Extending with :gen-class a class that exposes its naked fields

Question

Suppose there is a Java class that doesn't provide getters and setters for all its fields, and I have to extend it with :gen-class and fiddle with them.

How do I access the superclass fields?

The quickest (and perhaps cleanest...) solution that comes to my mind right now is to create a java class that extends my super class, and extend it instead, but I was wondering if there is an alternative that sounds more direct.

Thanks!

Answer 1

The methods in generated classes can access base class fields with the help of the :exposes option of gen-class. :exposes expects a map where keys are symbols matching base class field names; values are also maps like {:get getterName, :set setterName}. Clojure generates those getter and setter methods automatically. They can be used to read and modify base class fields. This is documented in the docstring for gen-class.

This approach works for public and protected fields. It does not work for private fields.

Assuming the Java base class like this:

package fields;

class Base {
    public String baseField = "base";
}

The Clojure code to generate a subclass would be:

(ns fields.core
  (:gen-class
   :extends fields.Base
   :methods [[bar [] String]
             [baz [String] Object]]
   :exposes { baseField { :get getField :set setField }}))

(defn -bar [this]
  (str (.getField this) "-sub"))

(defn -baz [this val]
  (.setField this val)
  this)

(defn -main
  [& args]
  (println (.. (fields.core.) (bar)))
  (println (.. (fields.core.) (baz "new-base") (bar))))

Assuming all this is AOT compiled and ran, the output is:

base-sub
new-base-sub