Archive for May, 2012

Maven Best Practices


Love or hate it… he will stay for a moment.
So let’s apply the best practices to our poms and maven builds.

Make the build reproducible
  - Always specify a version for Maven2 plugins
  - Minimize number of SNASPHOT dependencies
  - Use dependency management section
  - Beware of relocation in maven repo
  - After a dependency modification, double check the produced artifacts
Use and abuse of modules
  - more “technical/layered”
  - more business oriented
Make the build maintainable
  - Prefer default directory layout
  - Avoid duplication by moving common tags to parent pom
  - Always specify a version of dependencies in a parent pom
  - Use Properties Liberally
  - Minimize the number of Profiles
Make the build portable
  - Don’t commit eclipse and maven artifacts
  - Don't modify pom/artifactsin your "enterprise" repository

Make the build reproducible

Always specify a version for Maven2 plugins

Wrong way


Correct way


This matters because if you leave out the version, maven2 defaults to LATEST  available.

So this can mean that

  • the result of the plugin can become unpredictable if the implementation of the plugin has changed (new/removed feature, …)
  • your project unknowingly became automatic beta testers for third party plugins if the latest version is a SNAPSHOT

Newer version fo m2clipse will this show as warnings in the console. See also the maven enforcer plugin

Minimize number of SNASPHOT dependencies

It’s strictly unrecommended to use SNAPSHOT version in your project dependencies since it’s never guaranteed that a SNAPSHOT version is available in any repository. This can lead to well-know build errors due to missing dependencies . So for projects that don’t belong to ‘you’… it’s preferable to use a real version.

how to detect if you have a snapshot version

Use dependency management section

Transitive dependencies is great feature… but in the end you want to be sure of the version you are using and shipping to production

Dependency Management allows to consolidate and centralize the management of dependency versions without adding dependencies which are inherited by all children. This is especially useful when you have a set of projects (i.e. more than one) that inherits a common parent.

Another extremely important use case of dependencyManagement is the control of versions of artifacts used in transitive dependencies. This is hard to explain without an example. Luckily, this is illustrated in the documentation.


Beware of relocation in maven repo

Relocating an artifact is changing the “maven id” (groupId:artifactId) of a project.


One common pitfall in maven relocation is having double jars even with correct usages of dependencyManagement.
Use the m2clipse and his dependency hierarchy views, to detect and exclude the undesired artifacts.

After a dependency modification, double check the produced artifacts

A good habit is to double check your war/ear produced after the addition of a new dependency or the upgrade of an existing one.
2 greats to tools to help you in this

Use and abuse of modules

the modules can be more “technical”

  <module>mymodule_api</module> <!-- service interface, value objects, exception -->
  <module>mymodule_impl</module> <!-- service & dao implementation -->
  <module>mymodule_web</module> <!-- web components, controllers, templates.. -->


more business oriented


Make the build maintainable


Prefer default directory layout

this will make the plugin configuration more easy :

src/main/java         Application/Library sources
src/main/resources    Application/Library resources
src/main/webapp       Web application sources (for war packaging)
src/test/java         Test sources
src/test/resources    Test resources

Avoid duplication by moving common tags to parent pom

Do you really want to say that you compile for 1.5 jdk in all your projects ?

    <!-- Use Java 1.5 -->

move this in a parent pom !

for example, in

  corporate_base_pom  ->  app_basecom  ->       app_module1         -> app_sub_modules

    jdk 1.5                    technical                application                     
 enterprise repo             dependencies               dependencies
                             (spring,...)               (app_module2,app_module3,..)

Always specify a version of dependencies in a parent pom

prefer a central place for version definition.
Don’t specify version in a specific sub-modules

Use Properties Liberally

Grouping Dependencies with properties… to avoid copy/pasting the version everywhere.
and help upgrading easily to 3.0.5.RELEASE 😉
And move this to a parent pom : cfr Avoid duplication move to parent pom



Minimize the number of Profiles

Profiles can help a lot… but will inevitability complexify the process.
So don’t use profiles if for example adding a new project with and assembly will do the trick. best practices

  • The build must pass when no profile has been activated
  • Use profiles to adapt to build-time context, not run-time context, and not (with rare exceptions) to produce alternative versions of your artifact

Remark: The support for profiles outside of the POM or the settings.xml has been removed in Maven 3.x.

Make the build portable

don’t commit eclipse and maven artifacts

to make the checkout easier… avoid commiting the following files and directory


these files are often :

  • referencing local settings like JRE name/path/…
  • specific to a version of plugins (wtp,…)

so let m2clipse handle this and maintain/generate the .project, .classpath,…

see svnignore/cvs ignore

Don’t modify pom/artifacts in your “enterprise” repository

It’s always tempting to fix a pom or a jar in the central repository… don’t do this.
Identify it a special version in your repo ‘artefact-1.0.5.corporatepath’ or manage the correct exclusions.
see log4j example


Jenkins : diskspace requirement tips

Jenkins is great tool but I already wrote the default value don’t help to keep it running for a long time without terrabytes of disks. So let’s manage our diskspace requirement for maven builds using the various option of jenkins and the system groovy scripts.

Disable maven artefact archiving

This option will tell jenkins to collect pom,jars,wars,ears as they are produced by maven. This is rarely usefull when you use an enterprise repository. This option is enabled by default… so if you aren’t using it… disable it !

to do so you need to go in each job definition and check :

Build > Advanced > Disable automatic artifact archiving

As lazy programmer, you may be know that jenkins offer a jenkins script console.
So you can fix artefact archiving in a single batch with the following script :

String format ='%-45s | %-20s | %-10s | %-10s | %-30s'
def readonly = false
activeJobs = hudson.model.Hudson.instance.items.findAll
    {job -> job.isBuildable() && job instanceof hudson.maven.MavenModuleSet}
def oneline= { str ->   if (str==null)     return "";  str.replaceAll("[\n\r]", " - ")}
println String.format(format , "job", "scm trigger","last status"," logrot","archiving")
println "-------------------------------------------------------------------------------------------------------------------------------"
activeJobs.each{run ->
    println String.format(format ,,oneline(run.getTrigger(hudson.triggers.Trigger.class)?.spec), run?.lastBuild?.result, run.logRotator.getDaysToKeep()+" "+run.logRotator.getNumToKeepStr(), ""+run.isArchivingDisabled()) ; 
    if (!run.isArchivingDisabled() && !readonly ) {        

adjust the readonly variable to true to fix them automatically 😉

job                  | scm trigger          | last status |  logrot  | archiving                    
myproject_ci         | 24 * * * *           | SUCCESS    | -1 10     | true

Discard Old Build

you can easily locate the jobs leaking logs

noLogRotation = hudson.model.Hudson.instance.items.findAll
                   {job -> job.isBuildable() && job.logRotator==null}
noLogRotation.each() {println}

and fix them also by providing a logRotator

def jobs = hudson.model.Hudson.instance.items.findAll
                                         { !it.logRotator && !it.disabled }
jobs.each { job ->
   // days to keep, num to keep, artifact days to keep, num to keep 
   job.logRotator = new hudson.tasks.LogRotator ( 30, 40, 1, 1) 
   println "$ fixed " 

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