Spring 框架参考文档(六)-Integration之Cache 抽象_integrationcache

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:

	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.

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 deprecatedorg.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 8, Spring 表达式语言 (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.

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

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

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(value="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) {...}

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

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

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

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(value="book", key="#isbn")
public Book updateBook(ISBN isbn, BookDescriptor descriptor)

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.

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(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 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.

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 an 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)

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 customCacheResolver. 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:

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:

	<cache:annotation-driven/> only looks for @Cacheable/@CachePut/@CacheEvict/@Caching 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 16.2, “The DispatcherServlet” for more information.

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.

	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.

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(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.

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:

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(value="books", cacheResolverFactory=MyCacheResolverFactory.class)
public Book findBook(ISBN isbn)

	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(value="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)

Nothing specific needs to be done to enable the JSR-107 support alongside Spring’s declarative annotation support. Both @EnableCaching and thecache: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.

	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.

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

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

<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.

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.

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.

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

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" .../>

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.