二级缓存机制原理详解

1. 整体架构

MyBatis-Plus二级缓存采用装饰器模式实现，核心组件包括：

‌Cache接口‌：定义缓存基本操作
‌PerpetualCache‌：基础缓存实现（HashMap）
‌装饰器‌：如LruCache、FifoCache等
‌TransactionalCache‌：事务缓存管理器

2. 工作流程

‌初始化阶段‌：
- 解析Mapper XML中的<cache>配置
- 创建基础缓存实例
- 根据配置添加装饰器

‌查询流程‌：

sequenceDiagramparticipant Clientparticipant SqlSessionparticipant Executorparticipant Cacheparticipant DBClient->>SqlSession: 执行查询SqlSession->>Executor: query()Executor->>Cache: 检查缓存alt 缓存命中Cache-->>Executor: 返回缓存结果else 缓存未命中Executor->>DB: 执行查询DB-->>Executor: 返回结果Executor->>Cache: 缓存结果endExecutor-->>SqlSession: 返回结果SqlSession-->>Client: 返回结果

‌更新流程‌：

sequenceDiagramparticipant Clientparticipant SqlSessionparticipant Executorparticipant Cacheparticipant DBClient->>SqlSession: 执行更新SqlSession->>Executor: update()Executor->>DB: 执行SQLDB-->>Executor: 返回影响行数Executor->>Cache: 清除相关缓存Executor-->>SqlSession: 返回结果SqlSession-->>Client: 返回结果

3. 关键实现细节

‌缓存键生成‌：基于Mapper ID + 方法参数 + SQL + 分页等生成唯一键
‌事务支持‌：通过TransactionalCacheManager管理事务提交/回滚时的缓存操作
‌序列化‌：默认使用JVM序列化，可配置为其他序列化方式

生产案例详细实现步骤

案例1：电商商品缓存系统

‌基础配置‌

<!-- mybatis-config.xml -->
<configuration><settings><setting name="cacheEnabled" value="true"/><!-- 配置缓存序列化方式 --><setting name="defaultCacheType" value="com.example.MyCustomCache"/></settings>
</configuration>

‌Mapper配置‌

<!-- ProductMapper.xml -->
<mapper namespace="com.example.mapper.ProductMapper"><cache type="org.mybatis.caches.redis.RedisCache"eviction="LRU"flushInterval="300000"size="1024"readOnly="false"/><select id="selectById" resultType="Product" useCache="true">SELECT * FROM product WHERE id = #{id}</select>
</mapper>

‌服务层实现‌

@Service
public class ProductServiceImpl implements ProductService {@Autowiredprivate ProductMapper productMapper;// 带缓存穿透保护的查询public Product getProductWithCache(Long id) {// 1. 先查缓存Product product = productMapper.selectById(id);if (product != null) {return product;}// 2. 缓存不存在，查数据库product = productMapper.selectFromDb(id);if (product == null) {// 防止缓存穿透：缓存空对象product = new Product();product.setId(id);product.setName("NULL_OBJECT");productMapper.cacheNullObject(product);} else {// 放入缓存productMapper.cacheProduct(product);}return product;}@Transactionalpublic void updateProduct(Product product) {// 1. 更新数据库productMapper.updateById(product);// 2. 清除缓存productMapper.clearCache(product.getId());// 3. 异步重建缓存CompletableFuture.runAsync(() -> {Product freshProduct = productMapper.selectFromDb(product.getId());productMapper.cacheProduct(freshProduct);});}
}

‌自定义Redis缓存实现‌

public class CustomRedisCache implements Cache {private final String id;private final RedisTemplate<String, Object> redisTemplate;public CustomRedisCache(String id) {this.id = id;this.redisTemplate = SpringContextHolder.getBean("redisTemplate");}@Overridepublic String getId() {return this.id;}@Overridepublic void putObject(Object key, Object value) {// 自定义序列化逻辑redisTemplate.opsForValue().set(generateRedisKey(key), serialize(value),30, TimeUnit.MINUTES // 设置TTL);}// 其他方法实现...
}

案例2：多级缓存策略

‌配置多级缓存‌

@Configuration
public class CacheConfig {@Beanpublic Cache productCache() {// 一级缓存：本地缓存（Caffeine）CaffeineCache localCache = new CaffeineCache("localProductCache",Caffeine.newBuilder().maximumSize(1000).expireAfterWrite(5, TimeUnit.MINUTES).build());// 二级缓存：Redis缓存RedisCache redisCache = new RedisCache("redisProductCache");// 构建多级缓存return new MultiLevelCache(localCache, redisCache);}
}

