mybatis源码分析(四) mybatis与spring事务管理分析

mybatis源码分析(四) mybatis与spring事务管理分析

 

一丶从jdbc的角度理解什么是事务

　　从mysql获取一个连接之后, 默认是自动提交, 即执行完sql之后, 就会提交事务. 这种事务的范围是一条sql语句. 　　

　　将该连接设置非自动提交, 可以执行多条sql语句, 然后由程序决定是提交事务, 还是回滚事务. 这也是我们常说的事务.

Connection connection = dataSource.getConnection();

// connection.setTransactionIsolation(level.getLevel()); //设置事务隔离级别

// 设置是否自动提交, 如果不是自动提交, 则是"开启"事务
connection.setAutoCommit(desiredAutoCommit);

// connection预编译statement, 并执行sql
Statement stmt=connection.preparedStatement();
stmt.execute(sql);

// 提交事务, 或者回滚
connection.commit();
//connection.rollback();

 

　　从jdbc使用事务的角度来看, 事务主要是围绕connection展开的, 所以谁可获得connection, 即可控制事务.

 

 

二丶mybatis是如何使用事务的

 　　mybatis将jdbc中的事务操作抽象封装成Transaction,用于管理connection的生命周期--创建, 准备, 提交/回滚 和关闭.

/**
 * Wraps a database connection.
 * Handles the connection lifecycle that comprises: its creation, preparation, commit/rollback and close.
 *
 * @author Clinton Begin
 */
public interface Transaction {

  /**
   * Retrieve inner database connection.
   * @return DataBase connection
   * @throws SQLException
   */
  Connection getConnection() throws SQLException;

  /**
   * Commit inner database connection.
   * @throws SQLException
   */
  void commit() throws SQLException;

  /**
   * Rollback inner database connection.
   * @throws SQLException
   */
  void rollback() throws SQLException;

  /**
   * Close inner database connection.
   * @throws SQLException
   */
  void close() throws SQLException;

  /**
   * Get transaction timeout if set.
   * @throws SQLException
   */
  Integer getTimeout() throws SQLException;

}

 　　mybatis提供了两种事务实现,一种是完全由jdbc实现的事务JdbcTransaction,包括实现提交和回滚.一种是供容器管理整个生命周期的事务ManagedTransaction,其中将忽略提交和回滚事务的请求, 将提交和回滚事务由容器实现, 但其实这种事务很少用.

　　JdbcTransaction:

public class JdbcTransaction implements Transaction {

  private static final Log log = LogFactory.getLog(JdbcTransaction.class);

  protected Connection connection;
  protected DataSource dataSource;
  protected TransactionIsolationLevel level;
  protected boolean autoCommit;

  public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
    dataSource = ds;
    level = desiredLevel;
    autoCommit = desiredAutoCommit;
  }

  public JdbcTransaction(Connection connection) {
    this.connection = connection;
  }

  @Override
  public Connection getConnection() throws SQLException {
    if (connection == null) {
      openConnection();
    }
    return connection;
  }

  @Override
  public void commit() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Committing JDBC Connection [" + connection + "]");
      }
      connection.commit();
    }
  }

  @Override
  public void rollback() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Rolling back JDBC Connection [" + connection + "]");
      }
      connection.rollback();
    }
  }

  @Override
  public void close() throws SQLException {
    if (connection != null) {
      resetAutoCommit();
      if (log.isDebugEnabled()) {
        log.debug("Closing JDBC Connection [" + connection + "]");
      }
      connection.close();
    }
  }

  protected void setDesiredAutoCommit(boolean desiredAutoCommit) {
    try {
      if (connection.getAutoCommit() != desiredAutoCommit) {
        if (log.isDebugEnabled()) {
          log.debug("Setting autocommit to " + desiredAutoCommit + " on JDBC Connection [" + connection + "]");
        }
        connection.setAutoCommit(desiredAutoCommit);
      }
    } catch (SQLException e) {
      // Only a very poorly implemented driver would fail here,
      // and there's not much we can do about that.
      throw new TransactionException("Error configuring AutoCommit.  "
          + "Your driver may not support getAutoCommit() or setAutoCommit(). "
          + "Requested setting: " + desiredAutoCommit + ".  Cause: " + e, e);
    }
  }

  protected void resetAutoCommit() {
    try {
      if (!connection.getAutoCommit()) {
        // MyBatis does not call commit/rollback on a connection if just selects were performed.
        // Some databases start transactions with select statements
        // and they mandate a commit/rollback before closing the connection.
        // A workaround is setting the autocommit to true before closing the connection.
        // Sybase throws an exception here.
        if (log.isDebugEnabled()) {
          log.debug("Resetting autocommit to true on JDBC Connection [" + connection + "]");
        }
        connection.setAutoCommit(true);
      }
    } catch (SQLException e) {
      if (log.isDebugEnabled()) {
        log.debug("Error resetting autocommit to true "
            + "before closing the connection.  Cause: " + e);
      }
    }
  }

  protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommit);
  }

  @Override
  public Integer getTimeout() throws SQLException {
    return null;
  }

