I mentioned in the terminology post that one of the new things to get used to in Java EE is the concept of a "Servlet Container" or "Application Server", and I think that this concept is worth a bit of going in to.
In a general sense, we've been working with this concept for a good while now: Domino is an application server in several senses, and (for the most part) the NSFs are the applications it houses. It blurs the lines in a couple ways by virtue of NSFs also being data stores, but an XPages application is a pretty direct match for a .war file deployed to an application server, code-wise.
So, conceptually we have a match, but a remaining huge different is choice. If you want to run an XPages app, you (more or less) just use Domino. For Servlet apps, there are a lot of options. And, importantly, there are two main types: basic servlet containers and full-fledged EE application servers.
The primary normal servlet container you're likely to have heard of is Apache Tomcat. It's very commonly used as an embedded server with Java web apps like Artifactory. Jetty is the other common one to know about, and it serves a similar role, and apparently has found its place as embedded plumbing inside complicated systems.
Those server won't inherently run a Java EE application, however. For example - if you try to deploy the app from the previous entry to Tomcat, it won't load up the JAX-RS services, since the undergirding infrastructure won't be there to pick up on them. You can, though, kind of piece together your own app server on top of Tomcat: if you include a JAX-RS implementation as a Maven dependency, you could make it work. This is a nice ability for if you only one to use one technology or are trying out something bleeding-edge that hasn't made it into a full server yet.
We'll largely be working with full EE servers, though - it's just more convenient to have these things provided by the container rather than having to pick from the buffet of implementations and provide connective glue to make them work together. The ones you're most likely to run into here (unless you fork over tons of cash for a commercial one) are GlassFish (the reference implementation, on its way to Eclipse), its cousin Payara, TomEE (conceptually Tomcat with bundled EE addons), WildFly, and Open Liberty (which I'm partial to).
Hypothetically, all of the EE servers are mutually compatible when it comes to the specs, and this plays out well for the most part. If you write a JAX-RS-based application like our example, it doesn't matter too much if you're running on WildFly (which uses RESTEasy for JAX-RS) or Liberty (which uses Apache CXF): neither needs additional configuration and they'll both do the same thing.
As usual, things get fiddly around the edges. For example, file handling in JAX-RS has historically been something of a mess, with each implementation having their own custom extension to the standard. Sometimes, it's more subtle: when working on my blog app, I discovered that Liberty serves up
text/html responses from JAX-RS with a header specifying ISO-8859-1 encoding, leading me to write a filter class to override this.
The larger your deployment, the more you'll care about the differences, too. Each server has different mechanisms for management/administration, clustering, and so forth. If you're in a situation where you or your company has already thrown in with one, you may as well stick with that, or pick one as closely related to what you do as possible. That's one of the reasons I've been using Liberty so much: since it has a common ancestry with the XPages runtime, some bits are familiar. Otherwise, unless you already have a business relationship with one of the commercial vendors, you can just pick whichever one suits your fancy.
I think that the next post will dive back into some actual code, specifically the pretty-important topic of actually serving up web pages.