Why does release:prepare change versions back to SNAPSHOT?

I'm using the maven-release-plugin and I'm noticing that it appears to change the released version BACK to -SNAPSHOT at the end.

Here's the command line args that I'm passing to maven:

--batch-mode release:prepare -DupdateWorkingCopyVersions=false -Darguments="-DskipTests -Djava.awt.headless=true -Dmaven.javadoc.failOnError=false"

Everything appears to go well - and I notice a commit and a push from the plugin where all of the proper version numbers are indeed updated to NOT have the -SNAPSHOT.

[WARNING] Ignoring unrecognized line: ?? myProject/pom.xml.releaseBackup

I see a few of these warnings ^ in the log file (not sure if this is relevant, but appears to be related to the release plugin)

Finally I see that the plugin modifies the POMs again:

[INFO] Transforming 'myProject POM'...
[INFO]   Updating my-project to 6.2-SNAPSHOT

Following this the plugin makes and pushes a commit with the following:

[maven-release-plugin] rollback changes from release preparation of myProject-6.2

Why is this last bit happening?

I don't understand the lifecycle of release:prepare or release:perform. Why does all of my code appear to be built twice?

Any help appreciated. Thanks

Solution

As I understand it, release:prepare

Updates the POM files to the version that was specified
performs the build, which runs the tests
performs other validations (ie, no dependencies to SNAPSHOT artifacts)
Commits the changes (which should be restricted to the POM files and their versions)
Tags the source that was just committed
Updates the pom files back to the nominated SNAPSHOT version
Commits those changes

Note: I'm not sure if that's the specific order. The above list is not an exhaustive list, it's just my recollection.

At this point, the release doesn't exist. That's what release:perform does. Release:perform checks out the code against the TAG and builds the artifacts against the tagged source.

This is why release:prepare moves it back to the snapshot version. It's simply preparing the source to have a release built against it. Developers working on that branch, once the tag is cut, can continue to commit changes without affecting release:perform. It also allows a release to be re-cut at a later date, as it is built against the tag.

Of course, there are variations to this plugin which is documented. But, as I understand it, this is a common workflow for this.