mybatis源码分析（8）-----事务（mybatis管理、spring管理）

写在前面

　　接口：MyBatis的事务Transaction的接口有一下实现类

　　　　JdbcTransaction 由jdbc管理的事务（即利用Connection对象完成对事务的提交（commit()）、回滚（rollback()）、关闭（close()）等）

　　　　ManagedTransaction 由容器管理事务 （这种机制MyBatis自身不会去实现事务管理，而是让程序的容器如（JBOSS，Weblogic）来实现对事务的管理）

　　　  SpringManagedTransaction（位于mybatis-spring包中） 由spring来管理事务 myBatis单独为spring的事务集成，设计的类。本文章也是重要讲解此类的原理。

　　设计：工厂模式

　　 Transaction---TransactionTransactionFactory

　　　　JdbcTransaction ---JdbcTransactionFactory

　　　　ManagedTransaction ---ManagedTransactionFactory

　　 　  SpringManagedTransaction---SpringManagedTransactionFactory

　  持有者：

　　　　Executor（执行器）即对于事务的所有操作，都是由执行器来调度

　　　　　　Transaction

　　　Transaction接口

public interface Transaction {
  //获取链接
  Connection getConnection() throws SQLException;
  //提交
  void commit() throws SQLException;
  //回滚
  void rollback() throws SQLException;
  //关闭
  void close() throws SQLException;
}

　　TransactionFactory接口

public interface TransactionFactory {
  //相关属性设置
  void setProperties(Properties props);
  //通过链接开启获取一个事务对象
  Transaction newTransaction(Connection conn);
  //通过datasource获取一个事务对象
  Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit);
}

spring事务相关

　　TransactionSynchronizationManager： spring 管理事务线程的抽象类，所有属性都是threadlocal和当前线程相关

public abstract class TransactionSynchronizationManager {
　　 private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<Map<Object, Object>>("Transactional resources");
    private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations = new NamedThreadLocal<Set<TransactionSynchronization>>("Transaction synchronizations");
    private static final ThreadLocal<String> currentTransactionName = new NamedThreadLocal<String>("Current transaction name");
    private static final ThreadLocal<Boolean> currentTransactionReadOnly = new NamedThreadLocal<Boolean>("Current transaction read-only status");
    private static final ThreadLocal<Integer> currentTransactionIsolationLevel = new NamedThreadLocal<Integer>("Current transaction isolation level");
    private static final ThreadLocal<Boolean> actualTransactionActive = new NamedThreadLocal<Boolean>("Actual transaction active");
}

　　

　　当 spring开启事务管理（声明式事务、注解式事务）：会初始化一个synchronizations，初始化一个空的Set<TransactionSynchronization>集合存放在当前线程中。

    public static void initSynchronization() throws IllegalStateException {
        if (isSynchronizationActive()) {
            throw new IllegalStateException("Cannot activate transaction synchronization - already active");
        }
        logger.trace("Initializing transaction synchronization");
        synchronizations.set(new LinkedHashSet<TransactionSynchronization>());
    }

　　

　　spring判断synchronizations是否存在，暴露的API

    public static boolean isSynchronizationActive() {
        return (synchronizations.get() != null);
    }

myBatis单独管理

　　前提假设myBatis 和spring集成，且满足spring没有开启事务管理（声明式事务、注解式事务）

　　 在获取sqlSession的时候，会判断TransactionSynchronizationManager中的synchronizations是否存在：

　　　　如果不存在，则不会注册SqlSessionHolder到resources中，也不会将SqlSessionSynchronization（myBatis对于TransactionSynchronization接口的实现类）注册到synchronizations中。

  private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,
      PersistenceExceptionTranslator exceptionTranslator, SqlSession session) {
    SqlSessionHolder holder;
    //判断spring是否管理事务，即synchronizations是否存在值
    if (TransactionSynchronizationManager.isSynchronizationActive()) {
      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);
        //将SqlSessionFactory和SqlSessionHolder 绑定，注册到TransactionSynchronizationManager.resources中
        TransactionSynchronizationManager.bindResource(sessionFactory, holder);
        //注册sqlSessionSynchronization到TransactionSynchronizationManager.synchronizations中
        TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
        holder.setSynchronizedWithTransaction(true);
        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");
      }
    }

　　sqlSession调用Executor-->statmentHandler 执行sql的流程后，判断是否commit，已经sqlSession是否关闭。

  private class SqlSessionInterceptor implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
        //sqlSession目标方法执行
        Object result = method.invoke(sqlSession, args);
        //判断resources中是否存在绑定的sqlSession、sqlSessionFactory。不存在直接提交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) {
          // 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) {
          //改方法中会判断resources中是否存在绑定的sqlSession、sqlSessionFacotory。如果不存在，关闭sqlSession
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }

  public static void closeSqlSession(SqlSession session, SqlSessionFactory sessionFactory) {
    notNull(session, NO_SQL_SESSION_SPECIFIED);
    notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);

    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
    if ((holder != null) && (holder.getSqlSession() == session)) {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Releasing transactional SqlSession [" + session + "]");
      }
      holder.released();
    } else {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Closing non transactional SqlSession [" + session + "]");
      }
      session.close();
    }
  }

mybatis交由spring管理事务

　　　如果myBatis和spring集成，且spring开启了事务管理（声明式事务、注解式事务）

　　　myBatis会将sqlSession交给spring来管理。会在一个可能的事务方法中，将sqlSession交由spring管理。

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

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

    //每一次获取sqlSession。都提前从resources中获取sqlSessionHolder
    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);

    SqlSession session = sessionHolder(executorType, holder);
    //如果spring管理了事务，则直接获取sqlSession
    if (session != null) {
      return session;
    }

    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug("Creating a new SqlSession");
    }
    //通过sqlSessionFactory 创建新的事务
    session = sessionFactory.openSession(executorType);
    registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
    return session;
  }