一、添加一个mapper.xml

select * from table_test where id = #{id}

二、再mybatis-config.xml中引入mapper

扩展：mybatis加载mapper的方式有四种：

package -> resource -> url -> class

三、通过sqlSessionFactory 操作查询：

        String resource = "mybatis-config.xml";InputStream inputStream = Resources.getResourceAsStream(resource);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);Object selectOne = sqlSessionFactory.openSession().selectOne("com.example.demo23.mapper.TestMapper.selectBlog",1L);

四、执行sql：源码

Object selectOne = sqlSessionFactory.openSession().selectOne("com.example.demo23.mapper.TestMapper.selectBlog",1L);

进入selectOne方法

  public  T selectOne(String statement, Object parameter) {// Popular vote was to return null on 0 results and throw exception on too many.List list = this.selectList(statement, parameter);if (list.size() == 1) {return list.get(0);}if (list.size() > 1) {throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());} else {return null;}}

断点至：

private  List selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {try {//从configuration中获取mapperStatement数据MappedStatement ms = configuration.getMappedStatement(statement);dirty |= ms.isDirtySelect();return executor.query(ms, wrapCollection(parameter), rowBounds, handler);} catch (Exception e) {throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);} finally {ErrorContext.instance().reset();}
}

执行表达式： configuration.getMappedStatement(statement)

，可以看到对应的statement保存的信息。 

 拿到ms对象后，交给：

executor.query(ms, wrapCollection(parameter), rowBounds, handler)

拓展：mybatid  executor有几种执行器？通过接口类图可以看见有三中执行器 

断点进入

public  List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler)throws SQLException {//从MappedStatement  获取当前绑定的sqlBoundSql boundSql = ms.getBoundSql(parameterObject);CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

 拿到bondsql后继续执行：

  public  List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler,CacheKey key, BoundSql boundSql) throws SQLException {
//获取缓存Cache cache = ms.getCache();
//判断是否有缓存if (cache != null) {flushCacheIfRequired(ms);if (ms.isUseCache() && resultHandler == null) {ensureNoOutParams(ms, boundSql);@SuppressWarnings("unchecked")List list = (List) tcm.getObject(cache, key);if (list == null) {list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);tcm.putObject(cache, key, list); // issue #578 and #116}return list;}}return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}

执行doQuery

public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,BoundSql boundSql) throws SQLException {Statement stmt = null;try {Configuration configuration = ms.getConfiguration();StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler,boundSql);stmt = prepareStatement(handler, ms.getStatementLog());return handler.query(stmt, resultHandler);} finally {closeStatement(stmt);}
}

查询数据

private  List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds,ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {List list;localCache.putObject(key, EXECUTION_PLACEHOLDER);try {list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);} finally {localCache.removeObject(key);}//放入缓存中localCache.putObject(key, list);if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);}return list;
}

执行完毕。