How to effectively use SNAPSHOT

Recently I showed how you can use the maven-release-plugin to create a release for your project. Now I would like to explain how to use maven to effectively use SNAPSHOT.

First what do I mean by SNAPSHOT? SNAPSHOT is a special version in maven that indicates the latest code; typically TRUNK or HEAD in your source control. With this version, maven will automatically grab the latest SNAPSHOT every time you build. On the other hand, when you are using 2.0, once maven has downloaded this artifact, it never tries to get a new 2.0.

Why should you use SNAPSHOT? You should use SNAPSHOT for rapidly moving code where bug fixes and enhancements are coming fast. Another reason is you should never require users of your project to checkout and build your code. EVER! I would venture to say that 50% of the open source projects I have had to checkout and build, have not initially built.

How to create a SNAPSHOT?
First you have to find a webserver to host the artifacts. I have typically used apache running on linux. I first create a folder under / called snapshots (owner:group equal to repoman:repoman) and then create a softlink (ln -s) under /var/www/html called snapshots (owner:group being root:root). So now you should have /var/www/html/snapshots --> /snapshots. For some examples of some public SNAPSHOT repositories check out Apache's and Codehaus.

Second you need to update your parent POM in order to properly use the maven-deploy-plugin. First make sure the version you are using includes -SNAPSHOT. For example something like 0.9.2-SNAPSHOT.

Third add the distributionManagement section to your parent POM to look something like this:

<distributionManagement>
<snapshotRepository>
<id>maven2-snapshot-repository</id>
<name>Maven2 Snapshot Repository</name>
<url>scp://host/snapshots/</url>
<uniqueVersion>false</uniqueVersion>
</snapshotRepository>
</distributionManagement>

This will use scp to upload the artifacts to the url //host/snapshots. The element that needs to receive the most of your attention is <uniqueVersion>. This value can either be true or false. When it is false the artifacts created will not be unique and will look something like this each time: myproject-0.9.2-SNAPSHOT. Use this to reduce disk usage. When it is set to true a unique version is created everytime. For example myproject-0.9.2-20070922.011633-1.jar and then the next build that day would be myproject-0.9.2-20070922.011633-2.jar.

Finally, the last thing is to set the username and password of the repository in your $USER_HOME/.m2/settings.xml file. This will be used by scp for authentication. To do this add the following in your settings.xml file:

<servers>
<server>
<id>maven2-snapshot-repository</id>
<username>repoman</username>
<password>mypassword</password>
</server>
<servers>

That is it. Now you are ready to run mvn deploy.