheable(cacheNames="book", condition="#name.length < 32", unless="#result.hardback")
public Book findBook(String name)

Available caching SpEL evaluation context

Each SpEL expression evaluates again a dedicated context. In addition to the build in parameters, the framework provides dedicated caching related metadata such as the argument names. The next table lists the items made available to the context so one can use them for key and conditional computations:

Table 36.1. Cache SpEL available metadata

NameLocationDescriptionExample

methodName

root object

The name of the method being invoked

#root.methodName

method

root object

The method being invoked

#root.method.name

target

root object

The target object being invoked

#root.target

targetClass

root object

The class of the target being invoked

#root.targetClass

args

root object

The arguments (as array) used for invoking the target

#root.args[0]

caches

root object

Collection of caches against which the current method is executed

#root.caches[0].name

argument name

evaluation context

Name of any of the method arguments. If for some reason the names are not available (e.g. no debug information), the argument names are also available under the #a<#arg> where #arg stands for the argument index (starting from 0).

#iban or #a0 (one can also use #p0 or #p<#arg> notation as an alias).

result

evaluation context

The result of the method call (the value to be cached). Only available in unless expressions, cache put expressions (to compute the key), or cache evict expressions (when beforeInvocation is false).

#result


36.3.2 @CachePut annotation

For cases where the cache needs to be updated without interfering with the method execution, one can use the @CachePut annotation. That is, the method will always be executed and its result placed into the cache (according to the @CachePut options). It supports the same options as @Cacheable and should be used for cache population rather than method flow optimization:

@CachePut(cacheNames="book", key="#isbn")
public Book updateBook(ISBN isbn, BookDescriptor descriptor)
[Important]Important

Note that using @CachePut and @Cacheable annotations on the same method is generally strongly discouraged because they have different behaviors. While the latter causes the method execution to be skipped by using the cache, the former forces the execution in order to execute a cache update. This leads to unexpected behavior and with the exception of specific corner-cases (such as annotations having conditions that exclude them from each other), such declaration should be avoided. Note also that such condition should not rely on the result object (i.e. the #result variable) as these are validated upfront to confirm the exclusion.

36.3.3 @CacheEvict annotation

The cache abstraction allows not just population of a cache store but also eviction. This process is useful for removing stale or unused data from the cache. Opposed to @Cacheable, annotation @CacheEvict demarcates methods that perform cache eviction, that is methods that act as triggers for removing data from the cache. Just like its sibling, @CacheEvict requires specifying one (or multiple) caches that are affected by the action, allows a custom cache and key resolution or a condition to be specified but in addition, features an extra parameter allEntries which indicates whether a cache-wide eviction needs to be performed rather then just an entry one (based on the key):

@CacheEvict(cacheNames="books", allEntries=true)
public void loadBooks(InputStream batch)

This option comes in handy when an entire cache region needs to be cleared out - rather then evicting each entry (which would take a long time since it is inefficient), all the entries are removed in one operation as shown above. Note that the framework will ignore any key specified in this scenario as it does not apply (the entire cache is evicted not just one entry).

One can also indicate whether the eviction should occur after (the default) or before the method executes through the beforeInvocation attribute. The former provides the same semantics as the rest of the annotations - once the method completes successfully, an action (in this case eviction) on the cache is executed. If the method does not execute (as it might be cached) or an exception is thrown, the eviction does not occur. The latter ( beforeInvocation=true) causes the eviction to occur always, before the method is invoked - this is useful in cases where the eviction does not need to be tied to the method outcome.

It is important to note that void methods can be used with @CacheEvict - as the methods act as triggers, the return values are ignored (as they don’t interact with the cache) - this is not the case with @Cacheable which adds/updates data into the cache and thus requires a result.

36.3.4 @Caching annotation

There are cases when multiple annotations of the same type, such as @CacheEvict or @CachePut need to be specified, for example because the condition or the key expression is different between different caches. @Caching allows multiple nested @Cacheable, @CachePut and @CacheEvict to be used on the same method:

@Caching(evict = { @CacheEvict("primary"), @CacheEvict(cacheNames="secondary", key="#p0") })
public Book importBooks(String deposit, Date date)

36.3.5 @CacheConfig annotation

So far we have seen that caching operations offered many customization options and these can be set on an operation basis. However, some of the customization options can be tedious to configure if they apply to all operations of the class. For instance, specifying the name of the cache to use for every cache operation of the class could be replaced by a single class-level definition. This is where @CacheConfig comes into play.

