Clojure: import java class dynamically

Question

Suppose I define such clojure function:

(defn print-this [this] (println "print this: " this))

If I use the function in repl:

(print-this that)

I would end up with:

print this: that
nil

as output.

Now if I make such definition:

(defn import-by-name [name] (import (some.package.value name)))

and use the function in repl:

(import-by-name "SomeClassName")

I get

java.lang.ClassNotFoundException: some.package.value.name (NO_SOURCE_FILE:0)

where I would expect that "name" be replaced by "SomeClassName" instead. If I type:

(import (some.package.value SomeClassName))

everything works as expected.

Why is it that [name] is not interpreted in the import-by-name function above? Is it possible to dynamically import a java class from a variable value? If so how? thanks!

Jared314 · Accepted Answer · 2013-10-08 21:29:59Z

6

import is a macro, so any symbols you pass to it will be taken literally.

(macroexpand '(import (some.package.value name)))

;; => (do (clojure.core/import* "some.package.value.name"))

Literals

For string literals, and by extension collection literals, you can use a macro to accomplish what you are describing.

(defmacro import-by-name [name] `(import '[some.package.value ~name]))

(import-by-name "ClassName") ;; => nil

Vars

For importing classes from a var, you have to start dipping into the internals of namespaces.

(defn import-by-name [n]
  (.importClass (the-ns *ns*)
                (clojure.lang.RT/classForName (str "some.package.value." n))))

There might be a cleaner way, and I will update this answer if I find one.

edited Oct 8, 2013 at 21:29

answered Oct 8, 2013 at 19:55

Jared314

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6 Comments

Jared314 Over a year ago

Just be aware, there are limits. Passing it a string works, but passing it a var will just append the symbol. I'm not sure what exactly you are trying to do.

Sebastien Over a year ago

Is there anyway to do a similar thing with a var? The goal is to interface clojure code with a large xjc-generated java library from a fairly complex xsd.

Jared314 Over a year ago

Are you trying to implement the equivalent of import some.package.*, or are you reading this list from a file?

Sebastien Over a year ago

No. It could be list from a file or a command-line argument. I want to import a small subset of classes from some.package.value but specify from input (file or command line). Thanks!

Jared314 Over a year ago

I've updated my answer to include an attempt at your scenario. Let me know if it works for you.

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