Clojure & Java Interop

About a year ago I got a phone call asking if I wanted to join another team at DRW. The team supports a (primarily) Java application, but the performance requirements would also allow it to be written in a higher level language. I'd been writing Clojure (basically) full-time at that point - so my response was simple: I'd love to join, but I'm going to want to do future development using Clojure.

A year later we still have plenty of Java, but the vast majority of the new code I add is Clojure. One of the big reasons I'm able to use Clojure so freely is the seamless interop with Java.

Execute Clojure from Java
Calling Clojure from Java is as simple as loading the .clj file and invoking a method from that file. I used the same example years ago, but I'll inline it here for simplicity.

; interop/core.clj
(ns interop.core)

(defn print-string [arg]
  (println arg))

// Java calling code
RT.loadResourceScript("interop/core.clj");
RT.var("interop.core", "print-string").invoke("hello world");

note: examples from this blog entry are available in this git repo. The commit with the code from the previous example is available here and I'm running the example from the command line with:

lein jar && java -cp "interop-1.0.0.jar:lib/*" interop.Example

Execute Java from Clojure
At this point we have Java executing some Clojure code, and we also have Clojure using an object that was created in Java. Even though we're in Clojure we can easily call methods on any Java object.

(ns interop.core)

(defn print-string [arg]
  (println arg "is" (.length arg) "characters long"))

commit

The above code (using the length method of a String instance) produces the following output.

hello world is 11 characters long

Calling a Java method and passing in additional arguments is also easy in Clojure.

(ns interop.core)

(defn print-string [arg]
  (println (.replace arg "hello" "goodbye")))

commit

The above code produces the following output.

goodbye world

There are a few other things to know about calling Java from Clojure. The following examples show how to call static methods, use enums, and use inner classes.

(ns interop.core)

(defn print-string [arg]
  ;;; calling a static method
  (println (String/valueOf true))

  ;;; using an enum
  (println (java.util.concurrent.TimeUnit/SECONDS))

  ;;; using a Java nested (inner) class. Note, in Clojure you
  ;;; use a $ instead of a .
  (println (java.util.AbstractMap$SimpleEntry. "key" "val")))

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And, the output:

true
#< SECONDS>
#<SimpleEntry key=val>

Create Java objects in Clojure
When working with Clojure you'll likely want to interact with existing Java objects, but you'll probably also want to create new instances of Java objects. You might have noticed the dot at the end of Abstract$SimpleEntry. in the previous example - that's how you instruct Clojure to create an instance of a Java object. The following example shows the dot notation for calling a constructor of the String class.

(ns interop.core)

(defn print-string [arg]
  (println (String. arg)))

commit

At this point our output is back to the original output.

hello world

When creating Java objects it's often beneficial to know which Java interfaces the Clojure data structures implement. The following examples demonstrate how you can create Java objects while passing Clojure datastructures (and functions) as constructor arguments.

(ns interop.core)

(defn print-string [arg]
  ;;; pass a Clojure vector where Java expects a java.util.Collection
  (println (java.util.HashSet. ["1" "2"]))

  ;;; pass a Clojure map where Java expects a java.util.Map
  (println (java.util.LinkedHashMap. {1 "1" 2 "2"}))

  ;;; pass a Clojure function where Java expects a Runnable
  (println (Thread. (fn [] (println "clojure fns are runnables (and callables)")))))

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The output shows the constructed Java objects.

#<HashSet [2, 1]>
#<LinkedHashMap {1=1, 2=2}>
#<Thread Thread[Thread-1,5,main]>

Calling constructors in Clojure is very easy, but that's not always an option when creating a Java object. At times you will likely need to create an instance of a Java interface. Clojure provides both proxy and reify for creating instances of Java interfaces. The following example demonstrates the syntax for using either proxy or reify.

(ns interop.core)

(defn proxy-coll []
  (proxy [java.util.Collection] []
    (add [o]
         (println o)
         true)))

(defn reify-coll []
  (reify java.util.Collection
    (add [this o]
         (println o)
         (println this)
         true)))

(defn main []
  (.add (proxy-coll) "this string is printed on proxied.add")
  (.add (reify-coll) "this string is printed on reified.add"))

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note, I also changed Example.java (the details are available in the above linked commit). The syntax for proxy and reify are fairly similar, and both offer additional options that are worth looking into. The primary differences between these two simple examples are:

• The proxy implementation requires an empty vector where we could specify constructor arguments (if this were an abstract class instead of an interface).
• The arg list for all methods of reify will specify the reified instance as the first argument. In our example the Collection.add method only takes one argument, but in our reify we also get the instance of the collection.

You might have also noticed that both implementations of add have "true" at the end - in our example we're hard-coding the return value of add to always return true. The following output is the result of running the current example code.

this string is printed on proxied.add
this string is printed on reified.add
#<core$reify_coll$reify__11 interop.core$reify_coll$reify__11@556917ee>

It's worth reading the docs to determine whether you want proxy or reify; however, if you don't see a clear choice I would opt for reify.

Returning objects from Clojure to Java
Our current Example.java returns something from the call to invoke on the clojure.lang.Var that is returned from RT.var("interop.core", "main"), but we're ignoring it so we have no idea what's returned.* Let's change the code and return something on purpose.

// interop/Example.java
package interop;

import clojure.lang.RT;

public class Example {
    public static void main(String[] args) throws Exception {
        RT.loadResourceScript("interop/core.clj");
        System.out.println(RT.var("interop.core", "main").invoke());
    }
}

; interop/core.clj
(ns interop.core)

(defn main []
  {:a "1" :b "2"})

Running our changes produces the following output.

{:a "1", :b "2"}

commit

At this point we are back in Java land after making a quick trip to Clojure to get a value. Returning most objects will be pretty straightforward; however, at some point you may want to return a Clojure function. This turns out to be fairly easy as well, since Clojure functions are instances of the IFn interface. The following code demonstrates how to return a Clojure function and call it from within Java.

// interop/Example.java
package interop;

import clojure.lang.RT;

public class Example {
    public static void main(String[] args) throws Exception {
        RT.loadResourceScript("interop/core.clj");
        clojure.lang.IFn f = (clojure.lang.IFn) RT.var("interop.core", "main").invoke();
        f.invoke("hello world");
    }
}

// interop/core.clj
(ns interop.core)

(defn main [] println)

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The above example returns the println function from interop.core/main and then invokes the println function from within Java. I only chose to pass one argument to invoke; however, the IFn.invoke method has various overrides to allow you to pass several arguments. The above code works, but it can be simplified to the following example.

package interop;

import clojure.lang.RT;

public class Example {
    public static void main(String[] args) throws Exception {
        clojure.lang.IFn f = (clojure.lang.IFn) RT.var("clojure.core", "println");
        f.invoke("hello world");
    }
}

commit

It seems like a fitting end that our final output is the same as our original output.

hello world

*actually, it's the last thing that's returned, or "true" for this specific case.