You can use newArrayList method from Guava library
@GET
@Produces({ MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML })
public Iterable<Complex> getISS(){
DB db = ConnectionHelper.client.getDB("testdb");
Jongo jongo = new Jongo(db);
MongoCollection complex = jongo.getCollection("COMPLEX");
Iterable<Complex> all = complex.find().as(Complex.class);
return Lists.newArrayList(all);
}
or copy iterable into a new List, see https://stackoverflow.com/a/10117051/122975 for more information
BTW you should no instanciate Jongo each time you handle a new request.