SqlsessionTemplate线程安全解密

springboot中使用的是SqlsessionTemplate，而不是DefaultSqlsession，这个类是单例的，如何保证线程安全呢？

类图

源码解析

org.mybatis.spring.SqlSessionTemplate#selectList(java.lang.String)，委托给代理

@Override
  public <E> List<E> selectList(String statement) {
    return this.sqlSessionProxy.<E> selectList(statement);
  }

org.mybatis.spring.SqlSessionTemplate.SqlSessionInterceptor#invoke，代理类核心方法

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
	// 获取Sqlsession
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
		  // 执行代理方法
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          // force commit even on non-dirty sessions because some databases require
          // a commit/rollback before calling close()
			// 处理事务相关
          sqlSession.commit(true);
        }
        return result;
      } catch (Throwable t) {
        Throwable unwrapped = unwrapThrowable(t);
        if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
          // release the connection to avoid a deadlock if the translator is no loaded. See issue #22
          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);
        }
      }
    }

org.mybatis.spring.SqlSessionUtils#getSqlSession(org.apache.ibatis.session.SqlSessionFactory, org.apache.ibatis.session.ExecutorType, org.springframework.dao.support.PersistenceExceptionTranslator)产生sqlSession方法

public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {

    notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);
    notNull(executorType, NO_EXECUTOR_TYPE_SPECIFIED);

    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);

    SqlSession session = sessionHolder(executorType, holder);
    if (session != null) {
      return session;
    }

    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug("Creating a new SqlSession");
    }
// 产生DefaultSqlsession
    session = sessionFactory.openSession(executorType);

    registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);

    return session;
  }