Spring Remoting is an RMI type of facility built into the Spring framework. Basically you define an interface and an implementation on a remote application. Spring then places a proxy in your application and when it is called, goes over HTTP to the remote implementation and returns it as if the implementation was local. It's really quite easy to implement using Spring. A few lines of configuration of where to find the remote implementation, a few lines to expose the remote implementation over HTTP and you're set. This ends up being a very cheap way to start having services exposed in your applications. There are definitely some downsides to this approach. The first is that it's only Java. There are some options to use Hessian/Burlap extensions that you can use, but deeper object graphs have difficulty travelling across the wire. Another is the potential set of dependency problems that can occur when using an RMI-like solution.
RMI and Dependencies
Probably the most significant downside to RMI doesn't really occur until you have used it for a while. Maybe you just have a few services that need to be exposed, Spring Remoting seems easy, so you use it. But then it grows, maybe other applications use it and it becomes more critical. The question is what objects are being transferred over RMI? So if you call the service and try to find the address associated with user John Doe, how is the address returned? Probably this is some type of Address class. Then the big question. Where does Address class live? The problem is the Address class needs to be available to both the server (which knows how to look up addresses) and each of the clients calling it. Changes to the service or to the objects can have significant ripple effects in the application. The problem is easy to understand, but slowly creeps up on a project and becomes a dependency nightmare. I thought that this was a logical first step toward a true web service. The problem with this line of thinking is that if it stays this way too long, theres already too much damage and the refactor is too costly.
Why not start with a web service? Web services are somewhat expensive to create so you have to make sure it's necessary. First you must develop some form of input to be accepted. Maybe this is an XML, or a JSON object and whether or not there is a proper schema doesn't really matter. It still needs to be thought about and defined, formally or informally. Next, code needs to be written to translate between the request and the business objects of the back end system. The same translation needs to happen for the response. The client also needs to translate to/from this same intermediate format. There are obviously things that can make this easier like code generation and such, but it's still additional work. In early phases of a project where the inputs/outputs might be changing substantially, this can lead to a developers thrashing with the services and producing very little.
Hibernate and RMI
Another potential RMI gotcha is attempting to transfer Hibernate POJOs. First, Hibernate POJOs are special. They have lazy loaded collections and other proxied objects that are more complex than just your basic JDK objects. The immediate consequence of this is every caller of the RMI service not only needs to have the POJO classes in their classpath, but also the Hibernate jars. The more subtle consequence of this, is what happens when one of those lazily loaded collections is transferred to the caller? The objects can't be lazy loaded from the client, the client doesn't have the database connection etc. From here you really have three options. The first option is to enable remote lazy loading (example here). I've not used this, it seems far too complex and error prone. The second option involves just marking all associations non-lazy (or using joins). Lazy fetching is a nice performance feature of Hibernate and the service will no longer be able to leverage it. The third options is to add a custom object serializer to Spring remoting that will exchange the lazy collections for a real collection. This will remove the dependency on Hibernate and essentially force non-lazy loading of all associations. All of these solutions make RMI less attractive and all of them are a good indication that you should rethink the need for service remoting, or rethink using RMI over a proper web service.
Other workarounds - is it worth it?
There are several techniques that can be used that can reduce the symptoms of these problems. Aside from Hibernate, interfaces for each request and response for the data passed in and returned from the RMI service. This will reduce the amount of data available to the service, require well defined input and output and will be easier to refactor to a service later. All of this adds up to a decent amount of extra work. I think in the end, the extra time involved evens out or becomes more than a proper web service.
I think that the lesson I have learned is that Spring remoting does not give you cheap services. Rather it gives you services with a low cost of entry, but that cost climbs much more quickly. With web services, you pay more up front, and less over the long term. I think maybe the best of both worlds is to use RMI/Spring remoting for the very early stages of the project (i.e. before going to prod) so that the service can be ironed out. What input data is really needed? What should be returned? Do we know most of what the service needs to do? With answers to these questions (which will only be known after some development) we are better armed for creating a real web service. At this point, the RMI implementation can be swapped and refactored to a web service, hopefully avoiding the longer term RMI issues discussed above.