How to include a parent Spring project (using Git submodule)?

Question

I'm currently developing multiple web applications using Spring. I'm using Maven for building and Git for version control. At the moment I'm trying to find a way to split development of some things used by all webapps, e.g. I have some helper classes that are the same for all projects. The problem is, I don't want to use only classes, but also resource files and some sort of parent POM while still being independent from a repository and able to benefit from Git.

Although I'm not enthusiastic to change the build system, I'm not a real fan of Maven. Especially the concept of inheritance and aggregation is what constrains me right now. Maybe Ivy is an option?

I'd want to give you a quick overview of my setup:

There's some sort of parent project including some classes, Spring configuration files and other resources like templates, images and style sheets. Let's call it base. This one is not a complete Spring webapp and won't be deployed. There are several other projects which inherit from base and should be packed into a WAR. Let's call them webapp1 and webapp2.

base, webapp1 and webapp2 have their own Git repositories:

\
 |
 |- base.git       (base's repository)
 |
 |- webapp1.git    (webapp1's repository)
 | \
 |   base          (base used as a Git submodule)
 |
 |- webapp2.git    (webapp2's repository)
   \
     base          (base used as a Git submodule)

I want to be able to change bases code from inside the the webapps using a Git submodule and also be able to build a fully functional WAR of each webapp using mvn package inside the webapp`s directory.

Maven's parent or module don't allow a dynamic approach like this. I didn't find a way to use module like that at all and using parent for my needs is complex and static: Every change to base would require a new version to be pushed to the repository so that the webapp`s can inherit from it.

Maybe I didn't completely understand Maven's inheritance, but I'm pretty lost right now.

Did anyone achieve something similar with success? What build system did you use and how?

Answer 1

Your use case seems to be a good use case for overlays (also have a look at the examples).

Overlays are used to share common resources accross multiple web applications. In general, all dependencies of a WAR project are collected in WEB-INF/lib except for WAR artifacts that are overlayed on the WAR source.

But base wouldn't be "under" the webapp module, it would very likely be a sibling module (maybe it can be "in" the webapp using some Git black magic but this goes beyond my Git skills and I fail at thinking of how this could be handled with Maven). No, really, I think the Maven way would be to use overlays. And if this is not what you want, better use something else than Maven IMO.

Answer 2

I found a working solution by myself.

The basic solution of my problem is a small sub-element of the <parent> element called <relativePath>. It allows to search for the parent POM in the specified directory. Otherwise you would always need to deploy a new version before you could test it within your application.

<parent>
    <groupId>com.example</groupId>
    <artifactId>base</groupId>
    <version>1.0.0</groupId>
    <relativePath>base</relativePath>
</parent>

But that was only the starting point, allowing to work with a Git submodule. To get it really working, I needed to do the following:

• Create a WAR with classes and resources from both, the webapp and base, i.e. all files inside

  base/src/main/java
  base/src/main/resources
  base/src/main/webapp
  src/main/java
  src/main/resources
  src/main/webapp
  

  While resources are pretty easy to cover, a bit of configuration for org.apache.maven.plugin:maven-war-plugin is needed to get the webapp resources into the WAR. Compiling of two different source folders requires a plugin, so I need to use org.codehaus.mojo:build-helper-maven-plugin to get the classes of base into the WAR.

• Additionally, I used a lot of filtering for the resources, so there's almost no need for customization of the basic files to get a webapp up and running. For example I use ${project.artifactId} inside my main template file, so my HTML <head> will look something like this for a webapp called webapp1:

  <link href="stylesheets/base.css" rel="stylesheet" media="screen" type="text/css" />
  <link href="stylesheets/webapp1.css" rel="stylesheet" media="screen" type="text/css" />
  

It really took a lot of trial and error, but at last I got it working and I think this is the best way to achieve my target using Maven. This would've been a lot easier using a dynamic tool like Buildr, but sadly Buildr is slow on Windows (thanks to Ruby) and doesn't integrate very well into most IDEs.