@CacheConfig("books")
public class BookRepositoryImpl implements BookRepository {

    @Cacheable
    public Book findBook(ISBN isbn) {...}
}

@CacheConfig is a class-level annotation that allows to share the cache names, the custom KeyGenerator, the custom CacheManager and finally the custom CacheResolver. Placing this annotation on the class does not turn on any caching operation.

An operation-level customization will always override a customization set on @CacheConfig. This gives therefore three levels of customizations per cache operation:

  • Globally configured, available for CacheManager, KeyGenerator
  • At class level, using @CacheConfig
  • At the operation level

36.3.6 Enable caching annotations

It is important to note that even though declaring the cache annotations does not automatically trigger their actions - like many things in Spring, the feature has to be declaratively enabled (which means if you ever suspect caching is to blame, you can disable it by removing only one configuration line rather than all the annotations in your code).

To enable caching annotations add the annotation @EnableCaching to one of your @Configuration classes:

@Configuration
@EnableCaching
public class AppConfig {
}

Alternatively for XML configuration use the cache:annotation-driven element:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:cache="http://www.springframework.org/schema/cache"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/cache http://www.springframework.org/schema/cache/spring-cache.xsd">

        <cache:annotation-driven />

</beans>

Both the cache:annotation-driven element and @EnableCaching annotation allow various options to be specified that influence the way the caching behavior is added to the application through AOP. The configuration is intentionally similar with that of @Transactional:

[Note]Note

Advanced customizations using Java config require to implement CachingConfigurer, refer to the javadoc for more details.

Table 36.2. Cache annotation settings

XML AttributeAnnotation AttributeDefaultDescription

cache-manager

N/A (See CachingConfigurer javadocs)

cacheManager

Name of cache manager to use. A default CacheResolver will be initialized behind the scenes with this cache manager (or `cacheManager`if not set). For more fine-grained management of the cache resolution, consider setting the 'cache-resolver' attribute.

cache-resolver

N/A (See CachingConfigurer javadocs)

A SimpleCacheResolver using the configured cacheManager.

The bean name of the CacheResolver that is to be used to resolve the backing caches. This attribute is not required, and only needs to be specified as an alternative to the 'cache-manager' attribute.

key-generator

N/A (See CachingConfigurer javadocs)

SimpleKeyGenerator

Name of the custom key generator to use.

error-handler

N/A (See CachingConfigurer javadocs)

SimpleCacheErrorHandler

Name of the custom cache error handler to use. By default, any exception throw during a cache related operations are thrown back at the client.

mode

mode

proxy

The default mode "proxy" processes annotated beans to be proxied using Spring’s AOP framework (following proxy semantics, as discussed above, applying to method calls coming in through the proxy only). The alternative mode "aspectj" instead weaves the affected classes with Spring’s AspectJ caching aspect, modifying the target class byte code to apply to any kind of method call. AspectJ weaving requires spring-aspects.jar in the classpath as well as load-time weaving (or compile-time weaving) enabled. (See the section called “Spring configuration” for details on how to set up load-time weaving.)

proxy-target-class

proxyTargetClass

false

Applies to proxy mode only. Controls what type of caching proxies are created for classes annotated with the @Cacheable or @CacheEvict annotations. If the proxy-target-class attribute is set to true, then class-based proxies are created. If proxy-target-class is false or if the attribute is omitted, then standard JDK interface-based proxies are created. (See Section 11.6, “Proxying mechanisms” for a detailed examination of the different proxy types.)

order

order

Ordered.LOWEST_PRECEDENCE

Defines the order of the cache advice that is applied to beans annotated with @Cacheable or @CacheEvict. (For more information about the rules related to ordering of AOP advice, see the section called “Advice ordering”.) No specified ordering means that the AOP subsystem determines the order of the advice.


[Note]Note

<cache:annotation-driven/> only looks for @[email protected][email protected][email protected] on beans in the same application context it is defined in. This means that, if you put <cache:annotation-driven/> in a WebApplicationContext for a DispatcherServlet, it only checks for beans in your controllers, and not your services. See Section 22.2, “The DispatcherServlet” for more information.

