SpringBoot Cache缓存概念讲解
作者：wenxueliu@HW
Spring 3.1中引入了基于注解的Cache的支持，在spring-context包中定义了org.springframework.cache.CacheManager和org.springframework.cache.Cache接口，用来统一不同的缓存的技术。
CacheManager是Spring提供的各种缓存技术管理的抽象接口，而Cache接口包含缓存的增加、删除、读取等常用操作。针对CacheManager，Spring又提供了多种实现，比如基于Collection来实现的SimpleCacheManager、基于ConcurrentHashMap实现的ConcurrentMapCacheManager、基于EhCache实现的EhCacheCacheManager和基于JCache标准实现的JCacheCacheManager等。
Spring Cache提供了@CacheConfig、@Cacheable、@CachePut、@CacheEvict等注解来完成缓存的透明化操作，相关功能如下。
- @CacheConfig：用于类上，缓存一些公共设置。
- @Cacheable：用于方法上，根据方法的请求参数对结果进行缓存，下次读取时直接读取缓存内容。
- @CachePut：用于方法上，能够根据方法的请求参数对其结果进行缓存，和@Cacheable不同的是，它每次都会触发真实方法的调用
- @CacheEvict：用于方法上，清除该方法的缓存，用在类上清除整个类的方法的缓存。在了解了Spring Cache的基本作用的和定义之后，下面来看在Spring Boot中是如何对Cache进行自动配置的
Cache自动配置
在Spring Boot中，关于Cache的默认自动配置类只有CacheAutoConfiguration，主要用于缓存抽象的自动配置，当通过@EnableCaching启用缓存机制时，根据情况可创建CacheManager。对于缓存存储可以通过配置自动检测或明确指定。CacheAutoConfiguration同样在META-INF/spring.factories文件中配置注册。
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
下面先来看CacheAutoConfiguration类的注解部分代码实现。
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(CacheManager.class)
@ConditionalOnBean(CacheAspectSupport.class)
@ConditionalOnMissingBean(value = CacheManager.class, name = "cacheResolver")
@EnableConfigurationProperties(CacheProperties.class)
@AutoConfigureAfter({ CouchbaseAutoConfiguration.class, HazelcastAutoConfiguration.class,
HibernateJpaAutoConfiguration.class, RedisAutoConfiguration.class })
@Import(CacheConfigurationImportSelector.class)
public class CacheAutoConfiguration {
...
}
1、@ConditionalOnClass 指定需要在classpath下存在CacheManager类。
2、@ConditionalOnBean 指定需要存在CacheAspectSupport的Bean时才生效，换句话说，就是需要在使用了@EnableCaching时才有效。这是因为该注解隐式的导致了CacheInterceptor对应的Bean的初始化，而CacheInterceptor为CacheAspectSupport的子类。
3、@ConditionalOnMissingBean指定名称为cacheResolver的CacheManager对象不存在时生效。@EnableConfigurationProperties加载缓存的CacheProperties配置项，配置前缀为spring.cache。
4、@EnableConfigurationProperties加载缓存的CacheProperties配置项，配置前缀为spring.cache。@AutoConfigureAfter指定该自动配置必须在缓存数据基础组件自动配置之后进行，这里包括Couchbase、Hazelcast、HibernateJpa和Redis的自动配置。
5、@Import导入CacheConfigurationImportSelector，其实是导入符合条件的Spring Cache使用的各类基础缓存框架（或组件）的配置。
ImportSelector
static class CacheConfigurationImportSelector implements ImportSelector {
@Override
public String[] selectImports(AnnotationMetadata importingClassMetadata) {
CacheType[] types = CacheType.values();
String[] imports = new String[types.length];
for (int i = 0; i < types.length; i++) {
imports[i] = CacheConfigurations.getConfigurationClass(types[i]);
}
return imports;
}
}
导入类的获取是通过实现ImportSelector接口来完成的，具体获取步骤位于selectImports方法中。该方法中，首先获取枚举类CacheType中定义的缓存类型数据，CacheType中定义支持的缓存类型如下。
// 支持的缓存类型（按照优先级定义）
public enum CacheType {
// 使用上下文中的Cache Bean进行通用缓存
GENERIC,
// JCache（JSR-107）支持的缓存
JCACHE,
// EhCache支持的缓存
EHCACHE,
// Hazelcast支持的缓存
HAZELCAST,
// Infinispan支持的缓存
INFINISPAN,
// Couchbase支持的缓存
COUCHBASE,
// Redis支持的缓存
REDIS,
// Caffeine支持的缓存
CAFFEINE,
// 内存基本的简单缓存
SIMPLE,
// 不支持缓存
NONE
}
枚举类CacheType中定义了以上支持的缓存类型，而且上面的缓存类型默认是按照优先级从前到后的顺序排列的。selectImports方法中，当获取CacheType中定义的缓存类型数组之后，遍历该数组并通过CacheConfigurations的getConfigurationClass方法获得每种类型缓存对应的自动配置类（注解@Configuration的类）。CacheConfigurations相关代码如下。
