Maven - skip plugin if property is empty/null - maven

Question

I want to obtain the following behavior: when I specify a value for the property "my.prop", I want the dependency and clean plugins to be executed. If a value is not specified for that property, I want them to be skipped.
I created "my.prop" like this:
<properties>
<my.prop></my.prop>
</properties>
Then I read that profile activation works only for system properties, so I deleted the above and used the surefire plugin:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.17</version>
<configuration>
<systemPropertyVariables>
<my.prop></my.prop>
</systemPropertyVariables>
</configuration>
</plugin>
I tried using profiles, like this:
<profiles>
<profile>
<id>default</id>
<activation>
<activeByDefault>true</activeByDefault>
</activation>
<properties>
<skipDependecyAndCleanPlugins>false</skipDependecyAndCleanPlugins>
</properties>
</profile>
<profile>
<id>skip-dependency-and-clean-plugins</id>
<activation>
<property>
<name>my.prop</name>
<value></value>
<!-- I also tried: <value>null</value> without success.-->
</property>
</activation>
<properties>
<skipDependecyAndCleanPlugins>true</skipDependecyAndCleanPlugins>
</properties>
</profile>
</profiles>
Later, for each plugin I do something like this:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<version>2.9</version>
<configuration>
<skip>${skipDependecyAndCleanPlugins}</skip>
</configuration>
....
</plugin>
But the plugins are still executed...
How can I determine Maven to skip the executions of the plugins when "my.prop" is empty/null?

Answer 1

The simplest solution would be to use the activation in the following form:
<profiles>
<profile>
<activation>
<property>
<name>debug</name>
</property>
</activation>
...
</profile>
</profiles>
The above means you can define any value for debug which means -Ddebug is enough.
An empty value can't be defined a pom file cause <value></value> is equivalent to <value/> which means the same as not defined.
Update:
I would suggest to use a profile and NOT a property. So you can simply define on command line mvn -Pxyz install or leave it.

Answer 2

You can use my.prop property in plugin's configuration:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<version>2.9</version>
<configuration>
<skip>${my.prop}</skip>
</configuration>
....
</plugin>
Now when you execute:
mvn ... -Dmy.prop=true
then plugin will be skipped

Answer 3

You were very close. You can achieve what you described by using the !my.prop syntax in profile activation.
<build>
<plugins>
<plugin>
<artifactId>maven-clean-plugin</artifactId>
<configuration>
<skip>${skipDependecyAndCleanPlugins}</skip>
</configuration>
</plugin>
</plugins>
</build>
<profiles>
<profile>
<id>skip-dependency-and-clean-plugins</id>
<activation>
<property>
<name>!my.prop</name>
</property>
</activation>
<properties>
<skipDependecyAndCleanPlugins>true</skipDependecyAndCleanPlugins>
</properties>
</profile>
</profiles>
According to Maven documentation, the skip-dependency-and-clean-plugins profile will be activated when the system property my.prop is not defined at all.

Answer 4

Here is a solution that directly addresses the OP's original request: the ability to skip a plugin's execution if a POM property (not a system property) my.prop is not defined. This solution relies on the Apache Maven Help Plugin. It is a kludge, but given Maven's paucity of expression prowess, this is about the best you're gonna get. At least it relies on a well-known, hopefully-maintained plugin, and should work 100% of the time. Oh, and it may make your head explode. Or make you cry. Or both. You've been warned.
First declare the latest version of the Maven Help Plugin in the <build><pluginManagement> section:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>build-helper-maven-plugin</artifactId>
<version>3.3.0</version>
</plugin>
Then add this "secret sauce" in the <build><plugins> section, which will check to see if my.prop is defined:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>build-helper-maven-plugin</artifactId>
<executions>
<execution>
<id>set-is-skip-true-or-prefixed</id>
<phase>validate</phase>
<goals>
<goal>regex-property</goal>
</goals>
<configuration>
<name>is-skip</name>
<value>_${my.prop}</value>
<regex>_\$\{my.prop\}</regex>
<replacement>true</replacement>
<failIfNoMatch>false</failIfNoMatch>
</configuration>
</execution>
<execution>
<id>set-is-skip</id>
<phase>initialize</phase>
<goals>
<goal>regex-property</goal>
</goals>
<configuration>
<name>is-skip</name>
<value>${is-skip}</value>
<regex>_.*</regex>
<replacement>false</replacement>
<failIfNoMatch>false</failIfNoMatch>
</configuration>
</execution>
</executions>
</plugin>
Now you have a POM property (not a system property) named is-skip, which you can use in any later phase to disable a plugin—provided that plugin has a <skip> or similar option taking a Boolean value. If my.prop is not defined at all, is-skip will be set to true; otherwise is-skip will be set to false.
<plugin>
<groupId>org.example</groupId>
<artifactId>foobar-plugin</artifactId>
<executions>
<execution>
<id>foo</id>
<phase>package</phase>
<goals>
<goal>foo</goal>
</goals>
<configuration>
<skip>${is-skip}</skip>
</configuration>
</execution>
</executions>
</plugin>
There is one caveat: don't define my.prop at all, even to the empty string, or this solution will consider it defined and set is-skip to false. Setting is-skip to true if my.prop is set but empty would require an additional regex evaluation clause above. For my use case I didn't need it, as an empty my.prop isn't valid and I didn't define my.prop at all in the parent POM.
I'll leave it to you to understand how it works (my head exploded already when I was writing it), but I'll be happy to answer any questions.
In step #2 above, be sure and put the two regex evaluations in different phases, such as validate and initialize as used here. The reason is because Maven gets confused if you mix in the same plugins in in the same phase a child POM, and may scramble the execution order. (See MNG-5987.) My kludge of a solution relies on the order of evaluation.