What would be the correct way to serialize this Java Object in Clojure?

Question

I have a dummy Java Program, which I want to write in Clojure. It has a class which implements Serializable and a function which saves it. Since I have never written such programs in Clojure, I wanted to know what would be the correct way to approach this problem, what Clojure data structures, and api calls would you use?

import java. io. *; 

public class Box implements Serializable
{
private int width; private int height;
public void setWidth(int w)
   { width =w;}
public void setHeight(int h)
   {height = h;}
}

public static void main (String[] args)
{
   Box myBox =new Box();
  myBox.setWidth(50);
  myBox.setHeight(20) ;

try {
  FileoutputStream fs = new File("foo.ser");
  ObjectOUtputStream os = new ObjectOutputStream(fs);
  os.writeObject(myBox);
  os . close () ;

} catch (Exception ex) {} }

Answer 1

If you want to do it in pure Clojure, use the Clojure reader.

(use 'clojure.contrib.duck-streams)

(defn serialize [o filename]
  (binding [*print-dup* true]
   (with-out-writer filename
     (prn o))))

(defn deserialize [filename]
  (read-string (slurp* filename)))

Example:

user> (def box {:height 50 :width 20})
#'user/box
user> (serialize box "foo.ser")
nil
user> (deserialize "foo.ser")
{:height 50, :width 20}

This works for most Clojure objects already, but fails for most Java objects.

user> (serialize (java.util.Date.) "date.ser")
; Evaluation aborted.
No method in multimethod 'print-dup' for dispatch value: class java.util.Date

But you can add methods to the print-dup multimethod to allow Clojure to print other objects readably.

user> (defmethod clojure.core/print-dup java.util.Date [o w]
        (.write w (str "#=(java.util.Date. " (.getTime o) ")")))
#<MultiFn clojure.lang.MultiFn@1af9e98>
user> (serialize (java.util.Date.) "date.ser")
nil
user> (deserialize "date.ser")
#<Date Mon Aug 17 11:30:00 PDT 2009>

If you have a Java object with a native Java serialization method, you can just use that and not bother writing your own code to do it.