MyBatis工作原理源码分析（二）——获取SqlSession对象

亦凉 2022-03-19 14:38 421阅读 0赞

## 一、引言 ##

由上一篇知，手动使用Mybatis的四个步骤：

获取SqlSessionFactory对象
    获取sqlSession对象
    获取接口的代理对象（MapperProxy）
    执行增删改查方法

上一篇中[MyBatis工作原理源码分析（一）][MyBatis]，详细分析了第一步SqlSessionFactory的初始化过程，下面在此基础上，来分析SqlSession对象的获取。

## 二、获取SqlSession对象源码分析 ##

当获取到SqlSessionFactory之后，就可以通过SqlSessionFactory去获取SqlSession对象。下面来分析一下源码：

//2、获取sqlSession对象
     SqlSession openSession = sqlSessionFactory.openSession();

1、调用openSession()方法，获取sqlSession对象

@Override
    public SqlSession openSession() { 
       //从数据源中拿openSession,参数是得到默认的ExecutorType,默认是Simple,即简单的执行器
      return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
    }

2、调用openSessionFromDataSource()方法

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { 
        Transaction tx = null;
        try { 
          //通过Confuguration对象去获取Mybatis中的Environment对象，该包含了数据源和事务的配置
          final Environment environment = configuration.getEnvironment();
          //获取一些信息，创建事务
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          
          //从表面上来看，咱们是用sqlSession在执行sql语句， 实际呢，其实是通过excutor执行，excutor是对于Statement的封装
          final Executor executor = configuration.newExecutor(tx, execType);
          //关键在这，创建了一个DefaultSqlSession对象!!!!!!
          return new DefaultSqlSession(configuration, executor, autoCommit);
          
        } catch (Exception e) { 
         
        } finally { 
          ErrorContext.instance().reset();
        }
      }

3、根据Executor在全局配置中的类型，创建出SimpleExecutor/ReuseExecutor/BatchExecutor

//根据ExecutorType类型，创建一个Executor
    public Executor newExecutor(Transaction transaction, ExecutorType executorType) { 
       executorType = executorType == null ? defaultExecutorType : executorType;
       executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
       Executor executor;
       if (ExecutorType.BATCH == executorType) { 
       	 //创建一个BatchExecutor
         executor = new BatchExecutor(this, transaction);
       } else if (ExecutorType.REUSE == executorType) { 
         //创建一个ReuseExecutor
         executor = new ReuseExecutor(this, transaction);
       } else { 
         //创建一个SimpleExecutor
         executor = new SimpleExecutor(this, transaction);
       }
       //如果二级缓存如果开启，则创建一个CachingExecutor
       if (cacheEnabled) { 
         executor = new CachingExecutor(executor);
       }
       //很重要！很重要！很重要！每一个Executor创建都要走这一步。
       executor = (Executor) interceptorChain.pluginAll(executor);
       //返回包装后的Executor
       return executor;
     }

4、获取到所有拦截器，调用每一个拦截器的plugin方法，重新将executor包装一下

public Object pluginAll(Object target) { 
      for (Interceptor interceptor : interceptors) { 
        target = interceptor.plugin(target);
      }
      return target;
    }

说明：Executor是一个接口，里面有增删改查的方法，源码如下。

public interface Executor { 
    
      ResultHandler NO_RESULT_HANDLER = null;
    
      int update(MappedStatement ms, Object parameter) throws SQLException;
    
      <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
    
      <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
    
      <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
    
      List<BatchResult> flushStatements() throws SQLException;
    
      void commit(boolean required) throws SQLException;
    
      void rollback(boolean required) throws SQLException;
    
      CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
    
      boolean isCached(MappedStatement ms, CacheKey key);
    
      void clearLocalCache();
    
      void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
    
      Transaction getTransaction();
    
      void close(boolean forceRollback);
    
      boolean isClosed();
    
      void setExecutorWrapper(Executor executor);
    
    }

## 三、小结 ##

下面是上面分析的时序图：

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3p4ZDE0MzU1MTM3NzU_size_16_color_FFFFFF_t_70]  
总结：

1、返回SqlSession的实现类DefaultSqlSession对象，它里面包含了Executor和Configuration；

2、Executor会在这一步被创建，用于执行Sql。

[MyBatis]: https://blog.csdn.net/zxd1435513775/article/details/87300169
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3p4ZDE0MzU1MTM3NzU_size_16_color_FFFFFF_t_70]: /images/20220319/82c4fccd78694505ab72e7cd88773215.png