final class CacheConfigurations {
private static final Map<CacheType, Class<?>> MAPPINGS;
// 定义CacheType与@Configuration之间的对应关系
static {
Map<CacheType, Class<?>> mappings = new EnumMap<>(CacheType.class);
mappings.put(CacheType.GENERIC, GenericCacheConfiguration.class);
mappings.put(CacheType.EHCACHE, EhCacheCacheConfiguration.class);
mappings.put(CacheType.HAZELCAST, HazelcastCacheConfiguration.class);
mappings.put(CacheType.INFINISPAN, InfinispanCacheConfiguration.class);
mappings.put(CacheType.JCACHE, JCacheCacheConfiguration.class);
mappings.put(CacheType.COUCHBASE, CouchbaseCacheConfiguration.class);
mappings.put(CacheType.REDIS, RedisCacheConfiguration.class);
mappings.put(CacheType.CAFFEINE, CaffeineCacheConfiguration.class);
mappings.put(CacheType.SIMPLE, SimpleCacheConfiguration.class);
mappings.put(CacheType.NONE, NoOpCacheConfiguration.class);
MAPPINGS = Collections.unmodifiableMap(mappings);
}
// 根据CacheType类型获得对应的@Configuration类
static String getConfigurationClass(CacheType cacheType) {
Class<?> configurationClass = MAPPINGS.get(cacheType);
Assert.state(configurationClass != null, () -> "Unknown cache type " + cacheType);
return configurationClass.getName();
}
...
}我们会发现通过@Import注解，CacheAutoConfiguration导入了CacheType中定义的所有类型的自动配置，也就是Spring Boot目前支持的缓存类型。而具体会自动配置哪种类型的缓存，还需要看导入的自动配置类里面的生效条件。
GenericCacheConfiguration
// 实例化CacheManagerCustomizers
@Bean
@ConditionalOnMissingBean
public CacheManagerCustomizers cacheManagerCustomizers(
ObjectProvider<CacheManagerCustomizer<?>> customizers) {
return new CacheManagerCustomizers(
customizers.orderedStream().collect(Collectors.toList()));
}
cacheManagerCustomizers方法初始化了CacheManagerCustomizers对象的Bean，主要是将容器中存在的一个或多个CacheManagerCustomizer的Bean组件包装为CacheManager-Customizers，并将Bean注入容器。
CacheManagerValidator
cacheAutoConfigurationValidator方法初始化了CacheManagerValidator的Bean，该Bean用于确保容器中存在一个CacheManager对象，以达到缓存机制可以继续被配置和使用的目的，同时该Bean也用来提供有意义的异常声明。
// 实例化CacheManagerValidator
@Bean
public CacheManagerValidator cacheAutoConfigurationValidator(CacheProperties-
cacheProperties,
ObjectProvider<CacheManager> cacheManager) {
return new CacheManagerValidator(cacheProperties, cacheManager);
}
// CacheManagerValidator的具体定义，用于检查并抛出有意义的异常
static class CacheManagerValidator implements InitializingBean {
private final CacheProperties cacheProperties;
private final ObjectProvider<CacheManager> cacheManager;
CacheManagerValidator(CacheProperties cacheProperties, ObjectProvider<Cache-
Manager> cacheManager) {
this.cacheProperties = cacheProperties;
this.cacheManager = cacheManager;
}
@Override
public void afterPropertiesSet() {
Assert.notNull(this.cacheManager.getIfAvailable(),
() -> "No cache manager could be auto-configured, check your configuration (caching " + "type is '" + this.cacheProperties.getType() + "')");
}
}
Redis Cache
RedisCacheConfiguration
我们以RedisCacheConfiguration为例进行分析。源码如下
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(RedisConnectionFactory.class)
@AutoConfigureAfter(RedisAutoConfiguration.class)
