Ggtcf

I suspect this may be somewhat related to java-interop since I call a lot of java functionality in my code.

When I run the following in my REPL (via emacs) it works exactly as it should

 (def height 100)
 (def image (BufferedImage. width height BufferedImage/TYPE_INT_ARGB))
 (def graphics (.createGraphics image))

 (.setColor graphics Color/black)
 (for [x (range 0 width 10)]
 (.drawLine graphics x 0 x height ))
 (for [y (range 0 height 10)]
 (.drawLine graphics 0 y width y))
 (ImageIO/write image "png" (io/file "output.png"))

An image of a grid is properly generated.

However if I do C-c C-k, it generates a blank image.

Now, when I stick it in a function and I run it via lein run I get a warning I don't understand:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by clojure.lang.InjectedInvoker/1832669781 (file:/home/n/.m2/repository/org/clojure/clojure/1.10.0/clojure-1.10.0.jar) to method sun.java2d.SunGraphics2D.setColor(java.awt.Color)
WARNING: Please consider reporting this to the maintainers of clojure.lang.InjectedInvoker/1832669781
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

I know very little about clojure and even less about java, but I am running openjdk 10.

I believe my code is written correctly (albeit poorly), is this an issue with my code or is it clojure?

I suspect the problem you have is that “for” does not do what you think it does. It generates a lazy sequence. At the REPL the printer will generally evaluate these, but standalone code will not.

Try replacing the for with doseq. This will eagerly execute your side effects and should improve matters.

The illegal access warnings are a red herring. Since the Java module system came in there are certain patterns of interop which generate them. Details of how to resolve the warnings are given in the Clojure FAQ at https://clojure.org/guides/faq#illegal_access