‌多级缓存实现‌

public class MultiLevelCache implements Cache {private final Cache[] caches;public MultiLevelCache(Cache... caches) {this.caches = caches;}@Overridepublic Object getObject(Object key) {// 按顺序查询缓存for (Cache cache : caches) {Object value = cache.getObject(key);if (value != null) {// 填充上级缓存for (Cache upperCache : getUpperCaches(cache)) {upperCache.putObject(key, value);}return value;}}return null;}// 其他方法实现...
}

生产环境注意事项

‌缓存一致性解决方案‌：

使用消息队列实现缓存更新

@RabbitListener(queues = "cache.update.queue")
public void handleCacheUpdate(CacheUpdateMessage message) {if (message.getType().equals("PRODUCT")) {productMapper.clearCache(message.getId());}
}

‌监控指标采集‌：

public class MonitoredCache implements Cache {private final Cache delegate;private final CacheMetrics metrics;@Overridepublic Object getObject(Object key) {long start = System.currentTimeMillis();try {Object value = delegate.getObject(key);metrics.recordHit(value != null);metrics.recordLatency(System.currentTimeMillis() - start);return value;} catch (Exception e) {metrics.recordError();throw e;}}// 其他方法...
}

‌缓存预热策略‌：

@PostConstruct
public void preloadHotProducts() {List<Long> hotProductIds = productMapper.selectHotProductIds();hotProductIds.parallelStream().forEach(id -> {Product product = productMapper.selectById(id);// 主动放入缓存productMapper.cacheProduct(product);});
}

通过以上详细实现，可以构建一个高性能、高可用的二级缓存系统，适用于各种生产环境场景。

下面是springboot用法：


<?xml version="1.0" encoding="UTF-8"?>
<project><parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>3.1.4</version></parent><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><dependency><groupId>com.baomidou</groupId><artifactId>mybatis-plus-boot-starter</artifactId><version>3.5.3.1</version></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-data-redis</artifactId></dependency><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><optional>true</optional></dependency></dependencies>
</project>spring:datasource:url: jdbc:mysql://localhost:3306/testusername: rootpassword: 123456driver-class-name: com.mysql.cj.jdbc.Driverredis:host: localhostport: 6379password: database: 0mybatis-plus:configuration:cache-enabled: true@Configuration
public class RedisCacheConfig {@Beanpublic RedisTemplate<String, Object> redisTemplate(RedisConnectionFactory factory) {RedisTemplate<String, Object> template = new RedisTemplate<>();template.setConnectionFactory(factory);template.setKeySerializer(new StringRedisSerializer());template.setValueSerializer(new GenericJackson2JsonRedisSerializer());return template;}@Beanpublic RedisCacheManager cacheManager(RedisConnectionFactory factory) {RedisCacheConfiguration config = RedisCacheConfiguration.defaultCacheConfig().entryTtl(Duration.ofMinutes(30)).serializeKeysWith(RedisSerializationContext.SerializationPair.fromSerializer(new StringRedisSerializer())).serializeValuesWith(RedisSerializationContext.SerializationPair.fromSerializer(new GenericJackson2JsonRedisSerializer()));return RedisCacheManager.builder(factory).cacheDefaults(config).transactionAware().build();}
}@CacheNamespace(implementation = RedisCache.class, eviction = RedisCache.class)
public interface UserMapper extends BaseMapper<User> {@Options(useCache = true)@Select("SELECT * FROM user WHERE id = #{id}")User selectUserById(Long id);@CacheEvict@Update("UPDATE user SET name=#{name} WHERE id=#{id}")int updateUserName(@Param("id") Long id, @Param("name") String name);
}@Service
public class UserService {@Autowiredprivate UserMapper userMapper;@Cacheable(key = "#id", unless = "#result == null")public User getUserById(Long id) {return userMapper.selectById(id);}@Transactional@CacheEvict(key = "#user.id")public void updateUser(User user) {userMapper.updateById(user);}@CacheEvict(allEntries = true)public void clearAllCache() {// 清空所有缓存}
}@Configuration
@MapperScan("com.example.mapper")
public class MyBatisPlusConfig {@Beanpublic MybatisPlusInterceptor mybatisPlusInterceptor() {MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));return interceptor;}@Beanpublic ConfigurationCustomizer configurationCustomizer() {return configuration -> {configuration.setCacheEnabled(true);configuration.setLocalCacheScope(LocalCacheScope.SESSION);};}
}

@CacheNamespace（MyBatis注解）

作用：在Mapper接口级别声明启用二级缓存
核心属性：
- implementation：指定自定义缓存实现类（默认PerpetualCache）
- eviction：指定缓存淘汰策略（LRU/FIFO等）
- flushInterval：缓存刷新间隔（毫秒）
- size：缓存最大容量
示例：

javaCopy Code

@CacheNamespace(implementation = RedisCache.class, eviction = FifoCache.class, flushInterval = 60000) public interface UserMapper {...}