[erlang-questions] rpc from erlang to java

Vlad Dumitrescu <>
Wed Aug 15 09:26:07 CEST 2007


Hi all,

I'm not sure how many people are interacting with Java from Erlang,
but I am (for ErlIDE) and I just implemented a nice feature (if I may
say so myself) that might of interest: the ability to call Java code
from Erlang.

The reason I needed this is that I want to move as much erlide core
functionality from Java code to Erlang code, but doing it without rpcs
would have meant writing an awful lot of wrapper classes around the
Eclipse API.

So what I can do now (work is still in progress) is for example
  {ok, Map} = jrpc:call("java.util.HashMap", "HashMap", []),
  jrpc:call(Map, "put", "theKey", {some, kind, of value}),

The conversion layer converts between Erlang terms and Java objects,
almost seamlessly (atoms aren't easy to translate, for example) and
uses reflection to resolve and invoke the proper methods. Type
translation is trying to be clever, for example strings are
represented by native types, likewise integers. Lists correspond to
java.util.Lists and tuples to arrays.

Objects can also be sent back and forth, by storing them in a map and
converting to unique references.

Beside 'call's, there is support for 'cast's and 'event's. The latter
are async messages to which one can subscribe for notification on the
Java side.

The functionality is similar to what the corba interface provides, but
it's at the same time simpler (no idl descriptions are needed) and
more naive, and more powerful (since objects can be sent). I am
working on a tool to generate erlang stubs that would make the above
example look something like
  {ok, Map} = java_util_HashMap:'HashMap'(),
  java_util_HashMap:put(Map, "theKey", value),

Of course, this isn't a general solution, and it is very slow to use
as in the example above. One would still want to write special
interface classes that would do much more work per call.

There are also issues still to be resolved (like memory management and
concurrency issues), but I thought that someone might be interested
and find it useful. The code is part of Erlide but I tried to keep it
generic, so it can be extracted to a standalone library, if needed.

best regards,
Vlad



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