Maven `pre` and `post` phases - maven

Question

Are the pre and post phases always executed when I execute the associated phase? For example, if I do mvn clean, will this execute the mvn post-clean phase, too?
I was looking at https://maven.apache.org/guides/introduction/introduction-to-the-lifecycle.html#Lifecycle_Reference, in which it said:
The following lists all build phases of the default, clean and site
lifecycles, which are executed in the order given up to the point of
the one specified.
So strictly speaking, since post-clean comes after clean, it should not be executed if I just do mvn clean. But my gut feel is different - although, I didn't find a way to verify this, as the maven stdout doesn't print the phase it's executing.
Could anyone weigh in on the answer and how you verified?

Answer 1

You can bind a plugin (such as echo-maven-plugin) to the phases of the clean lifecycle to help verify if/when each phase is executed.
For example, given the following plugin definition:
<plugin>
<groupId>com.github.ekryd.echo-maven-plugin</groupId>
<artifactId>echo-maven-plugin</artifactId>
<version>1.2.0</version>
<executions>
<execution>
<id>pre-clean</id>
<phase>pre-clean</phase>
<goals>
<goal>echo</goal>
</goals>
<configuration>
<message>In 'pre-clean'</message>
</configuration>
</execution>
<execution>
<id>clean</id>
<phase>clean</phase>
<goals>
<goal>echo</goal>
</goals>
<configuration>
<message>In 'clean'</message>
</configuration>
</execution>
<execution>
<id>post-clean</id>
<phase>post-clean</phase>
<goals>
<goal>echo</goal>
</goals>
<configuration>
<message>In 'post-clean'</message>
</configuration>
</execution>
</executions>
</plugin>
Invoking mvn clean will result in the following output:
$ mvn clean
[INFO] Scanning for projects...
[INFO]
[INFO] --- echo-maven-plugin:1.2.0:echo (pre-clean) # sandbox ---
[INFO] In 'pre-clean'
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) # sandbox ---
[INFO] Deleting target
[INFO]
[INFO] --- echo-maven-plugin:1.2.0:echo (clean) # sandbox ---
[INFO] In 'clean'
So, there's no invocation of the post-clean phase there.
Invoking mvn clean compile will result in the following output:
$ mvn clean compile
[INFO] Scanning for projects...
[INFO]
[INFO] --- echo-maven-plugin:1.2.0:echo (pre-clean) # sandbox ---
[INFO] In 'pre-clean'
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) # sandbox ---
[INFO]
[INFO] --- echo-maven-plugin:1.2.0:echo (clean) # sandbox ---
[INFO] In 'clean'
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) # sandbox ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Copying 1 resource
[INFO]
[INFO] --- maven-compiler-plugin:3.5.1:compile (default-compile) # sandbox ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 5 source files to ...
Again, there's no invocation of the post-clean phase there. This implies that the maven-clean-plugin (and perhaps nothing else) is not bound to post-clean.
Invoking mvn post-clean will result in the post-clean phase being invoked ...
$ mvn post-clean
[INFO] Scanning for projects...
[INFO]
[INFO] --- echo-maven-plugin:1.2.0:echo (pre-clean) # sandbox ---
[INFO] In 'pre-clean'
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) # sandbox ---
[INFO] Deleting target
[INFO]
[INFO] --- echo-maven-plugin:1.2.0:echo (clean) # sandbox ---
[INFO] In 'clean'
[INFO]
[INFO] --- echo-maven-plugin:1.2.0:echo (post-clean) # sandbox ---
[INFO] In 'post-clean'
So, based on the above test I think the following statements are true:
post-clean is not invoked when you call clean
post-clean is only called when you explicitly invoke post-clean (note: invoking the pre- and post- phases directly is unusual)