[Tip]Tip

Spring recommends that you only annotate concrete classes (and methods of concrete classes) with the @Cache* annotation, as opposed to annotating interfaces. You certainly can place the @Cache* annotation on an interface (or an interface method), but this works only as you would expect it to if you are using interface-based proxies. The fact that Java annotations are not inherited from interfaces means that if you are using class-based proxies ( proxy-target-class="true") or the weaving-based aspect ( mode="aspectj"), then the caching settings are not recognized by the proxying and weaving infrastructure, and the object will not be wrapped in a caching proxy, which would be decidedly bad.

[Note]Note

In proxy mode (which is the default), only external method calls coming in through the proxy are intercepted. This means that self-invocation, in effect, a method within the target object calling another method of the target object, will not lead to an actual caching at runtime even if the invoked method is marked with @Cacheable - considering using the aspectj mode in this case. Also, the proxy must be fully initialized to provide the expected behaviour so you should not rely on this feature in your initialization code, i.e. @PostConstruct.

36.3.7 Using custom annotations

The caching abstraction allows you to use your own annotations to identify what method triggers cache population or eviction. This is quite handy as a template mechanism as it eliminates the need to duplicate cache annotation declarations (especially useful if the key or condition are specified) or if the foreign imports (org.springframework) are not allowed in your code base. Similar to the rest of the stereotype annotations, @Cacheable, @CachePut, @CacheEvict and @CacheConfig can be used as meta-annotations, that is annotations that can annotate other annotations. To wit, let us replace a common @Cacheable declaration with our own, custom annotation:

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
@Cacheable(cacheNames="books", key="#isbn")
public @interface SlowService {
}

Above, we have defined our own SlowService annotation which itself is annotated with @Cacheable - now we can replace the following code:

@Cacheable(cacheNames="books", key="#isbn")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

with:

@SlowService
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

Even though @SlowService is not a Spring annotation, the container automatically picks up its declaration at runtime and understands its meaning. Note that as mentioned above, the annotation-driven behavior needs to be enabled.

36.4 JCache (JSR-107) annotations

Since the Spring Framework 4.1, the caching abstraction fully supports the JCache standard annotations: these are @CacheResult, @CachePut, @CacheRemove and @CacheRemoveAll as well as the @CacheDefaults, @CacheKey and @CacheValue companions. These annotations can be used right the way without migrating your cache store to JSR-107: the internal implementation uses Spring’s caching abstraction and provides default CacheResolver and KeyGenerator implementations that are compliant with the specification. In other words, if you are already using Spring’s caching abstraction, you can switch to these standard annotations without changing your cache storage (or configuration, for that matter).

36.4.1 Features summary

For those who are familiar with Spring’s caching annotations, the following table describes the main differences between the Spring annotations and the JSR-107 counterpart:

Table 36.3. Spring vs. JSR-107 caching annotations

SpringJSR-107Remark

@Cacheable

@CacheResult

Fairly similar. @CacheResult can cache specific exceptions and force the execution of the method regardless of the content of the cache.

@CachePut

@CachePut

While Spring updates the cache with the result of the method invocation, JCache requires to pass it as an argument that is annotated with @CacheValue. Due to this difference, JCache allows to update the cache before or after the actual method invocation.

@CacheEvict

@CacheRemove

Fairly similar. @CacheRemove supports a conditional evict in case the method invocation results in an exception.

@CacheEvict(allEntries=true)

@CacheRemoveAll

See @CacheRemove.

@CacheConfig

@CacheDefaults

Allows to configure the same concepts, in a similar fashion.


JCache has the notion of javax.cache.annotation.CacheResolver that is identical to the Spring’s CacheResolver interface, except that JCache only supports a single cache. By default, a simple implementation retrieves the cache to use based on the name declared on the annotation. It should be noted that if no cache name is specified on the annotation, a default is automatically generated, check the javadoc of @CacheResult#cacheName() for more information.

CacheResolver instances are retrieved by a CacheResolverFactory. It is possible to customize the factory per cache operation:

@CacheResult(cacheNames="books", cacheResolverFactory=MyCacheResolverFactory.class)
public Book findBook(ISBN isbn)
[Note]Note

For all referenced classes, Spring tries to locate a bean with the given type. If more than one match exists, a new instance is created and can use the regular bean lifecycle callbacks such as dependency injection.