@ConditionalOnBean(RedisConnectionFactory.class)
@ConditionalOnMissingBean(CacheManager.class)
@Conditional(CacheCondition.class)
class RedisCacheConfiguration {
@Bean
RedisCacheManager cacheManager(CacheProperties cacheProperties, CacheManagerCustomizers cacheManagerCustomizers,
ObjectProvider<org.springframework.data.redis.cache.RedisCacheConfiguration> redisCacheConfiguration,
ObjectProvider<RedisCacheManagerBuilderCustomizer> redisCacheManagerBuilderCustomizers,
RedisConnectionFactory redisConnectionFactory, ResourceLoader resourceLoader) {
RedisCacheManagerBuilder builder = RedisCacheManager.builder(redisConnectionFactory).cacheDefaults(
determineConfiguration(cacheProperties, redisCacheConfiguration, resourceLoader.getClassLoader()));
List<String> cacheNames = cacheProperties.getCacheNames();
if (!cacheNames.isEmpty()) {
builder.initialCacheNames(new LinkedHashSet<>(cacheNames));
}
redisCacheManagerBuilderCustomizers.orderedStream().forEach((customizer) -> customizer.customize(builder));
return cacheManagerCustomizers.customize(builder.build());
}
private org.springframework.data.redis.cache.RedisCacheConfiguration determineConfiguration(
CacheProperties cacheProperties,
ObjectProvider<org.springframework.data.redis.cache.RedisCacheConfiguration> redisCacheConfiguration,
ClassLoader classLoader) {
return redisCacheConfiguration.getIfAvailable(() -> createConfiguration(cacheProperties, classLoader));
}
private org.springframework.data.redis.cache.RedisCacheConfiguration createConfiguration(
CacheProperties cacheProperties, ClassLoader classLoader) {
Redis redisProperties = cacheProperties.getRedis();
org.springframework.data.redis.cache.RedisCacheConfiguration config = org.springframework.data.redis.cache.RedisCacheConfiguration
.defaultCacheConfig();
config = config.serializeValuesWith(
SerializationPair.fromSerializer(new JdkSerializationRedisSerializer(classLoader)));
if (redisProperties.getTimeToLive() != null) {
config = config.entryTtl(redisProperties.getTimeToLive());
}
if (redisProperties.getKeyPrefix() != null) {
config = config.prefixCacheNameWith(redisProperties.getKeyPrefix());
}
if (!redisProperties.isCacheNullValues()) {
config = config.disableCachingNullValues();
}
if (!redisProperties.isUseKeyPrefix()) {
config = config.disableKeyPrefix();
}
return config;
}
}@ConditionalOnClass：当 RedisConnectionFactory 在存在时
@AutoConfigureAfter：在 RedisAutoConfiguration 之后加载
@ConditionalOnBean：RedisConnectionFactory 实例化之后
@ConditionalOnMissingBean：当 CacheManager 的 Bean 不存在时进行实例化操作
@Conditional：满足 CacheCondition 条件时，进行实例化操作
CacheCondition
class CacheCondition extends SpringBootCondition {
@Override
public ConditionOutcome getMatchOutcome(ConditionContext context, Annotated-
TypeMetadata metadata) {
String sourceClass = "";
if (metadata instanceof ClassMetadata) {
sourceClass = ((ClassMetadata) metadata).getClassName();
}
ConditionMessage.Builder message = ConditionMessage.forCondition("Cache",
sourceClass);
Environment environment = context.getEnvironment();
try {
// 创建指定环境的Binder，然后绑定属性到对象上
BindResult<CacheType> specified = Binder.get(environment).bind("spring.
