Clojure vars and Java static methods

Question

I'm a few days into learning Clojure and are having some teething problems, so I'm asking for advice.

I'm trying to store a Java class in a Clojure var and call its static methods, but it doesn't work.

Example:

user=> (. java.lang.reflect.Modifier isPrivate 1)
false
user=> (def jmod java.lang.reflect.Modifier)
#'user/jmod
user=> (. jmod isPrivate 1)
java.lang.IllegalArgumentException: No matching method found: isPrivate for class java.lang.Class (NO_SOURCE_FILE:0)
    at clojure.lang.Compiler.eval(Compiler.java:4543)

From the exception it looks like the runtime expects a var to hold an object, so it calls .getClass() to get the class and looks up the method using reflection. In this case the var already holds a class, so .getClass() returns java.lang.Class and the method lookup obviously fails.

Is there some way around this, other than writing my own macro?

In the general case I'd like to have either an object or a class in a varible and call the appropriate methods on it - duck typing for static methods as well as for instance methods.

In this specific case I'd just like a shorter name for java.lang.reflect.Modifier, an alias if you wish. I know about import, but looking for something more general, like the Clojure namespace alias but for Java classes. Are there other mechanisms for doing this?

Edit:

Maybe I'm just confused about the calling conventions here. I thought the Lisp (and by extension Clojure) model was to evaluate all arguments and call the first element in the list as a function.

In this case (= jmod java.lang.reflect.Modifier) returns true, and (.getName jmod) and (.getName java.lang.reflect.Modifier) both return the same string.

So the variable and the class name clearly evaluate to the same thing, but they still cannot be called in the same fashion. What's going on here?

Edit 2

Answering my second question (what is happening here), the Clojure doc says that

If the first operand is a symbol that resolves to a class name, the access is considered to be to a static member of the named class... Otherwise it is presumed to be an instance member

http://clojure.org/java_interop under "The Dot special form"

"Resolving to a class name" is apparently not the same as "evaluating to something that resolves to a class name", so what I am trying to do here is not supported by the dot special form.

Michał Marczyk Michał Marczyk · Accepted Answer · 2010-05-17T20:51:37

(Update: I've prepared something which might be acceptable as a solution... The original answer remains below a horizontal rule towards the end of the post.)

I've just written a macro to enable this:

(adapter-ns java.lang.reflect.Modifier jmod)
; => nil
(jmod/isStatic 1)
; => false
(jmod/isStatic 8)
; => true

The idea is to create a single-purpose namespace, import the statics of a given class as Vars into that namespace, then alias the namespace to some useful name. Convoluted, but it works! :-)

The code is looks like this:

(defmacro import-all-statics
  "code stolen from clojure.contrib.import-static/import-static"
  [c]
  (let [the-class (. Class forName (str c))
        static? (fn [x]
                  (. java.lang.reflect.Modifier
                     (isStatic (. x (getModifiers)))))
        statics (fn [array]
                  (set (map (memfn getName)
                            (filter static? array))))
        all-fields (statics (. the-class (getFields)))
        all-methods (statics (. the-class (getMethods)))
        import-field (fn [name]
                       (list 'def (symbol name)
                             (list '. c (symbol name))))
        import-method (fn [name]
                        (list 'defmacro (symbol name)
                              '[& args]
                              (list 'list ''. (list 'quote c)
                                    (list 'apply 'list
                                          (list 'quote (symbol name))
                                          'args))))]
    `(do ~@(map import-field all-fields)
         ~@(map import-method all-methods))))

(defmacro adapter-ns [c n]
  (let [ias (symbol (-> (resolve 'import-all-statics) .ns .name name)
                    "import-all-statics")]
    `(let [ns-sym# (gensym (str "adapter_" ~n))]
       (create-ns 'ns-sym#)
       (with-ns 'ns-sym#
         (clojure.core/refer-clojure)
         (~ias ~c))
       (alias '~n 'ns-sym#))))

The above looks up the Var holding the import-all-statics macro in a somewhat convoluted way (which is, however, guaranteed to work if the macro is visible from the current namespace). If you know which namespace it's going to be found in, the original version I've written is a simpler replacement:

(defmacro adapter-ns [c n]
  `(let [ns-sym# (gensym (str "adapter_" ~n))]
     (create-ns 'ns-sym#)
     (with-ns 'ns-sym#
       (clojure.core/refer-clojure)
       ;; NB. the "user" namespace is mentioned below;
       ;; change as appropriate
       (user/import-all-statics ~c))
     (alias '~n 'ns-sym#)))

(Original answer below.)

I realise that this is not really what you're asking for, but perhaps clojure.contrib.import-static/import-static will be useful to you:

(use 'clojure.contrib.import-static)

(import-static clojure.lang.reflect.Modifier isPrivate)

(isPrivate 1)
; => false
(isPrivate 2)
; => true

Note that import-static imports static methods as macros.

Clojure vars and Java static methods

3 Answers