Keys are generated by a javax.cache.annotation.CacheKeyGenerator that serves the same purpose as Spring’s KeyGenerator. By default, all method arguments are taken into account unless at least one parameter is annotated with @CacheKey. This is similar to Spring’s custom key generation declaration. For instance these are identical operations, one using Spring’s abstraction and the other with JCache:

@Cacheable(cacheNames="books", key="#isbn")
public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)

@CacheResult(cacheName="books")
public Book findBook(@CacheKey ISBN isbn, boolean checkWarehouse, boolean includeUsed)

The CacheKeyResolver to use can also be specified on the operation, in a similar fashion as the CacheResolverFactory.

JCache can manage exceptions thrown by annotated methods: this can prevent an update of the cache but it can also cache the exception as an indicator of the failure instead of calling the method again. Let’s assume that InvalidIsbnNotFoundException is thrown if the structure of the ISBN is invalid. This is a permanent failure, no book could ever be retrieved with such parameter. The following caches the exception so that further calls with the same, invalid ISBN, throws the cached exception directly instead of invoking the method again.

@CacheResult(cacheName="books", exceptionCacheName="failures"
             cachedExceptions = InvalidIsbnNotFoundException.class)
public Book findBook(ISBN isbn)

36.4.2 Enabling JSR-107 support

Nothing specific needs to be done to enable the JSR-107 support alongside Spring’s declarative annotation support. Both @EnableCaching and the cache:annotation-driven element will enable automatically the JCache support if both the JSR-107 API and the spring-context-support module are present in the classpath.

[Note]Note

Depending of your use case, the choice is basically yours. You can even mix and match services using the JSR-107 API and others using Spring’s own annotations. Be aware however that if these services are impacting the same caches, a consistent and identical key generation implementation should be used.

36.5 Declarative XML-based caching

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

<!-- 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 configuration above, the bookService is made cacheable. The caching semantics to apply are encapsulated in the cache:advice definition which instructs method findBooks to be used for putting data into the cache while method loadBooks for evicting data. Both definitions are working 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 Chapter 11, Aspect Oriented Programming with Spring). In the example above, all methods from the BookService are considered and the cache advice applied to them.

The declarative XML caching supports all of the annotation-based model so moving between the two should be fairly easy - further more 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 with 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, the AspectJ pointcut can be used to cherry pick the target methods and apply the appropriate caching functionality. However through XML, it is easier to apply a package/group/interface-wide caching (again due to the AspectJ pointcut) and to create template-like definitions (as we did in the example above by defining the target cache through the cache:definitions cache attribute).

36.6 Configuring the cache storage

Out of the box, the cache abstraction provides several storage integration. To use them, one needs to simply declare an appropriate CacheManager - an entity that controls and manages Caches and can be used to retrieve these for storage.

36.6.1 JDK ConcurrentMap-based Cache

The JDK-based Cache implementation resides under org.springframework.cache.concurrent package. It allows one to use ConcurrentHashMap as a backing Cache store.

<!-- simple cache manager -->
<bean id="cacheManager" class="org.springframework.cache.support.SimpleCacheManager">
    <property name="caches">
        <set>
            <bean class="org.springframework.cache.concurrent.ConcurrentMapCacheFactoryBean" p:name="default"/>
            <bean class="org.springframework.cache.concurrent.ConcurrentMapCacheFactoryBean" p:name="books"/>
        </set>
    </property>
</bean>

The snippet above uses the SimpleCacheManager to create a CacheManager for the two nested ConcurrentMapCache instances named default and books. Note that the names are configured directly for each cache.

As the cache is created by the application, it is bound to its lifecycle, making it suitable for basic use cases, tests or simple applications. The cache scales well and is very fast but it does not provide any management or persistence capabilities nor eviction contracts.

36.6.2 EhCache-based Cache

The EhCache implementation is located under org.springframework.cache.ehcache package. Again, to use it, one simply needs to declare the appropriate CacheManager:

<bean id="cacheManager"
      class="org.springframework.cache.ehcache.EhCacheCacheManager" p:cache-manager-ref="ehcache"/>

<!-- EhCache library setup -->
<bean id="ehcache"
      class="org.springframework.cache.ehcache.EhCacheManagerFactoryBean" p:config-location="ehcache.xml"/>

This setup bootstraps the ehcache library inside Spring IoC (through the ehcache bean) which is then wired into the dedicated CacheManager implementation. Note the entire ehcache-specific configuration is read from ehcache.xml.

