mybatis session 浅析

ゝ一纸荒年。 2022-05-20 09:13 176阅读 0赞

# mybatis session #

从源码的角度,去探索mybatis session的内容

SqlSession sqlSession = sqlSessionFactory.openSession();
        try {
          sqlSession.doXXX
        } finally {
            sqlSession.close();
        }

本节搞清楚两个问题  
1. openSession 创建的对象有什么  
2. close 关闭了什么

# 打开会话 #

直接去 DefaultSqlSessionFactory 去跟进 openSession 的内容:

public SqlSession openSession() {
        return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
      }

转发给 openSessionFromDataSource

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
        try {
          final Environment environment = configuration.getEnvironment();
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          final Executor executor = configuration.newExecutor(tx, execType);
          return new DefaultSqlSession(configuration, executor, autoCommit);
        } catch (Exception e) {
          closeTransaction(tx); // may have fetched a connection so lets call close()
          throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
        } finally {
          ErrorContext.instance().reset();
        }
      }

内容如下图

![会话内容][sessioncontent.png]

实际上,这里就是一个`事务`和`缓存`的管理器,执行时做事务处理和缓存处理

# 关闭会话 #

@Override
      public void close() {
      // 省略无关代码
          executor.close(isCommitOrRollbackRequired(false));
      }

跟进执行器的关闭

// 省略无关代码
        public void close(boolean forceRollback) {
            rollback(forceRollback);
            transaction.close();
        }
    
         public void rollback(boolean required) throws SQLException {
                clearLocalCache();
                if (required) {
                  transaction.rollback();
                }
         }
    
          public void clearLocalCache() {
               localCache.clear();
               localOutputParameterCache.clear();
          }

不难看出,关闭会话实际上关闭了什么  
1. 关闭事务  
2. 清空一级缓存

# 总结 #

最终得出结论:

mybatis 的session 就是一个事务和缓存的管理器.

[sessioncontent.png]: http://cdn.blog.shangwantong.com/mybatis/sessioncontent.png