Mybatis一级缓存和结合Spring Framework后失效的源码探究

1.在下面的案例中，执行两次查询控制台只会输出一次 SQL 查询：

mybatis-config.xml<?xml version='1.0' encoding='UTF-8' ?><!DOCTYPE configurationPUBLIC '-//mybatis.org//DTD Config 3.0//EN''http://mybatis.org/dtd/mybatis-3-config.dtd'><configuration> <environments default='development'><environment id='development'> <transactionManager type='JDBC'/> <dataSource type='POOLED'><property name='driver' value='com.mysql.jdbc.Driver'/><property name='url' value='jdbc:mysql://localhost:3306/xxx?useUnicode=true&characterEncoding=utf-8&autoReconnect=true'/><property name='username' value='xxx'/><property name='password' value='xxx'/> </dataSource></environment> </environments> <mappers><mapper resource='com/hrh/mapper/PersonMapper.xml'/> </mappers></configuration>

PersonMapper.xml<?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='com.hrh.mapper.PersonMapper'> <resultMap type='com.hrh.bean.Person'><id column='id' property='id' jdbcType='BIGINT'/><result column='name' property='name' jdbcType='VARCHAR'/><result column='age' property='age' jdbcType='BIGINT'/> </resultMap> <sql id='Base_Column_List'> id, name, age </sql> <select resultType='com.hrh.bean.Person'>select<include refid='Base_Column_List'/>from tab_person </select></mapper>

public interface PersonMapper { List<Person> list();}

String resource = 'mybatis-config2.xml';InputStream inputStream = Resources.getResourceAsStream(resource);SqlSessionFactory sqlSessionFactory =new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = sqlSessionFactory.openSession();//开启会话PersonMapper mapper = sqlSession.getMapper(PersonMapper.class);mapper.list();mapper.list();

之所以会出现这种情况，是因为 Mybatis 存在一级缓存导致的，下面 debug 探究下内部流程：

（1）mapper.list() 会进入 MapperProxy#invoke()：参数proxy是一个代理对象（每个 Mapper 接口都会被转换成一个代理对象），里面包含会话 sqlSession、接口信息、方法信息；method是目标方法（当前执行的方法），它里面包含了所属的哪个类（接口）、方法名、返回类型（List、Map、void 或其他）、参数类型等；args是参数；

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args); } else if (isDefaultMethod(method)) {return invokeDefaultMethod(proxy, method, args); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } //从方法缓存methodCache中获取到方法的信息：比如方法名、类型（select、update等）、返回类型 //如果获取中没有MapperMethod，则创建一个并放入methodCache中 final MapperMethod mapperMethod = cachedMapperMethod(method); //执行查询SQL并返回结果 return mapperMethod.execute(sqlSession, args); }

cacheMapperMethod：MapperMethod 包含方法名、类型（select、update等）、返回类型等信息

private MapperMethod cachedMapperMethod(Method method) { //缓存中获取 MapperMethod mapperMethod = methodCache.get(method); //没有则创建一个对象并放入缓存中供下次方便取用 if (mapperMethod == null) { mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()); methodCache.put(method, mapperMethod); } return mapperMethod; }

（2）MapperMethod#execute()根据 SQL 类型进入不同的查询方法

public Object execute(SqlSession sqlSession, Object[] args) { //返回结果 Object result; //判断语句类型 switch (command.getType()) { case INSERT: {//插入语句 Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));break; } case UPDATE: {//更新语句Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));break; } case DELETE: {//删除语句Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));break; } case SELECT://查询语句//返回空的查询if (method.returnsVoid() && method.hasResultHandler()) { executeWithResultHandler(sqlSession, args); result = null; //返回List的查询} else if (method.returnsMany()) { result = executeForMany(sqlSession, args); //返回Map的查询} else if (method.returnsMap()) { result = executeForMap(sqlSession, args); //返回游标的查询} else if (method.returnsCursor()) { result = executeForCursor(sqlSession, args);} else { Object param = method.convertArgsToSqlCommandParam(args); result = sqlSession.selectOne(command.getName(), param);}break; case FLUSH:result = sqlSession.flushStatements();break; default:throw new BindingException('Unknown execution method for: ' + command.getName()); } if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) { throw new BindingException('Mapper method ’' + command.getName() + ' attempted to return null from a method with a primitive return type (' + method.getReturnType() + ').'); } return result; }

