每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为中心的。SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。而 SqlSessionFactoryBuilder 则可以从 XML (mybatis-config.xml) 配置文件或一个预先定制的 Configuration 的实例构建出 SqlSessionFactory 的实例。

类图

其中SqlSession是比较重要的接口，提供基本的方法，比如：sql语句、事务等操作；

具体执行过程以<E> List<E> selectList(String statement);方法为例：SqlSession的实现类DefaultSqlSession调用configuration.getMappedStatement(statement);获取statement节点的sql，存放在MappedStatement，传给接口Executor的query方法。

1   @Override 2   public 
     
       List
      
        selectList(String statement, Object parameter, RowBounds rowBounds) { 3     try { 4       MappedStatement ms = configuration.getMappedStatement(statement); 5       return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); 6     } catch (Exception e) { 7       throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e); 8     } finally { 9       ErrorContext.instance().reset();10     }11   }
      
     

Executor处理MappedStatement的关键代码和注释如下

1   // 是否需要刷新缓存 2   if (queryStack == 0 && ms.isFlushCacheRequired()) { 3       clearLocalCache(); 4     } 5     List
     
       list; 6     try { 7       queryStack++; 8       list = resultHandler == null ? (List
      
       ) localCache.getObject(key) : null; 9       if (list != null) {10         // 如果缓存中有数据，直接拿到缓存中的数据11         handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);12       } else {13         // 否则从数据库中获取14         list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);15       }16     } finally {17       queryStack--;18     }
      
     

1   private 
     
       List
      
        queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { 2     List
       
         list; 3     localCache.putObject(key, EXECUTION_PLACEHOLDER); 4     try { 5       list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); 6     } finally { 7       localCache.removeObject(key); 8     } 9     // 把查询结果放入缓存后返回结果list10     localCache.putObject(key, list);11     if (ms.getStatementType() == StatementType.CALLABLE) {12       localOutputParameterCache.putObject(key, parameter);13     }14     return list;15   }
       
      
     

1   @Override 2   public 
     
       List
      
        doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException { 3     // 用来执行静态sql语句的对象 4     Statement stmt = null; 5     try { 6       Configuration configuration = ms.getConfiguration(); 7       StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql); 8       stmt = prepareStatement(handler, ms.getStatementLog()); 9       // StatementHandler中处理sql的执行和结果的返回10       return handler.
       
        query(stmt, resultHandler);11     } finally {12       closeStatement(stmt);13     }14   }
       
      
     

1   @Override2   public 
     
       List
      
        query(Statement statement, ResultHandler resultHandler) throws SQLException {3     String sql = boundSql.getSql();4     statement.execute(sql);5     return resultSetHandler.
       
        handleResultSets(statement);6   }
       
      
     

 

参考文档