How does plugin validation work in Maven, and why does it build my project with an invalid version? - maven

Question

I have a Maven project that builds fine even though I have specified a completely invalid plugin in my POM:
<build>
<plugins>
<plugin>
<groupId>bla</groupId>
<artifactId>bar</artifactId>
<version>1.9.553342342343</version>
<executions>
<execution>
<phase>compile</phase>
</execution>
</executions>
<configuration>
<project>
<inceptionYear>123123</inceptionYear>
<contributors>
asdad
</contributors>
</project>
</configuration>
</plugin>
</plugins>
</build>
I also don't see any errors in Eclipse and even after deleting the ~.m2\repository folder, it still builds fine. Has something changed in how Maven validates plugins? Or is it first when I declare a goal that it blows up?

Answer 1

Your question raises different matters, namely the different kind of validation checks that are performed by Maven, and when they are actually done. Sit tight, there is a lot to say.
Step 1: Model validation
The first set of validation is done right at the start of the build, when the model of the project is built. This process is done by the Model Builder component, and its goal is to parse the POM file into a Model object (so that later, a full MavenProject object can be created from it, performing notably dependency mediation). This validation step is actually splitted in 2 parts:
A raw model validation, which reasons on the POM, before any inheritance or anything is applied. It looks for missing required values, like the presence of a groupId, an artifactId or a version; that repositories have an id; or, in your case here, that a plugin has a groupId and an artifactId. It doesn't actually check if there is a version, because the version could be inherited, and that wasn't done yet.
An effective model validation, which is performed after inheritance, interpolation and profile/default injection. At this point, the model should be completely valid. Notably, it must have a packaging, each dependency must have a version, each plugin must have a version as well, etc. And the plugin version you have is actually perfectly valid, in the sense that 1.9.553342342343 is technically an accepted version number. In fact, practically all String qualify as a valid version number; the illegal characters are \\/:\"<>|?*. Also, the <configuration> of a plugin is not validated, simply because it can't: that is specific for each plugin, and one could potentially declare a <project> parameter. For the same reason, it doesn't check whether the plugin actually exists in a remote repository, or if any goals, phase, etc. are specified.
Therefore, at the end of this step, that POM is fully validated and perfectly OK.
Step 2: Project building
Then comes the step of actually building the MavenProject from it. Because Maven needs to perform dependency mediation on the dependencies of the project, it first has to download them. So if you have any invalid dependencies (i.e. dependencies that cannot be resolved with the configured remote repositories in the settings or the project itself), that'll stop right here.
But if we imagine that the dependencies are correctly resolved, the build will continue to invoke each plugin one by one. The important point is that plugins, and their respective dependencies, are only resolved if Maven detects that they are going to be invoked during the build. If not, Maven will not try to download anything. Furthermore, the validation of the configuration of the plugin is also done when the plugin is actually invoked and the values are injected into it, to be used by it.
Depending on the Maven command that was launched, not all plugins declared in the POM will have work to do. For example, if phases were entered by the user, like mvn clean package, then every plugin bound to a phase of the clean lifecycle, or the default lifecycle up to package, will be invoked; so any plugin bound to the install phase would not be invoked. Also, if the user entered a goal, like mvn org.apache.maven.plugins:maven-clean-plugin:3.0.0:clean then only that specific goal of that specific plugin will be invoked, and all the other plugins will be ignored.
This last part is why the POM in the question poses absolutely no trouble to Maven, and here are multiple points about it:
It is bound to the compile phase, but it doesn't have a <goal>, so even if that phase were to be executed, there is nothing the plugin could do, since no goals were defined. Maven knows about this, and doesn't try to resolve the plugin artifact.
Let's set a <goal> to foo and re-test, by adding <goals><goal>foo</goal></goals> to the plugin declaration. We have in the POM:
<executions>
<execution>
<phase>compile</phase>
<goals><goal>foo</goal></goals>
</execution>
</executions>
Running mvn clean, or mvn clean validate would still cause absolutely no issue: the compile phase was not executed. But now, if we run mvn compile, we'll finally get an error:
Plugin bla:bar:1.9.553342342343 or one of its dependencies could not be resolved
This is, after all, what we wanted. Since the plugin declaration has a phase of compile, and the command used would run that phase, Maven tries to download it (and fails).
So let's remove the phase. What would happen now?
<executions>
<execution>
<goals><goal>foo</goal></goals>
</execution>
</executions>
Actually, running any command with specific phases, like mvn clean or mvn validate, would now fail the build. The reason is that a plugin can have a default phase (see also the defaultPhase attribute on #Mojo annotated goal). Since each plugin has the discretion of providing a default phase to any of its goal, Maven has to download the plugin artifact, and find out if this particular plugin uses a default. So, our build will fail again, yay!
It's a different story if the user invokes a specific goal. Try mvn clean:clean with the above, and it will not fail. Actually, warnings are just going to get printed that Maven can't resolve the plugin artifact, but none of that is an error, since invoking clean:clean will just invoke the specific clean goal of the maven-clean-plugin. And actually, in theory, there shouldn't be any warnings here; Maven shouldn't try to download anything. It's a side-effect from the fact that using the prefix clean demands to checks to remote repositories in order to resolve it (refer to this answer to know how that works). But if you fully qualify it, without any plugin prefix resolution needed, with mvn org.apache.maven.plugins:maven-clean-plugin:3.0.0:clean, you're back to zero errors/warnings.
Finally, if we remove everything and end up with
<executions>
<execution>
</execution>
</executions>
it should be pretty clear, that nothing you'll do with result in an errors, because in no way can that plugin ever be executed. (You'll still get warnings if using a prefix).
Step 3: Plugin configuration
The last part of the question is the simple one: the configuration validation of the plugin. You'll notice that at no point this was mentioned here; this is because it only happens when the plugin is actually executed. And since it doesn't even exist, it's not likely to be executed.
Let's suppose it is, for the sake of the explanation. Each plugin is configured with a specific configurator. By default, it maps the XML elements to classes, fields, lists, maps, arrays, just like you would expect. You could provide your own configurator, but that's not a trivial task. There is actually no real validation performed: basically, if the configurator can wire the proper values in the mojo, it's done. You can check the different types of converters that are present by default, but it comes down to: not specifying a String "foo" to an expected integer value; passing a correct enumeration name if the plugin expects that; passing proper XML configuration for a custom class (i.e. each field with their own XML element)... Worth pointing out that setting "foo" to an expected boolean property is not a problem, it'll wire false into the value.
And finally, the XML configuration that did not map to any parameter of the mojo are completely ignored, so even if the bar plugin existed and didn't take any parameters, passing a <project> in the XML configuration would just be ignored, and wouldn't cause any errors.