public VariableInfo[] getVariableInfo(TagData data) 
{
 //$NON-NLS-1$
        List variables = new ArrayList(2);
        Object idObj = data.getAttribute(TagAttributeInfo.ID);
        // handle both the id and uid attributes
        if (idObj == null)
        {
            idObj = data.getAttribute("uid"); //$NON-NLS-1$
        }
        // avoid errors, but "id" and "id_rownum" will not be defined
        if (idObj != TagData.REQUEST_TIME_VALUE && idObj != null)
        {
            String tagId = idObj.toString();
            // don't try to add variables if id is not a valid java identifier.
            if (isJavaId(tagId))
            {
                // current row
                variables.add(new VariableInfo(tagId, Object.class.getName(), true, VariableInfo.NESTED));
                // current row number
                variables.add(new VariableInfo(
                    tagId + ROWNUM_SUFFIX,
                    Integer.class.getName(),
                    true,
                    VariableInfo.NESTED));
            }
        }
        return (VariableInfo[]) variables.toArray(new VariableInfo[]{});
}

Variables TableTag makes available in the pageContext.

 public static boolean isJavaId(String id) 
{
        if (id == null
            || id.length() == 0
            || Arrays.binarySearch(KEYWORDS, id)  >= 0
            || !Character.isJavaIdentifierStart(id.charAt(0)))
        {
            return false;
        }
        for (int j = 1; j <  id.length(); j++)
        {
            if (!Character.isJavaIdentifierPart(id.charAt(j)))
            {
                return false;
            }
        }
        return true;
}

isJavaId Returns true if the name is a valid java identifier.