I read that JCA is for legacy EIS integration. Is this spec geared towards vendors and not the application developer? I am curious to know of use cases where developers wrote JCA adapters to solve their technical or business problems.
I've written JCA connectors for all sorts of end systems (FTP, SFTP, File, financial systems).
This is mainly in the investment banking sector where I need to send trade and/or static data to various systems inside and outside the bank. Anything from RESTFul JSON/XML web services to socket calls to a mainframe can be involved in a business transaction.
So JCA comes in real handy is it provides a uniform programming model and can be managed by Application servers which help you with transactionality, pooling etc.
Want the FTP file containing the very expensive trade to arrive (transactionally guaranteed)? JCA is one technology you can use to tackle that.
<blatant plug> I'll add that I currently work on an open source project called Ikasan which has free JCA connectors </blatant plug> as do several other projects such as Mule and Spring Integration. So it's not often the average developer has to write their own.