This document summarizes how the Jinterface library allows Java code to interface with Erlang nodes and processes. It discusses how Jinterface models Erlang data types and nodes, allows sending messages between Java and Erlang processes, and supports features like linking, trapping exits, and remote execution. The goal of Jinterface is to integrate Java and Erlang so they can interact seamlessly.

1. Integrating Erlang and Java Dennis Byrne - ThoughtWorks [email_address] http://notdennisbyrne.blogspot.com/ © ThoughtWorks 2008

2. Overview Java vs. Erlang Jinterface Intro Erlang Data Types Nodes Message Passing Process Registration Linking Trapping Exits © ThoughtWorks 2008

3. Java vs. Erlang Strongly typed Object Oriented Pass by Reference Shared Memory Mutable State Nouns Dynamically Typed Functional Programming Pass by Value Non shared memory Immutable State Verbs © ThoughtWorks 2008 Both compiled to byte code

4. Introduction Created in Java 1.1 Little more than 10,000 LoC Primarily authored by Gordon Beaton OTP_DIR\lib\jinterface-X.X\priv\OtpErlang.jar Jinterface Nodes run in separate user space Different than JRuby, Jython, Groovy, etc. Speaks the wire protocol © ThoughtWorks 2008

5. Modeling Erlang Types OtpErlangObject OtpErlangPid OtpErlangRef OtpErlangList OtpErlangPort OtpErlangBinary OtpErlangBoolean OtpErlangByte OtpErlangTuple OtpErlangString OtpErlangFloat OtpErlangAtom © ThoughtWorks 2008

6. Modeling Nodes One JVM, more than one node Jinterface nodes are “hidden nodes” Hidden Node $ erl –hidden Non-transitive connections Not returned by erlang:nodes/0 © ThoughtWorks 2008

7. Modeling a Node, Long Names $ erl – name name > ‘ [email_address] ’ = node() . OtpNode node = new OtpNode("name@host.com"); assert "name@host.com".equals( node.node() ); © ThoughtWorks 2008

8. Modeling a Node, Short Name $erl -sname name > ‘ [email_address] ’ = node() . OtpNode node = new OtpNode( “ name ” ); assert “ [email_address] ” .equals( node.node() ); © ThoughtWorks 2008

9. Cookies, Command Line $ erl – name name – setcookie cookie > cookie = erlang:get_cookie() . OtpNode node = new OtpNode( “ name ” , “ cookie ” ); assert "cookie".equals( node.cookie() ); © ThoughtWorks 2008

10. Cookies, Programmatically $ erl – name name > erlang:set_cookie(node(), cookie) > cookie = erlang:get_cookie() . OtpNode node = new OtpNode( “ name ” ); node.setCookie("cookie") ; assert “ cookie ” .equals( node.cookie() ); © ThoughtWorks 2008

11. net_adm:ping/0 $ erl > pong = net_adm:ping(b@byrned) . > pang = net_adm:ping(c@byrned). OtpNode a = new OtpNode("a@byrned", "z"); OtpNode b = new OtpNode("b@byrned", "z"); OtpNode c = new OtpNode("c@byrned", "notz"); assert a.ping("b", 20); assert !a.ping("c", 20) & !a.ping("d", 20); © ThoughtWorks 2008

12. Local Status Changes OtpNode node = new OtpNode( “ node ” ); node.registerStatusHandler( new OtpNodeStatus() { public void localStatus(String node, boolean up, Object info) { // local initialization // or clean up logic } }); © ThoughtWorks 2008

13. Remote Status Changes, monitor_node/2 OtpNode node = new OtpNode( “ node ” ); node.registerStatusHandler( new OtpNodeStatus() { public void remoteStatus(String node, boolean up, Object info) { // peer node is up or down? } }); © ThoughtWorks 2008

14. Handling Connection Attempts OtpNode node = new OtpNode("a"); node.registerStatusHandler( new OtpNodeStatus() { public void connAttempt(String node, boolean incoming, Object info) { // handle failed connections } }); © ThoughtWorks 2008

15. Modeling Processes with OtpMbox $ erl – sname foo > Pid = self() , Msg = 999. > spawn ( fun() -> Pid ! Msg end ). > Msg = receive Any -> Any end . OtpNode node = new OtpNode("foo"); OtpMbox first = node.createMbox(); OtpMbox second = node.createMbox() ; OtpErlangInt msg = new OtpErlangInt(999); second.send( first.self() , msg); assert msg.equals( first.receive() ); © ThoughtWorks 2008

