Mybatis与Spring集成：SqlSessionTemplate[通俗易懂]

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Mybatis与Spring集成：SqlSessionTemplate

SqlSessionTemplate构造方法

  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
      PersistenceExceptionTranslator exceptionTranslator) { 
   

    notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
    notNull(executorType, "Property 'executorType' is required");

    this.sqlSessionFactory = sqlSessionFactory;
    this.executorType = executorType;
    this.exceptionTranslator = exceptionTranslator;
    //使用JDK动态代理，创建SqlSessionFactory的代理类的实例 
    this.sqlSessionProxy = (SqlSession) newProxyInstance(SqlSessionFactory.class.getClassLoader(),
        new Class[] { 
    SqlSession.class },new SqlSessionInterceptor());
  }

核心在SqlSessionInterceptor的invoke方法中

  private class SqlSessionInterceptor implements InvocationHandler { 
   
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 
   
        //重要：获取sqlSession，具体操作见下方详解
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try { 
   
        //调用真实SqlSession的操作方法
        Object result = method.invoke(sqlSession, args);
        //判断当前sqlSession是否被Spring托管；未被Spring托管则自动commit
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) { 
   
          // force commit even on non-dirty sessions because some databases require a commit/rollback before calling close()
          sqlSession.commit(true);
        }
        return result;
      } catch (Throwable t) { 
   
        Throwable unwrapped = unwrapThrowable(t);
        if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) { 
   
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
          sqlSession = null;
          Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
          if (translated != null) { 
   
            unwrapped = translated;
          }
        }
        throw unwrapped;
      } finally { 
   
        if (sqlSession != null) { 
   
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }

SqlSessionUtils.getSqlSession方法

  //从事务管理器中获取sqlSession或创建一个 
  public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) { 
   

    notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);
    notNull(executorType, NO_EXECUTOR_TYPE_SPECIFIED);
    //根据sqlSessionFactory从当前线程对应的资源map中获取SqlSessionHolder，当sqlSessionFactory创建了sqlSession，就会在事务管理器中添加一对映射：key为sqlSessionFactory，value为SqlSessionHolder，该类保存sqlSession及执行方式 
    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
    //返回holder中的sqlSession，见下方
    SqlSession session = sessionHolder(executorType, holder);
    if (session != null) { 
   
      return session;
    }

    if (LOGGER.isDebugEnabled()) { 
   
      LOGGER.debug("Creating a new SqlSession");
    }
    // 如果holder中返回的sqlSession为空，则创建sqlSession，并注册到holder中去
    session = sessionFactory.openSession(executorType);
    registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);

    return session;
  }

  private static SqlSession sessionHolder(ExecutorType executorType, SqlSessionHolder holder) { 
   
    SqlSession session = null;
    if (holder != null && holder.isSynchronizedWithTransaction()) { 
   
      // 在同一个事务中，执行类型不能变化，原因就是同一个事务中同一个sqlSessionFactory创建的sqlSession会被重用 
      if (holder.getExecutorType() != executorType) { 
   
        throw new TransientDataAccessResourceException("Cannot change the ExecutorType when there is an existing transaction");
      }

      holder.requested();

      if (LOGGER.isDebugEnabled()) { 
   
        LOGGER.debug("Fetched SqlSession [" + holder.getSqlSession() + "] from current transaction");
      }

      session = holder.getSqlSession();
    }
    return session;
  }

如何将sqlSession注册到holder中去？

  //开启了事务管理时（如springboot常用的的@Transactional），注册session
  //Register session holder if synchronization is active (i.e. a Spring TX is active).
  private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,
      PersistenceExceptionTranslator exceptionTranslator, SqlSession session) { 
   
    SqlSessionHolder holder;
    ///判断同步是否激活，只要SpringTX被激活，就是true 
    if (TransactionSynchronizationManager.isSynchronizationActive()) { 
   
      //加载环境变量，判断注册的事务管理器是否是Spring管理事务，如果是，则将sqlSession加载进事务管理的本地线程缓存中；否则报错
      Environment environment = sessionFactory.getConfiguration().getEnvironment();
      if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) { 
   
        if (LOGGER.isDebugEnabled()) { 
   
          LOGGER.debug("Registering transaction synchronization for SqlSession [" + session + "]");
        }
        holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
        // 以sessionFactory为key，hodler为value，加入到TransactionSynchronizationManager管理的本地缓存ThreadLocal<Map<Object, Object>>中 
        TransactionSynchronizationManager.bindResource(sessionFactory, holder);
        TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
        //设置当前holder和当前事务同步
        holder.setSynchronizedWithTransaction(true);
        //增加引用数 
        holder.requested();
      } else { 
   
        if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) { 
   
          if (LOGGER.isDebugEnabled()) { 
   
            LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because DataSource is not transactional");
          }
        } else { 
   
          throw new TransientDataAccessResourceException(
              "SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");
        }
      }
    } else { 
   
      if (LOGGER.isDebugEnabled()) { 
   
        LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because synchronization is not active");
      }
    }
}

这里用到了TransactionSynchronizationManager类：

TransactionSynchronizationManager.bindResource：在调用一个需要事务的组件的时候，管理器首先判断当前线程有没有事务，如果没有事务则启动一个事务，并把事务与当前线程绑定。Spring使用TransactionSynchronizationManager的bindResource方法将当前线程与一个事务绑定，采用的方式是ThreadLocal。

    public static void bindResource(Object key, Object value) throws IllegalStateException { 
   
        Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
        Assert.notNull(value, "Value must not be null");
        // 此处resources是一个全局变量： 
        //private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal("Transactional resources");
        Map<Object, Object> map = (Map)resources.get();
        if (map == null) { 
   
            map = new HashMap();
            resources.set(map);
        }
        //判断以此sessionFactory为key的Map是否存在。如果存在则此线程已经绑定了 
        Object oldValue = ((Map)map).put(actualKey, value);
        if (oldValue instanceof ResourceHolder && ((ResourceHolder)oldValue).isVoid()) { 
   
            oldValue = null;
        }

        if (oldValue != null) { 
   
            throw new IllegalStateException("Already value [" + oldValue + "] for key [" + actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
        } else { 
   
            if (logger.isTraceEnabled()) { 
   
                logger.trace("Bound value [" + value + "] for key [" + actualKey + "] to thread [" + Thread.currentThread().getName() + "]");
            }

        }
    }

可以看出：多线程获取SqlSession时，先从当前线程中获取SqlSessionHolder中的sqlSession；如果为空，再新建sqlSession。所以，多线程公用的是一个SqlSessionTemplate（默认bean的单例注入），但不共用一个SqlSession。