using maven to manage java dependencies in a jruby rails app

Question

I'm trying to write a pom.xml that will allow me to run a command locally and fetch all dependencies that my jruby Rails app has. I'm seeing two different configs though and I'm not totally sure which to use (as I'm not a java person whatsoever)

First, many Pom's i'm seeing just have a tag under the root of the pom.xml that list all dependencies. This doesn't however have any information about where these are stored etc... so I feel like this isn't what I want (I need to copy them to my rails lib dir)

Second option, I'm seeing in the mvn docs is to use the maven-dependency-plugin, which seems more like what i'm looking for. I assume then that my outputDirectory would be something like lib

So I don't fully understand what the purpose of the first option's dependency list is for. All I want is mvn to copy my jars locally (and then eventually when my CI server does a deploy). Can someone point me in the right direction?

First Option

<project>
  <dependencies> 
   <dependency>
     <groupId>commons-lang</groupId>
     <artifactId>commons-lang</artifactId>
     <version>2.4</version>
   </dependency>
</project>

Second Option

<project>
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-dependency-plugin</artifactId>
        <configuration>
          <artifactItems>
            <artifactItem>
              <groupId>[ groupId ]</groupId>
              <artifactId>[ artifactId ]</artifactId>
              <version>[ version ]</version>
              <type>[ packaging ]</type>
              <classifier> [classifier - optional] </classifier>
              <overWrite>[ true or false ]</overWrite>
              <outputDirectory>[ output directory ]</outputDirectory>
              <destFileName>[ filename ]</destFileName>
            </artifactItem>
          </artifactItems>
          <!-- other configurations here -->
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

Answer 1

First, many Pom's i'm seeing just have a tag under the root of the pom.xml that list all dependencies. This doesn't however have any information about where these are stored etc... so I feel like this isn't what I want (I need to copy them to my rails lib dir)

This is the traditional way to declare and use dependencies on a Java project. Dependencies declared under the <dependencies> element are downloaded from a "remote repository" and installed to your "local repository" (in ~/.m2/repository by default) and artifacts are then handled from there. Maven projects (at least the Java ones) don't use a local lib/ folder for their dependencies.

Second option, I'm seeing in the mvn docs is to use the maven-dependency-plugin, which seems more like what i'm looking for. I assume then that my outputDirectory would be something like lib

The maven dependency plugin allows to interact with artifacts and to copy/unpack them from the local or remote repositories to a specified location. So it can be used to get some dependencies and copy them in lets say a lib/ directory indeed. Actually, it has several goals allowing to do this:

• dependency:copy takes a list of artifacts defined in the plugin configuration section and copies them to a specified location, renaming them or stripping the version if desired. This goal can resolve the artifacts from remote repositories if they don't exist in local.
• dependency:copy-dependencies takes the list of project direct dependencies and optionally transitive dependencies and copies them to a specified location, stripping the version if desired. This goal can also be run from the command line.

The first goal would use the setup you described in your second option. The second goal would use the standard project dependencies that you described in your first option. Both approaches would work.

The problem here is that I don't know exactly what a JRuby Rails app is, what the development workflow is, how to build such an app, etc so I don't know exactly what you need to do and, consequently, what would be the best way to implement that with Maven.

So I googled a bit and found this post that shows another approach based on OS commands (using the maven exec plugin) and has a complete pom.xml doing some other things. Maybe you should look at it and use it as a starting point instead of reinventing everything. This is my suggestion actually.