Integrating Maven with Ivy

The problem: you have some resources in an Ivy repository (and only there) which you would like to use in a project based on Maven. Possible solutions:

• Migrate the repository to Maven (Nexus for example) since Ivy can easily use Maven-style repositories (so your Ivy clients can continue to use Ivy with some slight configuration changes and Maven clients will also work – also the push-to-repo process needs to be changed)
• Try JFrog Artifactory since it reportedly can serve the same resources to both Ivy and Maven (disclaimer: I haven’t tried to use it actually and I don’t know if the Open Source version includes this feature or not)
• or read on…

My goal for the solution (as complex as it may be) was:

• It should be as simple and self-explanatory as possible
• It should respect the DRY principle (Don’t Repeat Yourself)
• It shouldn’t have other dependencies than Maven itself

The solution looks like the following (for the full source check out the code-repo):

Have two Maven profiles: ivy-dependencies activates when the dependencies have already been downloaded and ivy-resolve when there are yet to download. This is based on checking the directory where the dependencies are to be copied ultimately:

...
<id>ivy-dependencies</id>
<activation>
  <activeByDefault>false</activeByDefault>
  <file>
    <exists>${basedir}/ivy-lib</exists>
  </file>
</activation>
...
<id>ivy-resolve</id>
<activation>
  <activeByDefault>false</activeByDefault>
  <file>
    <missing>${basedir}/ivy-lib</missing>
  </file>
</activation>
... 

Unfortunately there is a small repetition here, since Maven doesn’t seem to expand user-defined properties like ${ivy.target.lib.dir} in the profile activation section. The profiles also serve an other role: to avoid the consideration of the dependencies until they are actually resolved.

When the build is first run, it creates the target directory, writes the files needed for an Ivy build there (ivy.xml, ivysettings.xml and build.xml – for this example I’ve used some parts from corresponding files of the Red5 repo), runs the build and tries to clean up after itself. It also creates a dependencies.txt file containing the chunck of text which needs to be added to the dependencies list. Finally, it bails out (fails) instructing the user to run the command again.

On the second (third, fourth, etc) run the dependencies will already be present, so the resolution process won’t be run repeteadly. This approach was chosen instead of running the resolution at every build because – even though the resolution process is quick quick – it can take tens seconds in some more complicated cases and I didn’t want to slow the build down.

And, Ivy, the Apache BSF framework, etc are fetched from the Maven central repository, so they need not be preinstalled for build to complete successfully.

A couple of words about choosing ${ivy.target.lib.dir}: if you choose it inside your Maven tree (like it was chose in the example), you will receive warnings from Maven that this might not be supported in the future. Also, be sure to add the directory to the ignore mechanism of your VCS (.gitignore, .hgignore, .cvsignore, svn:ignore, etc), as to avoid accidentally committing the libraries to VCS.

If you need to add a new (Ivy) dependency to the project, the steps are as follows:

• Delete the current ${ivy.target.lib.dir} directory
• Update the part of your pom.xml which writes out the ivy.xml file to include the new dependency
• Run a build and watch the new dependency being resolved
• Update the dependencies section of the ivy-dependencies profile to include the new dependency (possibly copying from dependencies.txt)

One drawback of this method is the fact that advanced functionalities of systems based on Maven will not work with these dependencies (for example dependency analisys / graphing plugins, automated downloading of sources / javadocs, etc). A possible workaround (and a good idea in general) is to use this method for the minimal subset – just the jars which can’t be found in Maven central. All the rests (even if they are actually dependencies of the code fetched from Ivy) should be declared as a normal dependency, to be fetched from the Maven repository.

Finally I would like to say that this endeavour once again showed me how flexible both Maven and Ivy/Ant can be and clarified many cornercases (like how we escape ]] inside of CDATA – we split it in two). And it can also be further tweaked (for example: adding a clean target to the ivy-resolve profile, so you can remove the directory with mvn clean -P ivy-resolve or re-jar-ing all the downloaded jars into a single one for example like this, thus avoiding the need to modify the pom file every time the list of Ivy dependencies gets changed – then again signed JARs can’t be re-jarred so it is not an universal solution either).

Reference: Integrating Maven with Ivy from our JCG partners at Transylvania Java User Group.

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