Recent versions of Excel have added new kinds of Conditional Formatting, such as icon sets, colour gradients and data bars. These are supported in both .xlsx and .xls file formats, but only rendered on newer versions of Excel It would be good if POI was able to read and write these, plus have then demonstrated in the ConditionalFormats example program
As of r1690813, it's now possible to read XLS and XLSX files with most of these conditional formats in without error, see the conditional formats, and to get back the type of the format rules. Still to do - decoding the details of these rules, getters and setters for reading and altering them, methods to allow you to create them, and examples
As of r1691869, it's now possible to read and write DataBar, Icon Set and Colour Scale conditional formattings, for HSSF and XSSF, with examples. Office 2010 extensions aren't supported (#58149), and nor are Filter CFs (#58150), see the respective entries for those future enhancements
For people curious how this was implemented, see related commits below: r1690494 CF12 headers r1690503 CFHeaderBase r1690527 rearrange CFRuleRecord to CFRuleBase r1690530 unit tests r1690686 CFRule12 r1690783 CFRule12 r1690796 implementation and unit test r1690803 implementation r1690808 rearrange logic r1690813 unit test r1691100 Icon r1691101 Icon r1691111 Icon r1691113 Icon r1691472 more CF types r1691479 Icon r1691676 interface for two more CF types r1691677 enum r1691679 Icon r1691787 Icon unit tests r1691788 Icon unit tests r1691789 Color Scale unit tests r1691858 Thresholds r1691859 Color Scale r1691860 ColorScale support, unit tests r1691864 rearrange logic to base class r1691865 ColorScale color support r1691866 ColorScale Formattings r1691867 DataBar support r1691868 DataBar support r1691869 DataBar example and tests