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Declarative Annotation-based Caching

For caching declaration, Spring’s caching 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.

The `@Cacheable` Annotation

As the name implies, you can use @Cacheable to demarcate methods that are cacheable — that is, methods for which the result is stored in the cache so that, 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, as the following example shows:

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

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

All the other caches that do not contain the value are also updated, even though the cached method was not actually executed.

The following example uses @Cacheable on the findBook method:

@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. 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 than one param is given, return a SimpleKey that contains 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, you need to change the strategy.

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

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, considered only 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, the target methods are quite likely to have various signatures that cannot be readily 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). Consider the following 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. Furthermore, what if only one of the two is important while the other is not?

For such cases, the @Cacheable annotation lets you specify how the key is generated through its key attribute. You 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 works for all methods.

The following examples ise various SpEL declarations (if you are not familiar with SpEL, do yourself a favor and read Spring Expression Language):

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

If the algorithm responsible for generating the key is too specific or if it needs to be shared, you can define a custom keyGenerator on the operation. To do so, specify the name of the KeyGenerator bean implementation to use, as the following example shows:

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

The key and keyGenerator parameters are mutually exclusive and an operation that specifies both results in an exception.

Default Cache Resolution

The caching abstraction uses a simple CacheResolver that retrieves the caches defined at the operation level by using the configured CacheManager.

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

Custom Cache Resolution

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

For applications that work with several cache managers, you can set the cacheManager to use for each operation, as the following example shows:

@Cacheable(cacheNames="books", cacheManager="anotherCacheManager") (1)
public Book findBook(ISBN isbn) {...}

1	Specifying `anotherCacheManager`.

You can also replace the CacheResolver entirely in a fashion similar to that of replacing key generation. The resolution is requested for every cache operation, letting the implementation actually resolve the caches to use based on runtime arguments. The following example shows how to specify a CacheResolver:

@Cacheable(cacheResolver="runtimeCacheResolver") (1)
public Book findBook(ISBN isbn) {...}

1	Specifying the `CacheResolver`.

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 results in an exception. as a custom CacheManager is 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, you can use the sync attribute to instruct the underlying cache provider to lock the cache entry while the value is being computed. As a result, only one thread is busy computing the value, while the others are blocked until the entry is updated in the cache. The following example shows how to use the sync attribute:

@Cacheable(cacheNames="foos", sync=true) (1)
public Foo executeExpensiveOperation(String id) {...}

1	Using the `sync` attribute.

This is an optional feature, and your favorite cache library may not support it. All CacheManager implementations provided by the core framework support it. See 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, the method is executed every time no matter what values are in the cache or what arguments are used). For example, the following method is cached only if the argument name has a length shorter than 32:

@Cacheable(cacheNames="book", condition="#name.length() < 32") (1)
public Book findBook(String name)

1	Setting a condition on `@Cacheable`.

In addition to the condition parameter, you can use the unless parameter to veto the adding of a value to the cache. Unlike condition, unless expressions are evaluated after the method has been called. To expand on the previous example, perhaps we only want to cache paperback books, as the following example does:

@Cacheable(cacheNames="book", condition="#name.length() < 32", unless="#result.hardback") (1)
public Book findBook(String name)

1	Using the `unless` attribute to block hardbacks.

The cache abstraction supports java.util.Optional, using its content as the cached value only if it is present. #result always refers to the business entity and never a supported wrapper, so the previous example can be rewritten as follows:

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

Note that result still refers to Book and not Optional. As it might be null, we should use the safe navigation operator.

Available Caching SpEL Evaluation Context

Each SpEL expression evaluates against a dedicated context. In addition to the built-in parameters, the framework provides dedicated caching-related metadata, such as the argument names. The following table describes the items made available to the context so that you can use them for key and conditional computations:

Table 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 arguments. If the names are not available (perhaps due to having 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` (you 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`). For supported wrappers (such as `Optional`), `#result` refers to the actual object, not the wrapper.	`#result`

The `@CachePut` Annotation

When the cache needs to be updated without interfering with the method execution, you can use the @CachePut annotation. That is, the method is always executed and its result is 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. The following example uses the @CachePut annotation:

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

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 declarations should be avoided. Note also that such conditions should not rely on the result object (that is, the #result variable), as these are validated up-front to confirm the exclusion.

The `@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. As opposed to @Cacheable, @CacheEvict demarcates methods that perform cache eviction (that is, methods that act as triggers for removing data from the cache). Similarly to its sibling, @CacheEvict requires specifying one or more caches that are affected by the action, allows a custom cache and key resolution or a condition to be specified, and features an extra parameter (allEntries) that indicates whether a cache-wide eviction needs to be performed rather than just an entry eviction (based on the key). The following example evicts all entries from the books cache:

@CacheEvict(cacheNames="books", allEntries=true) (1)
public void loadBooks(InputStream batch)

1	Using the `allEntries` attribute to evict all entries from the cache.

This option comes in handy when an entire cache region needs to be cleared out. Rather than evicting each entry (which would take a long time, since it is inefficient), all the entries are removed in one operation, as the preceding example shows. Note that the framework ignores any key specified in this scenario as it does not apply (the entire cache is evicted, not only one entry).