cache.type", CacheType.class);
// 如果未绑定，则返回匹配
if (!specified.isBound()) {
return ConditionOutcome.match(message.because("automatic cache type"));
}
// 获取所需的缓存类型
CacheType required = CacheConfigurations.getType(((AnnotationMetadata)
metadata).getClassName());
// 如果已绑定，并且绑定的类型与所需的缓存类型相同，则返回匹配
if (specified.get() == required) {
return ConditionOutcome.match(message.because(specified.get() + " cache type"));
}
} catch (BindException ex) {
}
// 其他情况则返回不匹配
return ConditionOutcome.noMatch(message.because("unknown cache type"));
}
}CacheCondition的核心逻辑就是首先通过Binder进行指定属性和类的绑定，然后通过绑定结果（BindResult）进行判断：如果判断结果是未绑定，则直接返回条件匹配；否则，判断绑定的缓存类型与所需的缓存类型是否相等，如果相等则返回条件匹配；其他情况则返回条件不匹配。
RedisCacheManager
1、管理多个 RedisCache
2、每个 RedisCache 包含一个 name
3、每个 RedisCache 可以包含一个 RedisCacheConfiguration（可以有默认配置）
4、支持配置是否动态增加 Cache
public abstract class AbstractCacheManager implements CacheManager, InitializingBean {
private final ConcurrentMap<String, Cache> cacheMap = new ConcurrentHashMap<>(16);
private volatile Set<String> cacheNames = Collections.emptySet();
}
public class RedisCacheManager extends AbstractTransactionSupportingCacheManager {
private final RedisCacheWriter cacheWriter;
private final RedisCacheConfiguration defaultCacheConfig;
private final Map<String, RedisCacheConfiguration> initialCacheConfiguration;
private final boolean allowInFlightCacheCreation;
protected Collection<RedisCache> loadCaches() {
List<RedisCache> caches = new LinkedList();
Iterator var2 = this.initialCacheConfiguration.entrySet().iterator();
while(var2.hasNext()) {
Entry<String, RedisCacheConfiguration> entry = (Entry)var2.next();
caches.add(this.createRedisCache((String)entry.getKey(), (RedisCacheConfiguration)entry.getValue()));
}
return caches;
}
protected RedisCache getMissingCache(String name) {
return this.allowInFlightCacheCreation ? this.createRedisCache(name, this.defaultCacheConfig) : null;
}
public Map<String, RedisCacheConfiguration> getCacheConfigurations() {
Map<String, RedisCacheConfiguration> configurationMap = new HashMap(this.getCacheNames().size());
this.getCacheNames().forEach((it) -> {
RedisCache cache = (RedisCache)RedisCache.class.cast(this.lookupCache(it));
configurationMap.put(it, cache != null ? cache.getCacheConfiguration() : null);
});
return Collections.unmodifiableMap(configurationMap);
}
protected RedisCache createRedisCache(String name, @Nullable RedisCacheConfiguration cacheConfig) {
return new RedisCache(name, this.cacheWriter, cacheConfig != null ? cacheConfig : this.defaultCacheConfig);
}
}RedisCache
public class RedisCache extends AbstractValueAdaptingCache {
private static final byte[] BINARY_NULL_VALUE;
private final String name;
private final RedisCacheWriter cacheWriter;
private final RedisCacheConfiguration cacheConfig;
private final ConversionService conversionService;
}
RedisCache 的特点
1、key 支持非 String 类型，比如 Map 和 Collection
2、RedisCacheWriter 是更底层的实现，RedisCache 对 RedisCacheWriter 进行封装。
3、通过 ConversionService 对 key 进行转换
RedisCacheWriter
通过 RedisConnectionFactory 取一个 RedisConnection，然后执行命令。putIfAbsent 支持分布式锁。
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