Spring 是如何保证事务获取同一个Connection的

Spring 的事务管理是基于 JDBC 的事务的，如果要保证事务正确的执行，必须保证与数据库的链接是同一个

 

以JdbcTempalte为例：

public <T> T execute(StatementCallback<T> action) throws DataAccessException {
        Assert.notNull(action, "Callback object must not be null");

        Connection con = DataSourceUtils.getConnection(getDataSource());
        Statement stmt = null;
        try {
            Connection conToUse = con;
            if (this.nativeJdbcExtractor != null &&
                    this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativeStatements()) {
                conToUse = this.nativeJdbcExtractor.getNativeConnection(con);
            }
            stmt = conToUse.createStatement();
            applyStatementSettings(stmt);
            Statement stmtToUse = stmt;
            if (this.nativeJdbcExtractor != null) {
                stmtToUse = this.nativeJdbcExtractor.getNativeStatement(stmt);
            }
            T result = action.doInStatement(stmtToUse);
            handleWarnings(stmt);
            return result;
        }
        catch (SQLException ex) {
            // Release Connection early, to avoid potential connection pool deadlock
            // in the case when the exception translator hasn't been initialized yet.
            JdbcUtils.closeStatement(stmt);
            stmt = null;
            DataSourceUtils.releaseConnection(con, getDataSource());
            con = null;
            throw getExceptionTranslator().translate("StatementCallback", getSql(action), ex);
        }
        finally {
            JdbcUtils.closeStatement(stmt);
            DataSourceUtils.releaseConnection(con, getDataSource());
        }
    }

获取链接是由 DataSourceUtils 管理的：

public static Connection getConnection(DataSource dataSource) throws CannotGetJdbcConnectionException {
        try {
            return doGetConnection(dataSource);
        }
        catch (SQLException ex) {
            throw new CannotGetJdbcConnectionException("Could not get JDBC Connection", ex);
        }
    }

/**
 * Actually obtain a JDBC Connection from the given DataSource.
 * Same as {@link #getConnection}, but throwing the original SQLException.
 * <p>Is aware of a corresponding Connection bound to the current thread, for example
 * when using {@link DataSourceTransactionManager}. Will bind a Connection to the thread
 * if transaction synchronization is active (e.g. if in a JTA transaction).
 * <p>Directly accessed by {@link TransactionAwareDataSourceProxy}.
 * @param dataSource the DataSource to obtain Connections from
 * @return a JDBC Connection from the given DataSource
 * @throws SQLException if thrown by JDBC methods
 * @see #doReleaseConnection
 */

public static Connection doGetConnection(DataSource dataSource) throws SQLException {
   Assert.notNull(dataSource, "No DataSource specified");

   ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
   if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
      conHolder.requested();
      if (!conHolder.hasConnection()) {
         logger.debug("Fetching resumed JDBC Connection from DataSource");
         conHolder.setConnection(dataSource.getConnection());
      }
      return conHolder.getConnection();
   }
   // Else we either got no holder or an empty thread-bound holder here.

   logger.debug("Fetching JDBC Connection from DataSource");
   Connection con = dataSource.getConnection();

   if (TransactionSynchronizationManager.isSynchronizationActive()) {
      logger.debug("Registering transaction synchronization for JDBC Connection");
      // Use same Connection for further JDBC actions within the transaction.
      // Thread-bound object will get removed by synchronization at transaction completion.
      ConnectionHolder holderToUse = conHolder;
      if (holderToUse == null) {
         holderToUse = new ConnectionHolder(con);
      }
      else {
         holderToUse.setConnection(con);
      }
      holderToUse.requested();
      TransactionSynchronizationManager.registerSynchronization(
            new ConnectionSynchronization(holderToUse, dataSource));
      holderToUse.setSynchronizedWithTransaction(true);
      if (holderToUse != conHolder) {
         TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
      }
   }

   return con;
}

注意方法 getConnection 的注释：

Is aware of a corresponding Connection bound to the current thread, for example
 * when using {@link DataSourceTransactionManager}. Will bind a Connection to the thread
 * if transaction synchronization is active (e.g. if in a JTA transaction).

如果使用了事务，则把当前获取的链接 bind 到当前线程，看下 TransactionSynchronizationManager.getResource(dataSource)的源码：

public static Object getResource(Object key) {
        Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
        Object value = doGetResource(actualKey);
        if (value != null && logger.isTraceEnabled()) {
            logger.trace("Retrieved value [" + value + "] for key [" + actualKey + "] bound to thread [" +
                    Thread.currentThread().getName() + "]");
        }
        return value;
    }

    /**
     * Actually check the value of the resource that is bound for the given key.
     */
    private static Object doGetResource(Object actualKey) {
        Map<Object, Object> map = resources.get();
        if (map == null) {
            return null;
        }
        Object value = map.get(actualKey);
        // Transparently remove ResourceHolder that was marked as void...
        if (value instanceof ResourceHolder && ((ResourceHolder) value).isVoid()) {
            map.remove(actualKey);
            // Remove entire ThreadLocal if empty...
            if (map.isEmpty()) {
                resources.remove();
            }
            value = null;
        }
        return value;
    }

 

静态变量：

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

TransactionSynchronizationManager内部用ThreadLocal对象存储资源，ThreadLocal存储的为DataSource生成的actualKey为key值和ConnectionHolder作为value值封装成的Map。

在某个线程第一次调用时候，封装Map资源为：key值为DataSource生成actualKey【Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);】value值为DataSource获得的Connection对象封装后的ConnectionHolder。