You can also indicate whether the eviction should occur after (the default) or before the method executes by using 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 always occur before the method is invoked. This is useful in cases where the eviction does not need to be tied to the method outcome.

Note that void methods can be used with @CacheEvict - as the methods act as a trigger, the return values are ignored (as they do not interact with the cache). This is not the case with @Cacheable which adds or updates data into the cache and, thus, requires a result.

The `@Caching` Annotation

Sometimes, 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 lets multiple nested @Cacheable, @CachePut, and @CacheEvict annotations be used on the same method. The following example uses two @CacheEvict annotations:

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

The `@CacheConfig` annotation

So far, we have seen that caching operations offer many customization options and that you can set these options for each operation. 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 can be replaced by a single class-level definition. This is where @CacheConfig comes into play. The following examples uses @CacheConfig to set the name of the cache:

@CacheConfig("books") (1)
public class BookRepositoryImpl implements BookRepository {

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

1	Using `@CacheConfig` to set the name of the cache.

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

An operation-level customization always overrides a customization set on @CacheConfig. Therefore, this gives three levels of customizations for each cache operation:

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

Enabling 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 you can 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 https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/cache https://www.springframework.org/schema/cache/spring-cache.xsd">

		<cache:annotation-driven/>
</beans>

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

The default advice mode for processing caching annotations is proxy, which allows for interception of calls through the proxy only. Local calls within the same class cannot get intercepted that way. For a more advanced mode of interception, consider switching to aspectj mode in combination with compile-time or load-time weaving.

For more detail about advanced customizations (using Java configuration) that are required to implement CachingConfigurer, see the javadoc.

Table 2. Cache annotation settings
XML Attribute	Annotation Attribute	Default	Description
`cache-manager`	N/A (see the `CachingConfigurer` javadoc)	`cacheManager`	The name of the cache manager to use. A default `CacheResolver` is 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 the `CachingConfigurer` javadoc)	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 needs to be specified only as an alternative to the 'cache-manager' attribute.
`key-generator`	N/A (see the `CachingConfigurer` javadoc)	`SimpleKeyGenerator`	Name of the custom key generator to use.
`error-handler`	N/A (see the `CachingConfigurer` javadoc)	`SimpleCacheErrorHandler`	The name of the custom cache error handler to use. By default, any exception thrown during a cache related operation is thrown back at the client.
`mode`	`mode`	`proxy`	The default mode (`proxy`) processes annotated beans to be proxied by using Spring’s AOP framework (following proxy semantics, as discussed earlier, 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 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`, class-based proxies are created. If `proxy-target-class` is `false` or if the attribute is omitted, standard JDK interface-based proxies are created. (See 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 AOP advice, see Advice Ordering.) No specified ordering means that the AOP subsystem determines the order of the advice.

<cache:annotation-driven/> looks for @Cacheable/@CachePut/@CacheEvict/@Caching only on beans in the same application context in which it is defined. This means that, if you put <cache:annotation-driven/> in a WebApplicationContext for a DispatcherServlet, it checks for beans only in your controllers, not your services. See the MVC section for more information.

Method visibility and cache annotations

When you use proxies, you should apply the cache annotations only to methods with public visibility. If you do annotate protected, private, or package-visible methods with these annotations, no error is raised, but the annotated method does not exhibit the configured caching settings. Consider using AspectJ (see the rest of this section) if you need to annotate non-public methods, as it changes the bytecode itself.

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 use interface-based proxies. The fact that Java annotations are not inherited from interfaces means that, if you use class-based proxies (proxy-target-class="true") or the weaving-based aspect (mode="aspectj"), the caching settings are not recognized by the proxying and weaving infrastructure, and the object is not wrapped in a caching proxy.

In proxy mode (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 that calls another method of the target object) does not lead to actual caching at runtime even if the invoked method is marked with @Cacheable. Consider using the aspectj mode in this case. Also, the proxy must be fully initialized to provide the expected behavior, so you should not rely on this feature in your initialization code (that is, @PostConstruct).

Using Custom Annotations

Custom annotation and AspectJ

This feature works only with the proxy-based approach but can be enabled with a bit of extra effort by using AspectJ.

The spring-aspects module defines an aspect for the standard annotations only. If you have defined your own annotations, you also need to define an aspect for those. Check AnnotationCacheAspect for an example.

The caching abstraction lets you 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, which is especially useful if the key or condition are specified or if the foreign imports (org.springframework) are not allowed in your code base. Similarly to the rest of the stereotype annotations, you can use @Cacheable, @CachePut, @CacheEvict, and @CacheConfig as meta-annotations (that is, annotations that can annotate other annotations). In the following example, we replace a common @Cacheable declaration with our own custom annotation:

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

In the preceding example, 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)

The following example shows the custom annotation with which we can replace the preceding code:

@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 earlier, annotation-driven behavior needs to be enabled.