对于一名程序员，缓存真的很重要，而且缓存真的是老生常谈的一个话题拉。因为它在我们的开发过程中真的是无处不在。今天LZ带大家来看一下。Mybatis是怎么实现一级缓存和二级缓存的。(自带的缓存机制)

一级缓存

存在BaseExecutor中，是全局的缓存，每次查询后将值存入BaseExecutor的localCache中。key是由ms,parameter,rowBounds和boundSql一起生成的一个值。value就是查询出来的结果。一旦有任何更新变动，就删除整个localCache。

  @Override//生成一级缓存的key的函数，有兴趣的看看，LZ不详细解释了，不是重点。public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {if (closed) {throw new ExecutorException("Executor was closed.");}CacheKey cacheKey = new CacheKey();cacheKey.update(ms.getId());cacheKey.update(rowBounds.getOffset());cacheKey.update(rowBounds.getLimit());cacheKey.update(boundSql.getSql());List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();// mimic DefaultParameterHandler logicfor (ParameterMapping parameterMapping : parameterMappings) {if (parameterMapping.getMode() != ParameterMode.OUT) {Object value;String propertyName = parameterMapping.getProperty();if (boundSql.hasAdditionalParameter(propertyName)) {value = boundSql.getAdditionalParameter(propertyName);} else if (parameterObject == null) {value = null;} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {value = parameterObject;} else {MetaObject metaObject = configuration.newMetaObject(parameterObject);value = metaObject.getValue(propertyName);}cacheKey.update(value);}}if (configuration.getEnvironment() != null) {// issue #176cacheKey.update(configuration.getEnvironment().getId());}return cacheKey;}
//查询数据库并存入一级缓存的语句private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {List<E> list;//先放一个值占位localCache.putObject(key, EXECUTION_PLACEHOLDER);try {//查询数据list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);} finally {//删除占位的localCache.removeObject(key);}//将查询出来的结果存入一级缓存localCache.putObject(key, list);if (ms.getStatementType() == StatementType.CALLABLE) {//如果是存储过程把参数存入localOutputParameterCachelocalOutputParameterCache.putObject(key, parameter);}return list;}@Override//更新变动等语句删除缓存public int update(MappedStatement ms, Object parameter) throws SQLException {ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());if (closed) {throw new ExecutorException("Executor was closed.");}//删除一级缓存clearLocalCache();//执行语句return doUpdate(ms, parameter);}@Override//删除一级缓存public void clearLocalCache() {if (!closed) {localCache.clear();localOutputParameterCache.clear();}}

所以一级缓存的作用级别是SESSION级别的，因为一个session中存放一个Executor。而一级缓存放在Executor。

二级缓存

如果开启了二级缓存的话，你的Executor将会被装饰成CachingExecutor，二级缓存是MapperStatement级的缓存，也就是一个namespace就会有一个缓存，缓存是通过CachingExecutor来操作的。查询出来的结果会存在statement中的cache中，若有更新，删除类的操作默认就会清空该MapperStatement的cache（也可以通过修改xml中的属性，让它不执行），不会影响其他的MapperStatement

  @Overridepublic <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)throws SQLException {//获得该MappedStatement的cacheCache cache = ms.getCache();if (cache != null) {//看是否需要清除cache（在xml中可以配置flushCache属性决定何时清空cache）flushCacheIfRequired(ms);if (ms.isUseCache() && resultHandler == null) {//若开启了cache且resultHandler 为空ensureNoOutParams(ms, parameterObject, boundSql);@SuppressWarnings("unchecked")//从TransactionalCacheManager中取cacheList<E> list = (List<E>) tcm.getObject(cache, key);//若没值if (list == null) {//访问数据库list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);//存入cachetcm.putObject(cache, key, list); // issue #578 and #116}return list;}}return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}@Overridepublic int update(MappedStatement ms, Object parameterObject) throws SQLException {//看是否需要清除cache（在xml中可以配置flushCache属性决定何时清空cache）flushCacheIfRequired(ms);return delegate.update(ms, parameterObject);}private void flushCacheIfRequired(MappedStatement ms) {//获得cacheCache cache = ms.getCache();if (cache != null && ms.isFlushCacheRequired()) {      //若需要清除，则清除cachetcm.clear(cache);}}

因同一个namespace下的MappedStatement的cache是同一个，而TransactionalCacheManager中统一管理cache是里面的属性transactionalCaches,该属性以MappedStatement中的Cache为key，TransactionalCache对象为Value。即一个namespace对应一个TransactionalCache。

public class TransactionalCacheManager {private Map<Cache, TransactionalCache> transactionalCaches = new HashMap<Cache, TransactionalCache>();……
}
public class TransactionalCache implements Cache {private static final Log log = LogFactory.getLog(TransactionalCache.class);//就是对应的namespace中的cacheprivate Cache delegate;//提交的时候清除cache的标志位private boolean clearOnCommit;//待提交的集合private Map<Object, Object> entriesToAddOnCommit;//未查到的key存放的集合private Set<Object> entriesMissedInCache;
}

总结

一级缓存是sqlSession级别的缓存，存放在BaseExecutor中的localCache中。查询就将结果缓存进去，一旦有更新，删除，插入类的操作就清空缓存。不同的sqlSession之间的缓存是互相不影响的。
二级缓存是namespace级别的，可以理解为一个mapper.xml文件对应一个二级缓存（不同的sqlSession之间的缓存是共享的），然后对缓存的操作是在.xml文件中的标签文件进行控制的。比如下面的代码

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="mapper.TDemoMapper">
<!-- 一个namespace对应一个缓存 --><resultMap id="baseMap" type="entity.TDemo"><result property="id" column="id" jdbcType="INTEGER"></result><result property="name" column="name" jdbcType="VARCHAR"></result></resultMap><!-- 执行此语句不清空缓存 --><select id="getAll" resultType="entity.TDemo" useCache="true" flushCache="false" >select * from t_demo</select></mapper>