}

　　

　　mybatis事务执行流程:

　　1. 由于mybatis将事务抽取成一个接口, 便于管理, 所以可以在配置中配置事务管理的实现

　　2. 解析配置, 将事务管理对象, 保存到Configuration中

　　3. SqlSessionFactory创建SqlSession时, 将会同时注入tx对象

　　4. SqlSession执行sql语句时, 会委派给Executor执行, Executor处理主要的逻辑之外, 事务将会委派给事务对象处理, 如从事务对象中获取连接, 使用事务对象提交事务.

  //BaseExecutor
  // 在执行器里获取Connection , 最后是委派给Transaction获取,
  // 事务管理, 即是Connection是否设置自动提交, 以及将事务的回滚调用交给事务管理器管理
  protected Connection getConnection(Log statementLog) throws SQLException {
    Connection connection = transaction.getConnection();
    if (statementLog.isDebugEnabled()) {
      return ConnectionLogger.newInstance(connection, statementLog, queryStack);
    } else {
      return connection;
    }
  }

　　Transaction封装了connection,然后在transaction内部封装调用connection的操作,如提供不同的Transaction, 来管理connection的生命周期. 

　　

 

三丶spring是如何使用事务的

 　　srpingboot和mybatis整合,测试事务

 　　

　　1) 入口

　　在配置了DataSourceProperties属性之后, 会创建DataSource, 之后会创建DataSourceTransactionManager作为事务管理器

　　

　　 org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration

@Configuration
@ConditionalOnClass({ JdbcTemplate.class, PlatformTransactionManager.class })
@AutoConfigureOrder(Ordered.LOWEST_PRECEDENCE)
@EnableConfigurationProperties(DataSourceProperties.class)
public class DataSourceTransactionManagerAutoConfiguration {

    @Configuration
    @ConditionalOnSingleCandidate(DataSource.class)
    static class DataSourceTransactionManagerConfiguration {

        private final DataSource dataSource;

        private final TransactionManagerCustomizers transactionManagerCustomizers;

        DataSourceTransactionManagerConfiguration(DataSource dataSource,
                ObjectProvider<TransactionManagerCustomizers> transactionManagerCustomizers) {
            this.dataSource = dataSource;
            this.transactionManagerCustomizers = transactionManagerCustomizers.getIfAvailable();
        }

        @Bean
        @ConditionalOnMissingBean(PlatformTransactionManager.class)
        public DataSourceTransactionManager transactionManager(DataSourceProperties properties) {
　　　　　　　　// 默认使用DataSourceTransactionManager, 从使用的角度来说,具有通用性
            DataSourceTransactionManager transactionManager = new DataSourceTransactionManager(this.dataSource);
            if (this.transactionManagerCustomizers != null) {
                this.transactionManagerCustomizers.customize(transactionManager);
            }
            return transactionManager;
        }

    }

}

　　DataSourceTransactionManager继承于AbstractPlatformTransactionManager,而AbstractPlatformTransactionManager位于spring的tx子项目中　　

　　2) spring-tx子项目

　　该项目最主要是用于实现事务管理.

　　2.1) 最核心接口就是PlatformTransactionManager接口, 定义了事务管理器.

　　a) #getTransaction(TransactionDefinition): TransactionStatus

　　Return a currently active transaction or create a new one, according to the specified propagation behavior.

　　b) #commit(TransactionStatus): void

　　Commit the given transaction, with regard to its status. If the transaction has been marked rollback-only programmatically, perform a rollback.

　　c) #rollback(TransactionStatus): void

　　Perform a rollback of the given transaction.

 　　　　--更详细的文档则需要看源码或者API文档

 　2.2) TransactionDefinition 定义了事务的传播行为, 隔离界别, 事务超时时间等

　　2.3)TransactionStatus 定义了事务的状态, 以便于在提交事务或者回滚事务时决定如何后续行为.

 

 　　3)使用@Transactional注解声明事务

　　　　声明式事务,s是基于AOP实现的.Spring会对使用@Transactinal注解声明的方法进行动态代理, 生成使用org.springframework.transaction.interceptor.TransactionInterceptor增强对应方法的对象..

