While attending JavaZone recently, I went to a talk by Neal Ford. To be honest, the talk was not something mind-blowing, many tools he showed were either outdated or not best of breed, but he stated a very important fact: application servers are meant to address resources scarcity by sharing resources, while those resources are no more scarce in this day and age.
In fact, this completely matches my experience. Remember 10 years ago when we had to order hardware 6 months in advance? At that time, all webapps were deployed on the same application server - which weren’t always clustered. After some years, I noticed that application servers grew in number. Some were even required to be clustered because we couldn’t afford the service offered to be off. It was the time when people started to think about which application to deploy on which application server, because of their specific load profile. In the latest years, a new behavior appeared: deploying a single webapp to a single application server because it was deemed too critical to be potentially affected by other apps on the same application server. And that led sometimes to the practice of doing that for every application, whether the latter was seen as critical or not.
Nowadays, hardware is available instantly, in any required quantity and for nothing: they call it the Cloud. Then why are we still using application servers? I think that the people behind the Spring framework asked themselves the same question and came up with a radical answer: we don’t need them. Spring has always been about pragmatic development when JavaEE (called J2EE then) was still a big bloated standard, full of barbaric acronyms and a real pain in the ass to develop with (remember EJB 2.0?) - no wonder so many developers bitch about Java. Spring valued simple JSP/Servlet containers over fully-compliant JavaEE servers and now they finally crossed the Rubicon as no external application server is necessary anymore.
When I first heard about this, I was flabbergasted, but in the age of micro-services, I guess this makes pretty much sense. Imagine you just finished developing your application. Instead of creating a WAR, an EAR or whatever package you normally do, you just push to a Git repo. Then, a hook pushes the code to the server, stops the existing application and starts it again. Wouldn’t that be not only fun but really Agile/Devops/whatever-cool-concept-you-want? I think that would, and that’s exactly the kind of deployment Spring Boot. This is not the only feature of Spring Boot, it also provides a real convention over configuration, useful Maven POM, out-of-the-box metrics and health checks and much much more, but embedding Tomcat is the most important one (IMHO).
On the opposite side, big shops such as IBM, Oracle, even Red Hat still invest huge sums of money into developing their full profile Java EE compliant application servers. The funny stuff is that in order to be JavaEE compliant, you have to implement "interesting" bits such as Java Connector Architecture, something I’ve seen only once early in my career to connect to a CICS. Interestingly, the Web Profile defines a lightweight standard, leaving out JCA… but also JavaMail. However, it goes the way of going lightweight.
Now, only the future will tell what and how happens next, but I can see a trend forming there.