1. cloby.rb

require 'clojure'

class MyClojureObj < Clojure::Object
  def initialize
    dosync { @foo = 'foo' }
  end

  def foo; @foo; end
  def foo=(f); @foo = f; end
end

obj = MyClojureObj.new
puts "obj.foo = " + obj.foo

begin
  puts "Setting obj.foo to 'bar'"
  obj.foo = 'bar'
rescue ConcurrencyError
  puts "Oops, need a transaction"
end

puts "Trying again with a transaction"
dosync { obj.foo = 'bar' }
puts "Success"

puts "obj.foo = " + obj.foo

2. output

~/projects/jruby ➔ jruby -J-Xbootclasspath/a:build_lib/clojure-1.0.0.jar cloby.rb
obj.foo = foo
Setting obj.foo to 'bar'
Oops, need a transaction
Trying again with a transaction
Success
obj.foo = bar

3. ClojureLibrary.java

package org.jruby.clojure;

import clojure.lang.LockingTransaction;
import clojure.lang.Ref;
import java.io.IOException;
import java.util.concurrent.Callable;
import org.jruby.Ruby;
import org.jruby.RubyClass;
import org.jruby.RubyModule;
import org.jruby.RubyObject;
import org.jruby.anno.JRubyMethod;
import org.jruby.runtime.Block;
import org.jruby.runtime.ObjectAllocator;
import org.jruby.runtime.ThreadContext;
import org.jruby.runtime.builtin.IRubyObject;
import org.jruby.runtime.load.Library;

public class ClojureLibrary implements Library{
    public void load(Ruby runtime, boolean wrap) throws IOException {
        RubyModule cljModule = runtime.defineModule("Clojure");
        runtime.defineClassUnder("Object", runtime.getObject(), new ClojureObjectAllocator(), cljModule);

        runtime.getKernel().defineAnnotatedMethods(ClojureDosync.class);
    }

    public static class ClojureObjectAllocator implements ObjectAllocator {
        public IRubyObject allocate(Ruby runtime, RubyClass klazz) {
            return new ClojureObject(runtime, klazz);
        }
    }

    public static class ClojureObject extends RubyObject {
        public ClojureObject(Ruby runtime, RubyClass klass) {
            super(runtime, klass);
        }

        private synchronized Ref getRef(int index) {
            Ref ref = (Ref)super.getVariable(index);
            if (ref == null) {
                try {
                    ref = new Ref(getRuntime().getNil());
                } catch (Exception e) {
                    throw getRuntime().newConcurrencyError(e.getLocalizedMessage());
                }
                super.setVariable(index, ref);
            }
            return ref;
        }

        @Override
        public Object getVariable(int index) {
            return getRef(index).deref();
        }

        @Override
        public void setVariable(int index, Object value) {
            try {
                getRef(index).set(value);
            } catch (IllegalStateException ise) {
                throw getRuntime().newConcurrencyError(ise.getLocalizedMessage());
            }
        }
    }

    public static class ClojureDosync {
        @JRubyMethod
        public static IRubyObject dosync(final ThreadContext context, final IRubyObject self, final Block block) {
            final Ruby ruby = context.getRuntime();
            try {
                return (IRubyObject)LockingTransaction.runInTransaction(new Callable() {
                    public Object call() throws Exception {
                        // re-get transaction in case this gets run in different threads
                        return block.call(ruby.getCurrentContext());
                    }
                });
            } catch (Exception e) {
                throw ruby.newConcurrencyError(e.getLocalizedMessage());
            }
        }
    }
}