Spring 3.1 M1 is out with some very ​​useful features​​. One of the coolest feature in the latest release is comprehensive Caching support!

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.

The cache is applied to Java methods, reducing the number of executions based on the information available. Spring checks if the given method is already executed for given set of parameters. If the method is already executed, Spring uses the cache value and returns it to caller instead of calling the method again. This is a write through cache. 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.

## Adding Cache support to Spring project

In order to add Cache support to any Spring based project, one needs to declare the configuration using new Spring tag in the schema declaration.

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 12345678 ​​<​​​​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​​ ​​/>​​​​...​​​​​​

Note the ​​cache:annotation-driven​​ tag in above declaration enables the caching in given Spring project.

## Using @Cacheable and @CacheEvict annotations

Spring 3.1 M1 provides two very useful Java annotations: ​​@Cacheable​​ and ​​@CacheEvict​​ which allow methods to trigger cache population or cache eviction. Let us take a closer look at each annotation:

### @Cacheable annotation

This annotation mark a method cacheable. Thus the result from this method call will be stored into the cache on subsequent invocations with same arguments.

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 12 ​​@Cacheable​​​​(​​​​"persons"​​​​)​​​​public​​ ​​Person profile(Long personId) { ... }​​

In the above code snippet, method ​​profile​​ is marked cacheable using ​​@Cacheable​​ annotation. Also the method is associated with a cache named “​​persons​​“. Whenever method profile is called, the Spring framework will check if cached entry is available in persons cache and returns the same without calling profile method.

It is also possible to provide multiple cache names if you have multiple caches declared in your application. For example:

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 12 ​​@Cacheable​​​​({​​​​"persons"​​​​, ​​​​"profiles"​​​​})​​​​public​​ ​​Person profile(Long personId) { ... }​​

In above code snippet, we provide two cache names persons and profiles. Spring framework will check in all the caches if entry is available for given method call with argument personId, if at least one cache is hit, then the associated value will be returned.

### @CacheEvict annotation

Cache eviction is removing of any unused or stale 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):

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 12 ​​@CacheEvict​​ ​​(value = ​​​​"persons"​​​​, allEntries=​​​​true​​​​)​​​​public​​ ​​List listPersons()​​

This annotation is very useful when an entire cache region needs to be cleared out. The Spring framework will ignore any key specified in this scenario as it does not apply.

### Using Default key

The cache is nothing but a key-value store which stores the data based on certain key. In Spring framework based caching, the method arguments of cached method acts as the source of Key generation. Every key is essentially the Hash-code of these arguments. 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.

By default, all the method arguments are used in Key generation logic. In practice not all methods have only one argument or, worse yet, the parameters are not suitable as cache keys – take for example a variation of the method above:

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 12 ​​@Cacheable​​​​(value=​​​​"persons"​​​​, key=​​​​"personId"​​​​)​​​​public​​ ​​Person profile(Long personId, Long groundId) { ... }​​

Here we are using just ​​personId​​ in key generation ignoring groupId altogether.

### Understand Conditional caching

Spring framework also supports conditional caching letting user to cache certain methods based on some conditions. For example, in following code snippet we cache profiles only for those users who have profileId greater than 50:

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 12 ​​@Cacheable​​​​(value=​​​​"persons"​​​​, condition=​​​​"personId > 50"​​​​)​​​​public​​ ​​Person profile(Long personId) { ... }​​

## Currently supported libraries

There are probably hundreds of cache libraries available which can be used in your JEE project. For now the Spring framework supports following implementations:

1. JDK ConcurrentMap based Cache
2. Ehcache based Cache

### 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.

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 123456789 ​​​​​​<​​​​bean​​ ​​id​​​​=​​​​"cacheManager"​​ ​​class​​​​=​​​​"org.springframework.cache.support.SimpleCacheManager"​​​​>​​​​<​​​​property​​ ​​name​​​​=​​​​"caches"​​​​>​​​​<​​​​set​​​​>​​​​<​​​​bean​​ ​​class​​​​=​​​​"org.springframework.cache.concurrent.ConcurrentCacheFactoryBean"​​ ​​p:name​​​​=​​​​"default"​​​​/>​​​​<​​​​bean​​ ​​class​​​​=​​​​"org.springframework.cache.concurrent.ConcurrentCacheFactoryBean"​​ ​​p:name​​​​=​​​​"persons"​​​​/>​​​​​​​​​​​​​​

In above code snippet, we use ​​SimpleCacheManager​​ class to create a ​​CacheManager​​. Note that we have created two caches in our application, one is ​​default​​ and second is ​​persons​​.

### 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​​:

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 1234 ​​<​​​​bean​​ ​​id​​​​=​​​​"cacheManager"​​ ​​class​​​​=​​​​"org.springframework.cache.ehcache.EhcacheCacheManager"​​ ​​p:cache-manager​​​​=​​​​"ehcache"​​​​/>​​ ​​​​​​<​​​​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.