　　3.1) 事务切面方法实现

　　// org.springframework.transaction.interceptor.TransactionAspectSupport

    @Override
    @Nullable
    public Object invoke(MethodInvocation invocation) throws Throwable {
        // Work out the target class: may be {@code null}.
        // The TransactionAttributeSource should be passed the target class
        // as well as the method, which may be from an interface.
        Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

        // Adapt to TransactionAspectSupport's invokeWithinTransaction...
        return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
    }

/**
     * General delegate for around-advice-based subclasses, delegating to several other template
     * methods on this class. Able to handle {@link CallbackPreferringPlatformTransactionManager}
     * as well as regular {@link PlatformTransactionManager} implementations.
     * @param method the Method being invoked
     * @param targetClass the target class that we're invoking the method on
     * @param invocation the callback to use for proceeding with the target invocation
     * @return the return value of the method, if any
     * @throws Throwable propagated from the target invocation
     */
    @Nullable
    protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
            final InvocationCallback invocation) throws Throwable {

        // If the transaction attribute is null, the method is non-transactional.
        TransactionAttributeSource tas = getTransactionAttributeSource();
        final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
        final PlatformTransactionManager tm = determineTransactionManager(txAttr);
        final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

        if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
            // Standard transaction demarcation with getTransaction and commit/rollback calls.
            TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);

            Object retVal;
            try {
                // This is an around advice: Invoke the next interceptor in the chain.
                // This will normally result in a target object being invoked.
                retVal = invocation.proceedWithInvocation();
            }
            catch (Throwable ex) {
                // target invocation exception
                completeTransactionAfterThrowing(txInfo, ex);
                throw ex;
            }
            finally {
                cleanupTransactionInfo(txInfo);
            }
            commitTransactionAfterReturning(txInfo);
            return retVal;
        }

        else {
            final ThrowableHolder throwableHolder = new ThrowableHolder();

            // It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
            try {
                Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {
                    TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
                    try {
                        return invocation.proceedWithInvocation();
                    }
                    catch (Throwable ex) {
                        if (txAttr.rollbackOn(ex)) {
                            // A RuntimeException: will lead to a rollback.
                            if (ex instanceof RuntimeException) {
                                throw (RuntimeException) ex;
                            }
                            else {
                                throw new ThrowableHolderException(ex);
                            }
                        }
                        else {
                            // A normal return value: will lead to a commit.
                            throwableHolder.throwable = ex;
                            return null;
                        }
                    }
                    finally {
                        cleanupTransactionInfo(txInfo);
                    }
                });

                // Check result state: It might indicate a Throwable to rethrow.
                if (throwableHolder.throwable != null) {
                    throw throwableHolder.throwable;
                }
                return result;
            }
            catch (ThrowableHolderException ex) {
                throw ex.getCause();
            }
            catch (TransactionSystemException ex2) {
                if (throwableHolder.throwable != null) {
                    logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
                    ex2.initApplicationException(throwableHolder.throwable);
                }
                throw ex2;
            }
            catch (Throwable ex2) {
                if (throwableHolder.throwable != null) {
                    logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
                }
                throw ex2;
            }
        }
    }

    /**
     * Create a transaction if necessary based on the given TransactionAttribute.
     * <p>Allows callers to perform custom TransactionAttribute lookups through
     * the TransactionAttributeSource.
     * @param txAttr the TransactionAttribute (may be {@code null})
     * @param joinpointIdentification the fully qualified method name
     * (used for monitoring and logging purposes)
     * @return a TransactionInfo object, whether or not a transaction was created.
     * The {@code hasTransaction()} method on TransactionInfo can be used to
     * tell if there was a transaction created.
     * @see #getTransactionAttributeSource()
     */
    @SuppressWarnings("serial")
    protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
            @Nullable TransactionAttribute txAttr, final String joinpointIdentification) {

        // If no name specified, apply method identification as transaction name.
        if (txAttr != null && txAttr.getName() == null) {
            txAttr = new DelegatingTransactionAttribute(txAttr) {
                @Override
                public String getName() {
                    return joinpointIdentification;
                }
            };
        }

        TransactionStatus status = null;
        if (txAttr != null) {
            if (tm != null) {
                status = tm.getTransaction(txAttr); // 这里使用了配置的PlatformTransactionManager获取事务状态
            }
            else {
                if (logger.isDebugEnabled()) {
                    logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
                            "] because no transaction manager has been configured");
                }
            }
        }
        return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
    }

