XPages to Java EE, Part 2: Terminology

Jan 18, 2019, 1:09 PM

Tags: javaee
  1. XPages to Java EE, Part 1: Overview
  2. XPages to Java EE, Part 2: Terminology
  3. XPages to Java EE, Part 3: Hello, World
  4. XPages to Java EE, Part 4: Application Servers
  5. XPages to Java EE, Part 5: Web Pages
  6. XPages to Java EE, Part 6: Dependencies
  7. XPages to Java EE, Part 7: MVC
  8. XPages to Java EE, Part 8: IDE Server Integration
  9. XPages to Java EE, Part 9: IDE Features Grab Bag
  10. XPages to Java EE, Part 10: Data Storage
  11. XPages to Java EE, Part 11: Mixing MVC and an API
  12. XPages to Java EE, Part 12: Container Authentication
  13. XPages to Java EE, Part 13: Why Do This, Anyway?

Much like with my earlier series, I think it'll be useful to take a minute to go over some of the terms that we'll need to know for dealing with Java EE, beyond just the many names of the platform. Additionally, I think it'll be useful to go over some of the things we specifically need to not know when it comes to non-OSGi development.

What To Leave Behind

Looking at my earlier vocabulary list, everything other than "JAR" and (unsurprisingly) "Java EE" can be eschewed from our brains for now. Bundles, plugins, update sites, "Require-Bundle", "Import-Package", all that stuff - forget it. That's all specific to OSGi and, while you can write Java EE apps in an OSGi environment (like XPages), it's very uncommon.

Unfortunately, even outside of XPages specifically, Eclipse conflates OSGi and non-OSGi development a lot, doing things like offering to modify the project classpath instead of OSGi metadata in plug-in projects and vice-versa in non-plug-in ones. It took me a while when getting up to speed on Java to figure out what was "normal Java", what was OSGi, and what was just an Eclipse-ism.

Fortunately, that separation will be made easier by our Maven focus. If it doesn't exist in the Maven project, it doesn't exist at all, regardless of what Eclipse says.

What To Keep

One of the promises of XPages at the start was that it would be a bridge to "normal" Java technologies, and, though imperfectly, it did achieve this goal. A lot of the concepts and technologies we encountered in XPages are either the same in stock Java EE or are historically related.

For one, the normal Java runtime is the same - all the classes starting with java.*, like java.io.List and whatnot. Those are part of "Java SE", and they'll come with you wherever you go in Java.

Additionally, XPages uses Servlet as its basis like most other Java web tools. In XPages, you can access things like the HttpServletRequest and HttpServletResponse by way of #{facesContext.externalContext.request} et al, and those objects are the same in a normal web app.

The "WEB-INF" folder that shows up inside an XPage'd NSF is also an EE-ism, and is the holding pen for "app stuff": configuration, dependency libraries, classes, and other bits all go in here. In an NSF, this is tucked away under "WebContent" (which I think is a semi-standard structural location for resources in uncompiled projects), but the idea is the same. "WEB-INF/lib" in there holds third-party jars, while the hidden-in-Package-Explorer "WEB-INF/classes" holds the compiled classes for the application.

Thanks to the Extension Library, we've also had a surprisingly-smooth introduction to one of the most-important current Java EE technologies: JAX-RS. The ExtLib packaged up Apache Wink, a now-defunct implementation of the standard, and made it pretty easy to build on with OSGi plugins. Even though the version of JAX-RS Wink implemented is a little old, the core concepts are the same, and so, if you ever walked down that path, that knowledge will serve you directly.

What To Learn

There's potentially a whole ton to learn, but we'll start with a couple core concepts.

  • WEB-INF/web.xml- The web.xml file is the traditional core configuration point for a Java web app. You can specify configuration parameters, Servlet mappings, filters, and other bits here - though, over the years, annotation-based improvements have made it so that this file is now strictly optional during development. XPages doesn't have one of these in the NSF (it has kind of a pseudo implied one in the aether), but xsp.properties and faces-config.xml are conceptually related.
  • Web Application - I've been bandying around this term, and it's essentially the name of the finished product you put on a server. In XPages, an NSF is the main Web Application analogue: it's a contained blob of code that has its own internal configuration and identity.
  • Servlet Container, Web Container, or Application Server - These are varying terms for the software that loads and runs the web applications. In our case, that's Domino and its HTTP stack; in others, that will be Tomcat, WebSphere, GlassFish, or the like. Domino is technically a servlet container in two areas: the ancient "Java Servlets" support that haunts our server configuration documents and help docs to this day, and the hacked-apart subset of WebSphere that runs the XPages side of things.
  • Specs and Implementations - For cultural and historical reasons, the Java EE platform itself is a set of specifications, and each of those has at least one implementation, and one of THOSE is dubbed the "reference implementation" (usually developed with the spec and often coming from Oracle). So JAX-RS is a spec, while Jersey, Wink, CXF, and RESTEasy are implementations. For the most part, you don't need to care about the implementation, but you might if you want an extra feature that the spec doesn't provide or (as we'll talk about eventually) are deploying to a "bare bones" servlet container like Tomcat. When deploying to a Java EE server, you normally just write to the spec and the server will include some implementation to back it up.
  • Persistence - "Persistence" in this context basically means "databases". The most common database connection scheme for Java EE is JPA (Java Persistence API, you see) using JDBC to connect to a relational database. For NoSQL databases, the incubating project JNoSQL aims to behave similarly, though it doesn't have critical mass yet. With Domino, we never really had a layer like this - we either dealt with the lotus.domino API or xp:dominoDocument data sources, and those are much "closer to the metal", offering no object mapping or event hooks.
  • MicroProfile - Eclipse MicroProfile is a project started a couple years ago to take several of the most useful Java EE specifications, add a few new tricks, and create a small and speedy target without the huge code and political overhead of Java EE. Since it was started, Java EE went to Eclipse as well, and now Venn diagram of the two is getting closer together: MicroProfile picked up another EE spec or two and EE got its act together and shed a lot of the obligatory baggage. It can be thought of now as an "opinionated" subset of EE that's purpose-focused on microservices.
  • CDI - I've talked a bit before about CDI, and it deserves another mention here both because of its importance to EE development and because of how weird and "magic" its behavior is. At its core, CDI is "managed beans with super powers". While managed beans began their life in JSF (I believe), they're so useful as a concept that they were brought down the stack to become one of the underpinning technologies. Where things get weird is that, beyond just saying "I have a session-scoped bean named 'foo' with type SomeClass", CDI covers auto-injecting instances of classes into other objects and, in some cases, auto-creating implementations of interfaces via proxy objects. It can get really strange really fast, but the basics will hopefully be clear when we get to examples.

Next Steps

I figure that two posts of theory are enough for now, so, in the next post, I'll go through some steps to cover the creation of a new Java EE 8 application.

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