MyBatis源码探索

　　

　　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。

   @Override
   public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
     try {
       MappedStatement ms = configuration.getMappedStatement(statement);
       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();
     }
   }

Executor处理MappedStatement的关键代码和注释如下

   // 是否需要刷新缓存
   if (queryStack == 0 && ms.isFlushCacheRequired()) {
       clearLocalCache();
     }
     List<E> list;
     try {
       queryStack++;
       list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
       if (list != null) {
         // 如果缓存中有数据，直接拿到缓存中的数据
         handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
       } else {
         // 否则从数据库中获取
         list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
       }
     } finally {
       queryStack--;
     }

   private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
     List<E> list;
     localCache.putObject(key, EXECUTION_PLACEHOLDER);
     try {
       list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
     } finally {
       localCache.removeObject(key);
     }
     // 把查询结果放入缓存后返回结果list
     localCache.putObject(key, list);
     if (ms.getStatementType() == StatementType.CALLABLE) {
       localOutputParameterCache.putObject(key, parameter);
     }
     return list;
   }

   @Override
   public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
     // 用来执行静态sql语句的对象
     Statement stmt = null;
     try {
       Configuration configuration = ms.getConfiguration();
       StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
       stmt = prepareStatement(handler, ms.getStatementLog());
       // StatementHandler中处理sql的执行和结果的返回
       return handler.<E>query(stmt, resultHandler);
     } finally {
       closeStatement(stmt);
     }
   }

   @Override
   public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
     String sql = boundSql.getSql();
     statement.execute(sql);
     return resultSetHandler.<E>handleResultSets(statement);
   }

参考文档

MyBatis官方文档