（3）上面的案例是 select 语句，返回结果是List集合，所以进入 MapperMethod#executeForMany()：

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) { List<E> result; //获取参数 Object param = method.convertArgsToSqlCommandParam(args); //是否有分页查询 if (method.hasRowBounds()) { RowBounds rowBounds = method.extractRowBounds(args); result = sqlSession.<E>selectList(command.getName(), param, rowBounds); } else { result = sqlSession.<E>selectList(command.getName(), param); } // issue #510 Collections & arrays support //如果list中的泛型跟结果类型不一致，进行转换 if (!method.getReturnType().isAssignableFrom(result.getClass())) { if (method.getReturnType().isArray()) {return convertToArray(result); } else {return convertToDeclaredCollection(sqlSession.getConfiguration(), result); } } return result; }

（4）selectList执行了DefaultSqlSession#selectList()：

public <E> List<E> selectList(String statement, Object parameter) { return this.selectList(statement, parameter, RowBounds.DEFAULT); }

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { //SQL执行的信息：resource（xxMapper.xml）、id、sql、返回类型等 MappedStatement ms = configuration.getMappedStatement(statement); //执行查询 return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); } catch (Exception e) { throw ExceptionFactory.wrapException('Error querying database. Cause: ' + e, e); } finally { ErrorContext.instance().reset(); } }

（5）接下来调用缓存执行器的方法：CachingExecutor#query()

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { //获取到执行SQL BoundSql boundSql = ms.getBoundSql(parameterObject); //将SQL包装成一个缓存对对象，该对象和结果集组成键值对存储到缓存中，方便下次直接从缓存中拿而不需要再次查询 //createCacheKey：调用BaseExecutor#createCacheKey CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql); return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { //获取缓存 Cache cache = ms.getCache(); if (cache != null) { flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) {ensureNoOutParams(ms, boundSql);@SuppressWarnings('unchecked')List<E> list = (List<E>) tcm.getObject(cache, key);if (list == null) { list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578 and #116}return list; } } //没有缓存连接查询 return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }

（6）接下来执行 BaseExecutor#query()：从下面可以看到将结果缓存到localCache 中了

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { ErrorContext.instance().resource(ms.getResource()).activity('executing a query').object(ms.getId()); if (closed) { throw new ExecutorException('Executor was closed.'); } //如果不是嵌套查询（默认为0），且 <select> 的 flushCache=true 时清空缓存 if (queryStack == 0 && ms.isFlushCacheRequired()) { clearLocalCache(); } List<E> list; try { //嵌套查询层数+1 queryStack++; //从localCache缓存中获取 list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else {//连接查询list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { queryStack--; } //下面是延迟加载逻辑 if (queryStack == 0) { for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load(); } // issue #601 deferredLoads.clear(); if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache(); } } return list; }

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List<E> list; //缓存中添加占位符 localCache.putObject(key, EXECUTION_PLACEHOLDER); try { //连接查询获取到数据结果 list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { //删除占位符 localCache.removeObject(key); } //将结果缓存起来 localCache.putObject(key, list); //处理存储过程 if (ms.getStatementType() == StatementType.CALLABLE) { localOutputParameterCache.putObject(key, parameter); } return list; }

2.但当 Spring Framework + Mybatis 时，情况就不一样了，每次查询都会连接数据库查询，控制台都会打印 SQL 出来，如下案例：

@Servicepublic class PersonService { @Autowired PersonMapper personMapper; public List<Person> getList() {personMapper.list();personMapper.list();return personMapper.list(); }}

