Programmatically Compiling Java Code Using Clojure

Everytime I am using some cryptic Java code as reference I get the feeling that my life would be so much easier if I could just copy/paste parts of it into REPL and see what that variable called a is doing, so I spend some time playing with Java Compiler API hoping it would save me some time later. Below snippet allows you to compile and execute Java code from a string, 

(defn javac [src & interface]
  (let [name (gensym)
        kind javax.tools.JavaFileObject$Kind/SOURCE
        extension (.extension kind)
        uri (java.net.URI.  (str "string:///dummy/" name extension))
        class-path ["-classpath" (System/getProperty "java.class.path")]
        src (proxy [javax.tools.SimpleJavaFileObject] [uri kind]
              (getCharContent 
                [_] 
                (if (= :interface interface)
                  (str "package dummy;" "public interface " name " {" src "}")
                  (str "package dummy;" "public class " name " {" src "}"))))
        file-filter (proxy [java.io.FilenameFilter] []
                      (accept
                        [dir, fname]
                        (not (nil? (re-matches
                                    (re-pattern (str name ".*\\.class"))
                                    fname)))))
        dummy-dir (java.io.File. "./dummy")]
    (-> (javax.tools.ToolProvider/getSystemJavaCompiler)
        (.getTask nil nil nil class-path nil [src]) 
        .call)
    (if (not (.exists dummy-dir)) 
      (.mkdir dummy-dir))
    (doseq [fc (.listFiles (java.io.File. "./") file-filter)] 
      (.renameTo fc (java.io.File. dummy-dir (str fc))))
    name))

This scheme isn't perfect, compiled class will be written to the current working directory instead of the structure dictated by its package so it needs to be moved manually and once a class is loaded/used recompiling it won't reflect your changes so above method uses gensym to create a unique class each time you compile under the package dummy, 

(javac
  (str
   "  public static void main(String args[]) {"
   "    System.out.println(\"This is in another java file\");"
   "  }"))

user=> (dummy.G__7/main (into-array String [""]))
user=> This is in another java file
nil

(javac
  (str
   "  public int two() {"
   "    return 1+1;"
   "  }"))

user=> (.two (dummy.G__12.))
user=> 2
Tags: clojure java