前言

上一篇介绍了Mapper接口代理的实现原理,今天继续剖析SqlSession代理。希望看完大家能搞懂下面问题：

• Spring是如何管理Mapper的Bean，实现线程安全

• Mybatis自身的sqlSession是否线程安全

  
  
  spring-mybatis.png

源码分析

二. SqlSession代理

• 1、创建SqlSessionTemplate

在Spring扫描Mapper创建bean时，可以留意到不仅设置了className,beanClass，还设置了sqlSessionFactory属性。

public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {

    private MapperFactoryBean<?> mapperFactoryBean = new MapperFactoryBean<Object>();
    
    private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
        GenericBeanDefinition definition;
        for (BeanDefinitionHolder holder : beanDefinitions) {
            definition = (GenericBeanDefinition) holder.getBeanDefinition();
            //将mapper接口的名称添加到构造参数
         definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName());
            //设置BeanDefinition的class
            definition.setBeanClass(this.mapperFactoryBean.getClass());
            //添加属性addToConfig
            definition.getPropertyValues().add("addToConfig", this.addToConfig);
            //添加属性sqlSessionFactory
            definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
            ......
    }
}

MapperFactoryBean继承于SqlSessionDaoSupport，当设置sqlSessionFactory属性时会触发setSqlSessionFactory()方法。

public abstract class SqlSessionDaoSupport extends DaoSupport {

  private SqlSessionTemplate sqlSessionTemplate;

  /**
   * Set MyBatis SqlSessionFactory to be used by this DAO. Will automatically create SqlSessionTemplate for the given
   * SqlSessionFactory.
   *
   * @param sqlSessionFactory
   *          a factory of SqlSession
   */
  public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
    if (this.sqlSessionTemplate == null || sqlSessionFactory != this.sqlSessionTemplate.getSqlSessionFactory()) {
      this.sqlSessionTemplate = createSqlSessionTemplate(sqlSessionFactory);
    }
  protected SqlSessionTemplate createSqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
    return new SqlSessionTemplate(sqlSessionFactory);
  } 
}

最终会为每个Mapper Bean创建一个SqlSessionTemplate对象，因此SqlSessionTemplate在Mapper Bean之间是不共享的。

• 2、创建动态代理
  再看看SqlSessionTemplate的构造函数

public class SqlSessionTemplate implements SqlSession, DisposableBean {

  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,PersistenceExceptionTranslator exceptionTranslator) {
       ...
      this.sqlSessionFactory = sqlSessionFactory;
      this.executorType = executorType;
      this.exceptionTranslator = exceptionTranslator;
      //创建sqlSession代理
      this.sqlSessionProxy = (SqlSession) newProxyInstance(SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class }, new SqlSessionInterceptor());
  }

SqlSessionTemplate是实现了SqlSession接口的类。回顾上一篇，Mapper Bean实际调用的代理类MapperProxy，invoke()方法里用到的sqlSession就是SqlSessionTemplate。然而SqlSessionTemplate自己也不亲自操作，而是使用动态代理替自己完成sqlSession操作

public class SqlSessionTemplate implements SqlSession, DisposableBean {
  
  private final SqlSession sqlSessionProxy;
  ...
  @Override
  public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    this.sqlSessionProxy.select(statement, parameter, rowBounds, handler);
  }
  @Override
  public int insert(String statement) {
    return this.sqlSessionProxy.insert(statement);
  }
  ...
}

顺而言之，关注点来到处理类SqlSessionInterceptor。它是SqlSessionTemplate私有内部类。

 private class SqlSessionInterceptor implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      //获取当前线程事务的SqlSession
      SqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);
      try {
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          sqlSession.commit(true);
        }
        return result;
      } catch (Throwable t) {
        ...
      } finally {
        if (sqlSession != null) {
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }

• 3、sqlSession线程管理
  SqlSessionInterceptor的invoke方法获取Sqlsession的方式并不简单。奥妙在于getSqlSession()方法。它是Spring专门管理sqlSession的工具类SqlSessionUtils里的静态方法。

public final class SqlSessionUtils {

    public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType,PersistenceExceptionTranslator exceptionTranslator) {
        ...
        //从当前线程获取SqlSessionHolder
        SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
        //从SqlSessionHolder获取SqlSession，引用次数加1
        SqlSession session = sessionHolder(executorType, holder);
        if (session != null) {
          return session;
        }

        LOGGER.debug(() -> "Creating a new SqlSession");
        //如果当前线程没有sqlSession，创建一个
        session = sessionFactory.openSession(executorType);
       //将新的sqlSession注册当前线程
        registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);

        return session;
    }
}

可以看出sqlSession是由SqlSessionHolder管理的。只要搞懂sqlSession如何创建与注册，获取的事就再简单不过了。重心放在最后两行代码sessionFactory.openSession(executorType)和registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);。

• 4、openSession()
  实现了SqlSessionFactory的接口有两个：DefaultSqlSessionFactory和SqlSessionManager。
  SqlSessionManager还实现了SqlSession类，是供用户单独用Mybatis时使用的。MyBatis为了将SqlSession管理权交给Spring。将SqlSession由SqlSessionTemplate来实现，SqlSessionFactory使用DefaultSqlSessionFactory。

  
  
  SqlSession.png

public class DefaultSqlSessionFactory implements SqlSessionFactory {
      private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
        try {
          final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          // 创建事务
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          final Executor executor = configuration.newExecutor(tx, execType);
          //新建一个DefaultSqlSession对象作为SqlSession
          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();
        }
      }
}

DefaultSqlSessionFactory不仅创建sqlSession还创建了事务。

• 5、registerSessionHolder

private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,PersistenceExceptionTranslator exceptionTranslator, SqlSession session) {
      SqlSessionHolder holder;
        ...  
        Environment environment = sessionFactory.getConfiguration().getEnvironment();
        ...
        //将新的sqlSession与sqlSessionHolder绑定
        holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
        //将新的sqlSessionHolder与线程绑定
        TransactionSynchronizationManager.bindResource(sessionFactory, holder);
        TransactionSynchronizationManager
            .registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
        holder.setSynchronizedWithTransaction(true);
        //引用次数加一
        holder.requested();
        ...
      }

registerSessionHolder是SqlSessionUtils的私有静态方法，会创建新的SqlSessionHolder与当前线程绑定。

public abstract class TransactionSynchronizationManager {

    private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<>("Transactional resources");

    public static void bindResource(Object key, Object value) throws IllegalStateException {
        Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
        Assert.notNull(value, "Value must not be null");
        Map<Object, Object> map = resources.get();
        // set ThreadLocal Map if none found
        if (map == null) {
            map = new HashMap<>();
            resources.set(map);
        }
        Object oldValue = map.put(actualKey, value);
    }
}

bindResource()实现原理也很简单。将SessionFactory作为key，SqlSessionHolder为value存入一个map里，然后再将这个map存入ThreadLocal。因此，Spring调用Mybatis每个线程调用的是不同的SqlSessionHolder即不同的SqlSession，Spring就是这样管理sqlSession，实现线程安全的。

总结

经过两篇文章对mybatis源码的层层剖析，可知。Mapper Bean是通过MapperFactoryBean创建MapperProxy代理获取实现类。MapperProxy用到的SqlSession是SqlSessionTemplate，实际执行的是代理处理类SqlSessionInterceptor。执行时会从当前线程获取SqlSessionHolder中的SqlSession，实现线程安全。

MapperBean.png