@Configuration@ComponentScan('com.hrh')@MapperScan('com.hrh.mapper')public class MyBatisConfig { @Bean public SqlSessionFactoryBean sqlSessionFactory() throws Exception {SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();factoryBean.setDataSource(dataSource());factoryBean.setMapperLocations(resolveMapperLocations());return factoryBean; } public Resource[] resolveMapperLocations() {ResourcePatternResolver resourceResolver = new PathMatchingResourcePatternResolver();List<String> mapperLocations = new ArrayList<>();mapperLocations.add('classpath*:com/hrh/mapper/*Mapper*.xml');List<Resource> resources = new ArrayList();if (mapperLocations != null) { for (String mapperLocation : mapperLocations) {try { Resource[] mappers = resourceResolver.getResources(mapperLocation); resources.addAll(Arrays.asList(mappers));} catch (IOException e) { // ignore} }}return resources.toArray(new Resource[resources.size()]); } @Bean public DataSource dataSource() {DriverManagerDataSource driverManagerDataSource = new DriverManagerDataSource();driverManagerDataSource.setDriverClassName('com.mysql.jdbc.Driver');driverManagerDataSource.setUsername('xxx');driverManagerDataSource.setPassword('xxx');driverManagerDataSource.setUrl('jdbc:mysql://localhost:3306/xxx?useUnicode=true&characterEncoding=utf-8&autoReconnect=true');return driverManagerDataSource; }}

AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MyBatisConfig.class);PersonService bean = context.getBean(PersonService.class);bean.getList();

下面debug进入的步骤跟上面的（1）、（2）、（3）是一致的，但第四步却是进入SqlSessionTemplate#selectList()中【SqlSessionTemplate是mybatis-spring-xx.jar的，上文的DefaultSqlSession是属于mybatis-xx.jar的】：

public <E> List<E> selectList(String statement, Object parameter) { return this.selectList(statement, parameter, RowBounds.DEFAULT); }

接下来的selectList() 会被方法拦截：method.invoke() 会执行到 DefaultSqlSession#selectList()，重新回到上文的第四步并且继续下去，也就是在上文的（1）~（6）中插入了前后文，在其中做了关闭会话的操作；

private class SqlSessionInterceptor implements InvocationHandler { @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { //得到会话 SqlSession sqlSession = getSqlSession( SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator); try {//执行方法查询Object result = method.invoke(sqlSession, args);if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) { // force commit even on non-dirty sessions because some databases require // a commit/rollback before calling close() sqlSession.commit(true);//在关闭会话前提交和回滚}return result; } catch (Throwable t) {//有异常抛出异常并结束会话Throwable unwrapped = unwrapThrowable(t);if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) { // release the connection to avoid a deadlock if the translator is no loaded. See issue #22 closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory); sqlSession = null; Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped); if (translated != null) { unwrapped = translated; }}throw unwrapped; } finally {//关闭会话if (sqlSession != null) { closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);} } } }总结：

Mybatis 的一级缓存是会话级别的缓存（单线程的，特别鸡肋），Mybatis 每创建一个 SqlSession 会话对象，就表示打开一次数据库会话，在一次会话中，应用程序很可能在短时间内反复执行相同的查询语句，如果不对数据进行缓存，则每查询一次就要执行一次数据库查询，这就造成数据库资源的浪费。又因为通过 SqlSession 执行的操作，实际上由 Executor 来完成数据库操作的，所以在 Executor 中会建立一个简单的缓存，即一级缓存；将每次的查询结果缓存起来，再次执行查询的时候，会先查询一级缓存（默认开启的），如果命中，则直接返回，否则再去查询数据库并放入缓存中。

一级缓存的生命周期与 SqlSession 的生命周期相同，因此当 Mybatis 和Spring Framework 的集成包中扩展了一个 SqlSessionTemplate 类（它是一个代理类，增强了查询方法），所有的查询经过 SqlSessionTemplate 代理拦截后再进入到 DefaultSqlSession#selectList() 中，结束查询后把会话SqlSession 关了，所以导致了缓存失效。

那为什么要这么操作呢？

原始的 Mybatis 有暴露 SqlSession 接口，因此有 close 方法暴露出来供你选择使用，你可以选择关与不关，但在Mybatis 和Spring Framework 的集成包中，SqlSession 是交给了Spring Framework 管理的，没有暴露出来，为了稳妥决定，直接给你关了。

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