jetbrick-orm/src/main/java/jetbrick/dao/orm/DbHelper.java
public Transaction transaction() {
        if (transationHandler.get() != null) {
            if (ALLOW_NESTED_TRANSACTION) {
                return new JdbcNestedTransaction(transationHandler.get().getConnection());
            }
            throw new TransactionException("Can't begin a nested transaction.");
        }
        try {
            JdbcTransaction tx = new JdbcTransaction(dataSource.getConnection(), transationHandler);
            transationHandler.set(tx);
            return tx;
        } catch (SQLException e) {
            throw new TransactionException(e);
        }
    }
public Transaction transaction() {
        if (transationHandler.get() != null) {
            if (ALLOW_NESTED_TRANSACTION) {
                return new JdbcNestedTransaction(transationHandler.get().getConnection());
            }
            throw new TransactionException("Can't begin a nested transaction.");
        }
        try {
            JdbcTransaction tx = new JdbcTransaction(dataSource.getConnection(), transationHandler);
            transationHandler.set(tx);
            return tx;
        } catch (SQLException e) {
            throw new TransactionException(e);
        }
    }
启动一个事务(默认支持子事务)