Integrating Erlang and Java

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

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

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

Introduction
Created in Java 1.1
Little more than 10,000 LoC
Primarily authored by Gordon Beaton
OTP_DIRlibjinterface-X.XprivOtpErlang.jar
Jinterface Nodes run in separate user space
Different than JRuby, Jython, Groovy, etc.
Speaks the wire protocol

Modeling Erlang Types
OtpErlangObject
OtpErlangPid
OtpErlangRef
OtpErlangList
OtpErlangPort
OtpErlangBinary
OtpErlangBoolean
OtpErlangByte
OtpErlangTuple
OtpErlangString
OtpErlangFloat
OtpErlangAtom

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

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() );

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

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

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() );

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);

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
}
});

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

Handling Connection Attempts
OtpNode node = new OtpNode("a");
public void connAttempt(String node,
boolean incoming, Object info) {
// handle failed connections
}
});

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() );

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

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;

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 );

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)
});

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());

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

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

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");

Linking
> process_flag(trap_exit, true).
> F = fun() ->
receive after 200 -> exit(normal) end
end.
> Pid = spawn_link (F).
first. link (second.self());
second.close() ;

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());
}

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

Concurrent Programming
Fast Context Switching
Fast Message Passing
Fast Process Spawning

Synchronization
Sending and receiving messages
Mailbox creation
Linking and unlinking processes
Node creation

Thanks
Dennis Byrne
[email_address]
http://notdennisbyrne.blogspot.com/

Integrating Erlang and Java

by Dennis Byrne on Nov 23, 2008

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