 　　3.2) 查看springboot自动配置DataSourceTransactionManager实现

 

　　

    @Override
    protected Object doGetTransaction() {
        DataSourceTransactionObject txObject = new DataSourceTransactionObject();
        txObject.setSavepointAllowed(isNestedTransactionAllowed());
        ConnectionHolder conHolder =
                (ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
        txObject.setConnectionHolder(conHolder, false);
        return txObject; // 刚开始获取事务时, 由于没有开启事务, 所以为null
    }

    /**
     * This implementation sets the isolation level but ignores the timeout.
     */
    @Override
    protected void doBegin(Object transaction, TransactionDefinition definition) {
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
        Connection con = null;

        try {
            if (!txObject.hasConnectionHolder() ||
                    txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
                 // 从数据源中获取connection
                Connection newCon = obtainDataSource().getConnection();
                if (logger.isDebugEnabled()) {
                    logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
                }
                txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
            }

            txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
            con = txObject.getConnectionHolder().getConnection();

            Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
            txObject.setPreviousIsolationLevel(previousIsolationLevel);

            // Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
            // so we don't want to do it unnecessarily (for example if we've explicitly
            // configured the connection pool to set it already).
            if (con.getAutoCommit()) {
                txObject.setMustRestoreAutoCommit(true);
                if (logger.isDebugEnabled()) {
                    logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
                }
                con.setAutoCommit(false);
            }

            prepareTransactionalConnection(con, definition);
            txObject.getConnectionHolder().setTransactionActive(true);

            int timeout = determineTimeout(definition);
            if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
                txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
            }

            // Bind the connection holder to the thread.
            if (txObject.isNewConnectionHolder()) {
                // 将Connection和datasource关联, 交由事务同步管理器保存管理, 使用ThreadLocal隔离
                // TranscationSynchronizationManager也是Spring和mybatis-spring共同合作管理事务的桥梁
                // ThreadLocal与当前线程绑定, 即线程隔离, 并且使用了同一个DataSource作为key, 可以获取到同一个ConnectionHolder

                TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
            }
        }

        catch (Throwable ex) {
            if (txObject.isNewConnectionHolder()) {
                DataSourceUtils.releaseConnection(con, obtainDataSource());
                txObject.setConnectionHolder(null, false);
            }
            throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
        }
    }

 

四丶mybatis和spring的事务是如何结合使用的

 　　mybatis-spring项目,用于将Spring和mybatis整合

 　　mybatis源码分析(三) mybatis-spring整合源码分析

 

 　　mybatis-spring整合,需要配置SqlSessionFactoryBean构建生成SqlSessionFactory

 　　SqlSessionFactoryBean#buildSqlSessionFactory()

//如果为空,则使用默认的SpringManagedTransactionFactory生成SpringManagedTransaction
    targetConfiguration.setEnvironment(new Environment(this.environment,
        this.transactionFactory == null ? new SpringManagedTransactionFactory() : this.transactionFactory,
        this.dataSource));

 　　SpringManagedTransaction#openConnection()

 /**
   * Gets a connection from Spring transaction manager and discovers if this
   * {@code Transaction} should manage connection or let it to Spring.
   * <p>
   * It also reads autocommit setting because when using Spring Transaction MyBatis
   * thinks that autocommit is always false and will always call commit/rollback
   * so we need to no-op that calls.
   */
  private void openConnection() throws SQLException {
    // 从Spring transaction manager获取之前由Spring获取的connection
    this.connection = DataSourceUtils.getConnection(this.dataSource);
    this.autoCommit = this.connection.getAutoCommit();
    this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);

    LOGGER.debug(() ->
        "JDBC Connection ["
            + this.connection
            + "] will"
            + (this.isConnectionTransactional ? " " : " not ")
            + "be managed by Spring");
  }

　　有本文第二节可知, mybatis执行事务相关操作, 如获取Connection, 使用connection执行多条sql, 使用connection提交事务或者回滚事务, 都是委派给Transacation执行的,

要想将sql语句的执行由mybatis执行, 事务的提交或者回滚操作由Spring控制, 两者需要关联使用同一个connection, 在不同的方法中调用connection的相关方法操作,  (所以, Spring并没有直接使用mybatis sqlSession中提供的提交或者回滚方法) . 如何安全的获取同一个connection?这就需要使用TransactionSynchronizationManager

 

　　ThreadLocal<Map<Object, Object>> resources

　　保存connection资源,的 key为DataSource, value为ConnectionHolder

　　(所以, 事务只支持在一个数据源中, 0.0)

 

 

学习资料:

      Spring事务原理分析