How to use of the Maven Enforcer plugin?

I'd like to use the Maven Enforcer plugin to check to see if I have duplicate classes on my path.

I've tried the example from here.

But when I run it like this:

mvn enforcer:enforce

I get this error:

Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.0.1:enforce (default-cli) on project datapopulator: The parameters 'rules' for goal org.apache.maven.plugin开发者_StackOverflows:maven-enforcer-plugin:1.0.1:enforce are missing or invalid

Is there a way to use this correctly?

EDIT #1

If changing my config to this:

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-enforcer-plugin</artifactId>
            <version>1.0.1</version>
            <executions>
                <execution>
                    <id>enforce-versions</id>
                    <goals>
                        <goal>enforce</goal>
                    </goals>
                    <configuration>
                        <rules>
                            <AlwaysPass />
                        </rules>
                        <fail>true</fail>
                    </configuration>
                </execution>
            </executions>
        </plugin>

Produces the same error.

The reason why your first version did not work is because there is a difference between a plug-in configuration inside the execution tag and a plug-in configuration outside the execution tag. The execution is only used when your plug-in is triggered by a special phase of the complete Maven build.

The Maven guide to configuration explains it better:

Configurations inside the tag differ from those that are outside in that they cannot be used from a direct command line invocation. Instead they are only applied when the lifecycle phase they are bound to are invoked. Alternatively, if you move a configuration section outside of the executions section, it will apply globally to all invocations of the plugin.

Try this, moving the configuration outside executions, so it isn't bound to the life cycle phase.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-enforcer-plugin</artifactId>
    <version>1.0.1</version>
    <executions>
        <execution>
            <id>enforce-versions</id>
            <goals>
                <goal>enforce</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <rules>
            <AlwaysPass />
        </rules>
        <fail>true</fail>
    </configuration>
</plugin>

Now when you do mvn enforcer:enforce, it picks the rules from your pom.xml.

See these answers

You can use the special default command line execution id, default-cli to invoke it (see Maven Docs), see my example below. This works at least with 3.1.1 and the article cited says it should work with 2.2.0+

mvn enforcer:enforce

However if you are using above Maven 3.1.1 (I can confirm it works in 3.3.3 with enforcer v 1.4.1) you can specify the execution id you wish using the new @ syntax (see Maven JIRA and the answers above);

e.g. for the example below use

mvn enforcer:enforce@dependency-convergence

Here's a snippet from my pom;

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-enforcer-plugin</artifactId>
            <version>1.4.1</version>
            <executions>
                <execution>
                    <id>dependency-convergence</id>
                    <phase>install</phase>
                    <goals>
                        <goal>enforce</goal>
                    </goals>
                    <configuration>
                        <rules>
                            <DependencyConvergence />
                        </rules>
                        <fail>true</fail>
                    </configuration>
                </execution>
                <execution>
                    <id>default-cli</id>
                    <goals>
                        <goal>enforce</goal>
                    </goals>
                    <configuration>
                        <rules>
                            <DependencyConvergence/>
                        </rules>
                        <fail>true</fail>
                    </configuration>
                </execution>
            </executions>
        </plugin>
      ...

I don't know why it won't work with the config being in an execution, but this worked for me:

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-enforcer-plugin</artifactId>
            <version>1.0</version>
            <configuration>
                <rules>
                    <banDuplicateClasses>
                        <findAllDuplicates>true</findAllDuplicates>
                    </banDuplicateClasses>
                </rules>
                <fail>false</fail>
            </configuration>
            <dependencies>
                <dependency>
                    <groupId>org.codehaus.mojo</groupId>
                    <artifactId>extra-enforcer-rules</artifactId>
                    <version>1.0-alpha-1</version>
                </dependency>
            </dependencies>
        </plugin>