Write software as if you were writing a library

Writing an application tends to be very different than writing a library.  You often forgo any sort of API and you tend to couple the user interface with the implementation.  Your main() method often makes calls directly to your objects.  This can sometimes cause future maintenance issues.  It could also prevent future functionality.

I advocate always writing code as if it were for a library.  Once in library form, you can easily write a frontend to use the library.  If one day in the future you need to change the interface drastically, you can do that.  If someone wants to use your code in a way that it wasn't originally intended for, it is much easier to use code that was written for a library.

I recently needed to write a reporting system.  I wanted something that was really easy for the other developers to create a new report without training.  There were lots of reporting engines out there, but they all had their own syntax.  Everything required some training to create a new report.  I wanted something that looks like everything else we write.  My idea was to use the JSP engine as the report generating engine.  I could use the same charting software that I use for websites.  I don't have to use some complicated template engine.  I started looking at the Tomcat source code to look at the JSP compilation code.

This is where I ran into a problem.  The JSP engine looked like it was tightly coupled to the Tomcat codebase as a whole.  I know I would need to use the servlet engine under the JSP engine, bit it was a bit more than that.  There was no easy way to separate the JSP engine from the web container.  If the JSP engine was written as a library, then I would be able to use the JSP engine as a reporting engine as well.