The Eclipse Java Compiler (ecj) has a lot of advantages over the standard javac compiler. It is fast, and it has way more warnings and errors that can be configured, improving code quality. One of the most interesting things in the compiler is the addition of nullity null types inside the compiler: by annotating your code with @Nullable and @NotNull annotations you can force the Eclipse compiler to check null accesses at compile time instead of runtime. When applied rigorously this teaches you to code way more safe (by preventing null values) and it prevents NPE exceptions during testing or production.
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Passing arguments to the compiler is an utter trainwreck. See the separate section on that below here. In this example I use the properties setting which allows me to provide detailed settings on which errors and warnings I want to have when compiling things. By using the ${project.basedir} variable inside the parameter I have these settings per project: every project is required to have a .settings/org.eclipse.jdt.core.prefs file present (which is by happy chance the location where the Eclipse IDE leaves its compiler settings).
The dependency on plexus-codehaus-eclipse defines the plugin that knows how to run the Eclipse compiler. The 2.8.3 version was the latest at the time of writing but this version has a few issues. Version 2.8.4 comes should come with a rewritten interface to the compiler which fixes a lot of issues, but this version is still in the works at the time of writing. You can find details on the plugin here, so progress can be followed on new releases/code changes.
The other important dependency is the org.eclipse.jdt:ecj dependency: this one specifies the exact version of the ecj compiler to use. You should always specify it because otherwise build stability will suffer when the plugin decides to use another version of the compiler one day before you have a big release 
From the list of releases one might be able to find a version number and then check this maven repository for something that looks like it. But this repository only contains the old versions. When you need a more recent release you should apparently look here at this one - this is where Eclipse currently pushes its versions. This newer repository does away with the easily recognizable version numbers of the earlier one; it uses version numbers like 3.1x.x as seen above. Eclipse usually has a major release once every year plus one or two fix releases in between. The second part in the 3.13.x number corresponds to the internal versioning used inside the Eclipse Platform project for releases. It is hard to get by a list but at least these are known:
| Version | Eclipse compiler version | Compiler version |
|---|---|---|
| 3.13.0 | Oxygen Release | 4.7 |
| 3.13.50 | Oxygen 1A | 4.7.1a |
| 3.13.100 | Oxygen R2 | 4.7.2 |
The version always starts with 3, the 13 is more or less the "year" of the release. So when 13 is Oxygen (2017, 4.7) 14 will probably be Photon (2018, 4.8).
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