MyBatis源碼學習（一）SqlSession建立，mybatissqlsession

MyBatis源碼學習（一）SqlSession建立，mybatissqlsession

MyBatis封裝了JDBC操作資料庫的代碼，通過SqlSession來執行sql語句,那麼首先來看看MyBatis是怎麼建立SqlSession。 
MyBatis沒有託管給spring的時候，資料庫的配置資訊是在Configuration.xml檔案裡邊配置的 ，測試代碼如下

1 Reader reader =  Resources.getResourceAsReader("Configuration.xml");

Mybatis通過SqlSessionFactoryBuilder.build(Reader reader)方法建立一個SqlSessionFactory對象 build方法的參數就是剛才的reader對象，裡邊包含了設定檔的所有資訊，build方法有很多重載方法

 1 public SqlSessionFactory build(Reader reader, String environment, Properties properties) { 2     try { 3         //委託XMLConfigBuilder來解析xml檔案，並構建 4       XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties); 5       return build(parser.parse()); 6     } catch (Exception e) { 7       throw ExceptionFactory.wrapException("Error building SqlSession.", e); 8     } finally { 9       ErrorContext.instance().reset();10       try {11         reader.close();12       } catch (IOException e) {13   }14 public SqlSessionFactory build(Configuration config) {15     return new DefaultSqlSessionFactory(config);16   }

最後返回一個DefaultSqlSessionFactory對象，通過DefaultSqlSessionFactory的openSession()返回一個SqlSession對象

public SqlSession openSession() {    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);  }private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {    Transaction tx = null;    try {      final Environment environment = configuration.getEnvironment();      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);      //通過事務工廠來產生一個事務      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);      //產生一個執行器(事務包含在執行器裡)      final Executor executor = configuration.newExecutor(tx, execType);      //然後產生一個DefaultSqlSession      return new DefaultSqlSession(configuration, executor, autoCommit);    } catch (Exception e) {      //如果開啟事務出錯，則關閉它      closeTransaction(tx); // may have fetched a connection so lets call close()      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);    } finally {      //最後清空錯誤上下文      ErrorContext.instance().reset();    }  }

可以看到最後返回一個DefaultSqlSession即SqlSession對象，DefaultSqlSession中的selectOne(…) selectList(…) 
selectMap(…) update(…)等方法就是真正執行要執行sql的方法 
具體的執行由executor對象來執行

public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {    try {      MappedStatement ms = configuration.getMappedStatement(statement);      executor.query(ms, wrapCollection(parameter), rowBounds, handler);    } catch (Exception e) {      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);    } finally {      ErrorContext.instance().reset();    }  }