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.
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 | |
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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. |
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
Note that just like other services in 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. There are two integrations available out of the box, for JDK java.util.concurrent.ConcurrentMap
and EhCache - see Section 29.6, “Plugging-in different back-end caches” for more information on plugging in other cache stores/providers.
For caching declaration, the abstraction provides two Java annotations: @Cacheable
and @CacheEvict
which allow methods
to trigger cache population or cache eviction. Let us take a closer look at each 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 then 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 | |
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All the other caches that do not contain the method will be updated as well even though the cached method was not actually executed. |
@Cacheable({ "books", "isbns" }) public Book findBook(ISBN isbn) {...}
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 0.
If only one param is given, return that instance.
If more the one param is given, return a key computed from the hashes of all parameters.
This approach works well for objects with natural keys as long as the hashCode()
reflects that. If that is not the case then
for distributed or persistent environments, the strategy needs to be changed as the objects hashCode is not preserved.
In fact, depending on the JVM implementation or running conditions, the same hashCode can be reused for different objects, in the same VM instance.
To provide a different default key generator, one needs to implement the org.springframework.cache.KeyGenerator
interface.
Once configured, the generator will be used for each declaration that does not specify its own key generation strategy (see below).
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 favour and read Chapter 8, Spring Expression Language (SpEL):
@Cacheable(value="books", key="#isbn" public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed) @Cacheable(value="books", key="#isbn.rawNumber") public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed) @Cacheable(value="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.
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 conditional
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 then 32:
@Cacheable(value="book", condition="#name.length < 32") public Book findBook(String name)
In addition the conditional
parameter, the unless
parameter can be used to veto the adding of a value to the cache. Unlike
conditional
, unless
SpEL
expressions are evalulated after the method has been called. Expanding
on the previous example - perhaps we only want to cache paperback books:
@Cacheable(value="book", condition="#name.length < 32", unless="#result.hardback") public Book findBook(String name)
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(see next section) computations:
Table 29.1. Cache SpEL available metadata
Name | Location | Description | Example |
---|---|---|---|
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 argument. If for some reason the names are not available (ex: no debug information),
the argument names are also available under the a<#arg> where
#arg stands for the argument index (starting from 0). | ibanor a0(one can also use p0or 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 and 'cache evict '
expression (when beforeInvocation is false ). | #result |
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 then method flow optimization.
Note that using @CachePut
and @Cacheable
annotations on the same method is generally 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 declarations should be avoided.
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 one to specify one (or multiple) caches that are affected by the action, allows a
key 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(value = "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 entires 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/update data into the cache and thus requires a result.
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. Unfortunately Java does not support such declarations however there is a workaround - using a enclosing annotation, in this case,
@Caching
. @Caching
allows multiple nested @Cacheable
, @CachePut
and @CacheEvict
to be used on the same method:
@Caching(evict = { @CacheEvict("primary"), @CacheEvict(value = "secondary", key = "#p0") }) public Book importBooks(String deposit, Date date)
It is important to note that even though declaring the cache annotations does not automatically triggers 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 then 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
:
Table 29.2. Cache annotation settings
XML Attribute | Annotation Attribute | Default | Description |
---|---|---|---|
cache-manager | N/A (See CachingConfigurer Javadoc) | cacheManager | Name of cache manager to use. Only required
if the name of the cache manager is not
|
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 |
order | order | Ordered.LOWEST_PRECEDENCE | Defines the order of the cache advice that
is applied to beans annotated with
|
Note | |
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|
Tip | |
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Spring recommends that you only annotate concrete classes (and
methods of concrete classes) with the
|
Note | |
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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
|
The caching abstraction allows one to use her own annotations to identify what method trigger 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, both @Cacheable
and @CacheEvict
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(value=“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(value="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.
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 9, 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).
Out of the box, the cache abstraction provides integration with two storages - one on top of the JDK ConcurrentMap
and one
for ehcache library. To use them, one needs to simply declare an appropriate CacheManager
- an entity that controls and manages
Cache
s and can be used to retrieve these for storage.
The JDK-based Cache
implementation resides under org.springframework.cache.concurrent
package. It allows one to use
ConcurrentHashMap
as a backing Cache
store.
<!-- generic 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 Concurrent
Cache
implementations 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.
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 ehcache library inside Spring IoC (through bean ehcache
) which is then wired into the dedicated CacheManager
implementation. Note the entire ehcache-specific configuration is read from the resource ehcache.xml
.
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 GemFire reference documentation.
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 through 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 CacheManager
s 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.
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 then 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.
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.