mybatis开启二级缓存

例子

mybatis-config.xml

<configuration><settings><setting name="cacheEnabled" value="true"/></settings><environments default="development"><environment id="development"><transactionManager type="JDBC"/><dataSource type="POOLED"><property name="driver" value="com.mysql.cj.jdbc.Driver"/><property name="url" value="jdbc:mysql://localhost:3306/test"/><property name="username" value="xxx"/><property name="password" value="xxx"/></dataSource></environment></environments><mappers><mapper resource="mapper/UserMapper.xml"/></mappers>
</configuration>

UserMapper.xml
要加上"<cache></cache>"

<mapper namespace="com.db.dbdemo.mapper.UserMapper"><cache></cache><select id="selectAll" resultType="com.db.dbdemo.entity.User" >select user_id, name, age, gender, id_type, id_number from user</select>
</mapper>

@Testpublic void testCache() {Configuration configuration = sqlSessionFactory.getConfiguration();Cache cache = configuration.getCache("com.db.dbdemo.mapper.UserMapper");cache.putObject("hah", 1);cache.getObject("hah");}

输出：

mybatis cache涉及以下几个类：
Cache接口：

public interface Cache {/*** @return The identifier of this cache*/String getId();/*** @param key*          Can be any object but usually it is a {@link CacheKey}* @param value*          The result of a select.*/void putObject(Object key, Object value);/*** @param key*          The key* @return The object stored in the cache.*/Object getObject(Object key);/*** As of 3.3.0 this method is only called during a rollback* for any previous value that was missing in the cache.* This lets any blocking cache to release the lock that* may have previously put on the key.* A blocking cache puts a lock when a value is null* and releases it when the value is back again.* This way other threads will wait for the value to be* available instead of hitting the database.*** @param key*          The key* @return Not used*/Object removeObject(Object key);/*** Clears this cache instance.*/void clear();/*** Optional. This method is not called by the core.** @return The number of elements stored in the cache (not its capacity).*/int getSize();/*** Optional. As of 3.2.6 this method is no longer called by the core.* <p>* Any locking needed by the cache must be provided internally by the cache provider.** @return A ReadWriteLock*/default ReadWriteLock getReadWriteLock() {return null;}}

其下的实现类及装饰类

• SynchronizedCache：同步Cache，实现比较简单，直接使用synchronized修饰方法。
• LoggingCache：日志功能，装饰类，用于记录缓存的命中率，如果开启了DEBUG模式，则会输出命中率日志。
• SerializedCache：序列化功能，将值序列化后存到缓存中。该功能用于缓存返回一份实例的Copy，用于保存线程安全。
• LruCache：采用了Lru算法的Cache实现，移除最近最少使用的Key/Value。
• PerpetualCache： 作为为最基础的缓存类，底层实现比较简单，直接使用了HashMap。