EJBs are built on top of RMI. Both imply Java clients and beans. If your clients need to be written in something else (e.g., .NET, PHP, etc.) go with web services or something else that speaks a platform-agnostic wire protocol, like HTTP or XML over HTTP or SOAP.
If you choose RMI, you don’t need a Java EE EJB app server. You have to keep client and server JVMs in synch; you can’t upgrade the client without upgrading the server. You have to write all the services that the EJB app server provides for you (e.g., connection pooling, naming and directory services, pooling, request queuing, transactions, etc.).
RMI is quite low level when you think about it. Why would you drop all the way back to CORBA?
A better choice is EJB 3.0 versus Spring. It depends on whether you like POJO development, want a choice of relational technologies besides ORM and JPA, among other things.
You can pay for a Java EE app server (e.g., WebLogic, WebSphere) or use an open source one (JBOSS, Glassfish and OpenEJB and ActiveMQ), or you can stick to Spring and deploy on Tomcat, Jetty, Resin or any other servlet/JSP engine.
Spring offers a lot of choice by being technology agnostic: persistence (Hibernate, iBatis, JDBC, JDO, JPA, TopLink), remoting (HTTP, Hessian, Burlap, RMI, SOAP web service), etc.
EJB 3.0 is a spec with many vendors; Spring can only be had from Spring Source.
I would recommend Spring. It’s very solid, has lots of traction, isn’t going anywhere. It leaves all your options open.
Web services are great in theory, but there are some gotchas that you need to watch out for:
- Latency. Fowler’s first law of distributed objects: “Don’t!” An architecture consisting of lots of fine-grained distributed SOAP services will be elegant, beautiful, and slow as molasses. Think carefully before distributing.
- Marshalling from XML to objects and back consumes CPU cycles that aren’t providing any business value besides allowing your clients to speak a platform-agnostic protocol.
- SOAP is a standard that is becoming more bloated and complex every day, but it has lots of tool support. Vendors like it because it helps drive sales of ESBs. REST is simple but not as well understood. It’s not supported by tools.
Spring’s web service module is very good, but do be careful about choosing to deploy this way. Write in terms of POJO service interfaces. These will allow you to get the conceptual isolation you want, defer the deployment choice until the last moment, and let you change your mind if the first thought doesn’t perform well.