36.6.3 Caffeine Cache

Caffeine is a Java 8 rewrite of Guava’s cache and its implementation is located under org.springframework.cache.caffeine package and provides access to several features of Caffeine.

Configuring a CacheManager that creates the cache on demand is straightforward:

<bean id="cacheManager"
      class="org.springframework.cache.caffeine.CaffeineCacheManager"/>

It is also possible to provide the caches to use explicitly. In that case, only those will be made available by the manager:

<bean id="cacheManager" class="org.springframework.cache.caffeine.CaffeineCacheManager">
    <property name="caches">
        <set>
            <value>default</value>
            <value>books</value>
        </set>
    </property>
</bean>

The Caffeine CacheManager also supports customs Caffeine and CacheLoader. See the Caffeine documentation for more information about those.

36.6.4 Guava Cache

The Guava implementation is located under org.springframework.cache.guava package and provides access to several features of Guava.

Configuring a CacheManager that creates the cache on demand is straightforward:

<bean id="cacheManager"
      class="org.springframework.cache.guava.GuavaCacheManager"/>

It is also possible to provide the caches to use explicitly. In that case, only those will be made available by the manager:

<bean id="cacheManager" class="org.springframework.cache.guava.GuavaCacheManager">
    <property name="caches">
        <set>
            <value>default</value>
            <value>books</value>
        </set>
    </property>
</bean>

The Guava CacheManager also supports customs CacheBuilder and CacheLoader. See the Guava documentation for more information about those.

36.6.5 GemFire-based Cache

GemFire is a memory-oriented/disk-backed, elastically scalable, continuously available, active (with built-in pattern-based subscription notifications), globally replicated database and provides fully-featured edge caching. For further information on how to use GemFire as a CacheManager (and more), please refer to the Spring Data GemFire reference documentation.

36.6.6 JSR-107 Cache

JSR-107 compliant caches can also be used by Spring’s caching abstraction. The JCache implementation is located under org.springframework.cache.jcache package.

Again, to use it, one simply needs to declare the appropriate CacheManager:

<bean id="cacheManager"
      class="org.springframework.cache.jcache.JCacheCacheManager"
      p:cache-manager-ref="jCacheManager"/>

<!-- JSR-107 cache manager setup  -->
<bean id="jCacheManager" .../>

36.6.7 Dealing with caches without a backing store

Sometimes when switching environments or doing testing, one might have cache declarations without an actual backing cache configured. As this is an invalid configuration, at runtime an exception will be thrown since the caching infrastructure is unable to find a suitable store. In situations like this, rather then removing the cache declarations (which can prove tedious), one can wire in a simple, dummy cache that performs no caching - that is, forces the cached methods to be executed every time:

<bean id="cacheManager" class="org.springframework.cache.support.CompositeCacheManager">
    <property name="cacheManagers">
        <list>
            <ref bean="jdkCache"/>
            <ref bean="gemfireCache"/>
        </list>
    </property>
    <property name="fallbackToNoOpCache" value="true"/>
</bean>

The CompositeCacheManager above chains multiple CacheManagers and additionally, through the fallbackToNoOpCache flag, adds a no op cache that for all the definitions not handled by the configured cache managers. That is, every cache definition not found in either jdkCache or gemfireCache (configured above) will be handled by the no op cache, which will not store any information causing the target method to be executed every time.

36.7 Plugging-in different back-end caches

Clearly there are plenty of caching products out there that can be used as a backing store. To plug them in, one needs to provide a CacheManager and Cache implementation since unfortunately there is no available standard that we can use instead. This may sound harder than it is since in practice, the classes tend to be simple adapters that map the caching abstraction framework on top of the storage API as the ehcache classes can show. Most CacheManager classes can use the classes in org.springframework.cache.support package, such as AbstractCacheManager which takes care of the boiler-plate code leaving only the actual mapping to be completed. We hope that in time, the libraries that provide integration with Spring can fill in this small configuration gap.

36.8 How can I set the TTL/TTI/Eviction policy/XXX feature?

Directly through your cache provider. The cache abstraction is…​ well, an abstraction not a cache implementation. The solution you are using might support various data policies and different topologies which other solutions do not (take for example the JDK ConcurrentHashMap) - exposing that in the cache abstraction would be useless simply because there would no backing support. Such functionality should be controlled directly through the backing cache, when configuring it or through its native API.