1.在下面的案例中，執(zhí)行兩次查詢控制臺只會輸出一次 SQL 查詢：

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();//開啟會話PersonMapper mapper = sqlSession.getMapper(PersonMapper.class);mapper.list();mapper.list();

之所以會出現(xiàn)這種情況，是因為 Mybatis 存在一級緩存導致的，下面 debug 探究下內(nèi)部流程：

（1）mapper.list() 會進入 MapperProxy#invoke()：參數(shù)proxy是一個代理對象（每個 Mapper 接口都會被轉(zhuǎn)換成一個代理對象），里面包含會話 sqlSession、接口信息、方法信息；method是目標方法（當前執(zhí)行的方法），它里面包含了所屬的哪個類（接口）、方法名、返回類型（List、Map、void 或其他）、參數(shù)類型等；args是參數(shù)；

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); } //從方法緩存methodCache中獲取到方法的信息：比如方法名、類型（select、update等）、返回類型 //如果獲取中沒有MapperMethod，則創(chuàng)建一個并放入methodCache中 final MapperMethod mapperMethod = cachedMapperMethod(method); //執(zhí)行查詢SQL并返回結(jié)果 return mapperMethod.execute(sqlSession, args); }

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

private MapperMethod cachedMapperMethod(Method method) { //緩存中獲取 MapperMethod mapperMethod = methodCache.get(method); //沒有則創(chuàng)建一個對象并放入緩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()根據(jù) SQL 類型進入不同的查詢方法

public Object execute(SqlSession sqlSession, Object[] args) { //返回結(jié)果 Object result; //判斷語句類型 switch (command.getType()) { case INSERT: {//插入語句 Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));break; } case UPDATE: {//更新語句Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));break; } case DELETE: {//刪除語句Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));break; } case SELECT://查詢語句//返回空的查詢if (method.returnsVoid() && method.hasResultHandler()) { executeWithResultHandler(sqlSession, args); result = null; //返回List的查詢} else if (method.returnsMany()) { result = executeForMany(sqlSession, args); //返回Map的查詢} else if (method.returnsMap()) { result = executeForMap(sqlSession, args); //返回游標的查詢} 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 語句，返回結(jié)果是List集合，所以進入 MapperMethod#executeForMany()：

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) { List<E> result; //獲取參數(shù) Object param = method.convertArgsToSqlCommandParam(args); //是否有分頁查詢 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中的泛型跟結(jié)果類型不一致，進行轉(zhuǎn)換 if (!method.getReturnType().isAssignableFrom(result.getClass())) { if (method.getReturnType().isArray()) {return convertToArray(result); } else {return convertToDeclaredCollection(sqlSession.getConfiguration(), result); } } return result; }

（4）selectList執(zhí)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執(zhí)行的信息：resource（xxMapper.xml）、id、sql、返回類型等 MappedStatement ms = configuration.getMappedStatement(statement); //執(zhí)行查詢 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）接下來調(diào)用緩存執(zhí)行器的方法：CachingExecutor#query()

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { //獲取到執(zhí)行SQL BoundSql boundSql = ms.getBoundSql(parameterObject); //將SQL包裝成一個緩存對對象，該對象和結(jié)果集組成鍵值對存儲到緩存中，方便下次直接從緩存中拿而不需要再次查詢 //createCacheKey：調(diào)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 { //獲取緩存 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; } } //沒有緩存連接查詢 return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }

（6）接下來執(zhí)行 BaseExecutor#query()：從下面可以看到將結(jié)果緩存到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.'); } //如果不是嵌套查詢（默認為0），且 <select> 的 flushCache=true 時清空緩存 if (queryStack == 0 && ms.isFlushCacheRequired()) { clearLocalCache(); } List<E> list; try { //嵌套查詢層數(shù)+1 queryStack++; //從localCache緩存中獲取 list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else {//連接查詢list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { queryStack--; } //下面是延遲加載邏輯 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; //緩存中添加占位符 localCache.putObject(key, EXECUTION_PLACEHOLDER); try { //連接查詢獲取到數(shù)據(jù)結(jié)果 list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { //刪除占位符 localCache.removeObject(key); } //將結(jié)果緩存起來 localCache.putObject(key, list); //處理存儲過程 if (ms.getStatementType() == StatementType.CALLABLE) { localOutputParameterCache.putObject(key, parameter); } return list; }

2.但當 Spring Framework + Mybatis 時，情況就不一樣了，每次查詢都會連接數(shù)據(jù)庫查詢，控制臺都會打印 SQL 出來，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進入的步驟跟上面的（1）、（2）、（3）是一致的，但第四步卻是進入SqlSessionTemplate#selectList()中【SqlSessionTemplate是mybatis-spring-xx.jar的，上文的DefaultSqlSession是屬于mybatis-xx.jar的】：

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

接下來的selectList() 會被方法攔截：method.invoke() 會執(zhí)行到 DefaultSqlSession#selectList()，重新回到上文的第四步并且繼續(xù)下去，也就是在上文的（1）~（6）中插入了前后文，在其中做了關(guān)閉會話的操作；

private class SqlSessionInterceptor implements InvocationHandler { @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { //得到會話 SqlSession sqlSession = getSqlSession( SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator); try {//執(zhí)行方法查詢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);//在關(guān)閉會話前提交和回滾}return result; } catch (Throwable t) {//有異常拋出異常并結(jié)束會話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 {//關(guān)閉會話if (sqlSession != null) { closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);} } } }總結(jié)：

Mybatis 的一級緩存是會話級別的緩存（單線程的，特別雞肋），Mybatis 每創(chuàng)建一個 SqlSession 會話對象，就表示打開一次數(shù)據(jù)庫會話，在一次會話中，應用程序很可能在短時間內(nèi)反復執(zhí)行相同的查詢語句，如果不對數(shù)據(jù)進行緩存，則每查詢一次就要執(zhí)行一次數(shù)據(jù)庫查詢，這就造成數(shù)據(jù)庫資源的浪費。又因為通過 SqlSession 執(zhí)行的操作，實際上由 Executor 來完成數(shù)據(jù)庫操作的，所以在 Executor 中會建立一個簡單的緩存，即一級緩存；將每次的查詢結(jié)果緩存起來，再次執(zhí)行查詢的時候，會先查詢一級緩存（默認開啟的），如果命中，則直接返回，否則再去查詢數(shù)據(jù)庫并放入緩存中。

一級緩存的生命周期與 SqlSession 的生命周期相同，因此當 Mybatis 和Spring Framework 的集成包中擴展了一個 SqlSessionTemplate 類（它是一個代理類，增強了查詢方法），所有的查詢經(jīng)過 SqlSessionTemplate 代理攔截后再進入到 DefaultSqlSession#selectList() 中，結(jié)束查詢后把會話SqlSession 關(guān)了，所以導致了緩存失效。

那為什么要這么操作呢？

原始的 Mybatis 有暴露 SqlSession 接口，因此有 close 方法暴露出來供你選擇使用，你可以選擇關(guān)與不關(guān)，但在Mybatis 和Spring Framework 的集成包中，SqlSession 是交給了Spring Framework 管理的，沒有暴露出來，為了穩(wěn)妥決定，直接給你關(guān)了。

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