16. receive $ erl > receive Any -> Any after 1000 -> true end. OtpMbox mailBox = node.createMbox(); mailBox.receive(1000); © ThoughtWorks 2008

17. Pass By Value OtpNode node = new OtpNode("value"); OtpMbox from = node.createMbox(); OtpMbox to = node.createMbox(); OtpErlangLong sent = new OtpErlangLong(9); from.send(to.self(), sent); Object received = to.receive(); assert sent != received; © ThoughtWorks 2008

18. User Defined Classes OtpMbox to = node.createMbox(); OtpMbox from = node.createMbox(); Customer customer = new Customer(); customer.name = "Dennis"; from.send(to.self(), new OtpErlangBinary(customer)); OtpErlangBinary bin = (OtpErlangBinary) to.receive(); Customer object = (Customer)bin.getObject(); assert "Dennis".equals( object.name ); © ThoughtWorks 2008

19. Remote Execution OtpErlangObject[] call = new OtpErlangObject[] { new OtpErlangAtom("call"), new OtpErlangAtom("erlang"), /* mod */ new OtpErlangAtom("node"), /* fun */ new OtpErlangList(), /* arg */ new OtpErlangAtom("user") }; OtpErlangTuple msg = new OtpErlangTuple( new OtpErlangObject[] { mailBox.self(), new OtpErlangTuple(call) }); © ThoughtWorks 2008

20. Remote Execution mailBox.send("rex", "d@byrned", msg); OtpErlangTuple response = (OtpErlangTuple) mailBox.receive(); OtpErlangObject proc = response.elementAt(0); assert "rex".equals(proc.toString()); OtpErlangObject from = response.elementAt(1); assert "d@byrned".equals(from.toString()); © ThoughtWorks 2008

21. register/2 $ erl > register(first, self()). > Msg = 999. > spawn(fun() -> first ! Msg end). Msg = receive Any -> Any end. OtpMbox first = node.createMbox(); OtpMbox second = node.createMbox(); OtpErlangInt msg = new OtpErlangInt(999); first.registerName("first"); second.send("first", msg); assert msg.equals(first.receive()); © ThoughtWorks 2008

22. whereis/2 $ erl > Pid = self(). > register(w, Pid). > spawn(fun() -> Pid = whereis(w) end). OtpNode node = new OtpNode( “ whereis ” ); OtpMbox first = node.createMbox( “ w ” ); OtpMbox second = node.createMbox(); assert first.self() == second.whereis( “ w ” ) ; © ThoughtWorks 2008

23. registered/0 > register(a, self()). > F = fun() -> receive Any -> Any end end. > register(b, spawn(F)). > true = lists:member(a, registered() ). > true = lists:member(b, registered() ). OtpNode node = new OtpNode("node"); node.createMbox("a"); node.createMbox("b"); List names = Arrays. asList ( node.getNames() ); assert names.contains("a"); assert names.contains("b"); © ThoughtWorks 2008

24. Linking > process_flag(trap_exit, true). > F = fun() -> receive after 200 -> exit(normal) end end. > Pid = spawn_link (F). OtpMbox first = node.createMbox(); OtpMbox second = node.createMbox(); first. link (second.self()); second.close() ; © ThoughtWorks 2008

25. Trapping Exits {'EXIT', Pid, normal} = receive X -> X end. try { first.receive(); }catch(OtpErlangExit e) { assert second.self().equals(e.pid()); assert "normal".equals(e.getMessage()); } © ThoughtWorks 2008

26. Unlinking > Pid = spawn_link(Fun). > unlink(Pid). > nil = receive X -> received after 2000 -> nil end. OtpMbox first = node.createMbox(); OtpMbox second = node.createMbox(); first.link(second.self()); second.unlink(first.self()); second.exit("normal"); assert null == first.receive(1); © ThoughtWorks 2008

27. Concurrent Programming Fast Context Switching Fast Message Passing Fast Process Spawning © ThoughtWorks 2008

28. Synchronization Sending and receiving messages Mailbox creation Linking and unlinking processes Node creation © ThoughtWorks 2008

29. Thanks Dennis Byrne [email_address] http://notdennisbyrne.blogspot.com/ © ThoughtWorks 2008