Writing a custom rule

Custom rules are easy to make with the maven-enforcer-rule-api. These rules can then be invoked with the maven-enforcer-plugin.

Note: The files shown below may be downloaded here: custom-rule.zip

1. First make a new jar project starting with the sample pom below:

   <?xml version="1.0" encoding="UTF-8"?>
   <project xmlns="http://maven.apache.org/POM/4.0.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>
   
     <groupId>custom-rule</groupId>
     <artifactId>custom-rule-sample</artifactId>
     <packaging>jar</packaging>
     <version>1.0</version>
   
     <name>My Custom Rule</name>
     <description>This is my custom rule.</description>
   
     <properties>
       <api.version>1.4</api.version>
       <maven.version>2.0.9</maven.version>
     </properties>
   
     <dependencies>
       <dependency>
         <groupId>org.apache.maven.enforcer</groupId>
         <artifactId>enforcer-api</artifactId>
         <version>${api.version}</version>
       </dependency>
       <dependency>
         <groupId>org.apache.maven</groupId>
         <artifactId>maven-project</artifactId>
         <version>${maven.version}</version>
       </dependency>
       <dependency>
         <groupId>org.apache.maven</groupId>
         <artifactId>maven-core</artifactId>
         <version>${maven.version}</version>
       </dependency>
       <dependency>
         <groupId>org.apache.maven</groupId>
         <artifactId>maven-artifact</artifactId>
         <version>${maven.version}</version>
       </dependency>
       <dependency>
         <groupId>org.apache.maven</groupId>
         <artifactId>maven-plugin-api</artifactId>
         <version>${maven.version}</version>
       </dependency>
       <dependency>
         <groupId>org.codehaus.plexus</groupId>
         <artifactId>plexus-container-default</artifactId>
         <version>1.0-alpha-9</version>
       </dependency>
       <dependency>
         <groupId>junit</groupId>
         <artifactId>junit</artifactId>
         <version>3.8.2</version>
         <scope>test</scope>
       </dependency>
     </dependencies>
   
     <build>
     </build>
   </project>

2. Create your rule class. The rule must implement the EnforcerRule interface. The rule can get access to components and the log via the EnforcerRuleHelper interface. If the rule succeeds, it should just simply return. If the rule fails, it should throw an EnforcerRuleException with a descriptive message telling the user why the rule failed.

   There are several methods that must be implemented related to caching.

   Here's a sample class that shows how to access the helper methods and retrieve components by class name from the helper:

   package org.apache.maven.enforcer.rule;
   
   import org.apache.maven.artifact.resolver.ArtifactResolver;
   import org.apache.maven.enforcer.rule.api.EnforcerRule;
   import org.apache.maven.enforcer.rule.api.EnforcerRuleException;
   import org.apache.maven.enforcer.rule.api.EnforcerRuleHelper;
   import org.apache.maven.execution.MavenSession;
   import org.apache.maven.execution.RuntimeInformation;
   import org.apache.maven.plugin.logging.Log;
   import org.apache.maven.project.MavenProject;
   import org.codehaus.plexus.component.configurator.expression.ExpressionEvaluationException;
   import org.codehaus.plexus.component.repository.exception.ComponentLookupException;
   
   /**
    * @author <a href="mailto:brianf@apache.org">Brian Fox</a>
    */
   public class MyCustomRule
       implements EnforcerRule
   {
       /**
        * Simple param. This rule will fail if the value is true.
        */
       private boolean shouldIfail = false;
   
       public void execute( EnforcerRuleHelper helper )
           throws EnforcerRuleException
       {
           Log log = helper.getLog();
   
           try
           {
               // get the various expressions out of the helper.
               MavenProject project = (MavenProject) helper.evaluate( "${project}" );
               MavenSession session = (MavenSession) helper.evaluate( "${session}" );
               String target = (String) helper.evaluate( "${project.build.directory}" );
               String artifactId = (String) helper.evaluate( "${project.artifactId}" );
   
               // retrieve any component out of the session directly
               ArtifactResolver resolver = (ArtifactResolver) helper.getComponent( ArtifactResolver.class );
               RuntimeInformation rti = (RuntimeInformation) helper.getComponent( RuntimeInformation.class );
   
               log.info( "Retrieved Target Folder: " + target );
               log.info( "Retrieved ArtifactId: " +artifactId );
               log.info( "Retrieved Project: " + project );
               log.info( "Retrieved RuntimeInfo: " + rti );
               log.info( "Retrieved Session: " + session );
               log.info( "Retrieved Resolver: " + resolver );
   
               if ( this.shouldIfail )
               {
                   throw new EnforcerRuleException( "Failing because my param said so." );
               }
           }
           catch ( ComponentLookupException e )
           {
               throw new EnforcerRuleException( "Unable to lookup a component " + e.getLocalizedMessage(), e );
           }
           catch ( ExpressionEvaluationException e )
           {
               throw new EnforcerRuleException( "Unable to lookup an expression " + e.getLocalizedMessage(), e );
           }
       }
   
       /**
        * If your rule is cacheable, you must return a unique id when parameters or conditions
        * change that would cause the result to be different. Multiple cached results are stored
        * based on their id.
        * 
        * The easiest way to do this is to return a hash computed from the values of your parameters.
        * 
        * If your rule is not cacheable, then the result here is not important, you may return anything.
        */
       public String getCacheId()
       {
           //no hash on boolean...only parameter so no hash is needed.
           return ""+this.shouldIfail;
       }
   
       /**
        * This tells the system if the results are cacheable at all. Keep in mind that during
        * forked builds and other things, a given rule may be executed more than once for the same
        * project. This means that even things that change from project to project may still 
        * be cacheable in certain instances.
        */
       public boolean isCacheable()
       {
           return false;
       }
   
       /**
        * If the rule is cacheable and the same id is found in the cache, the stored results
        * are passed to this method to allow double checking of the results. Most of the time 
        * this can be done by generating unique ids, but sometimes the results of objects returned
        * by the helper need to be queried. You may for example, store certain objects in your rule
        * and then query them later.
        */
       public boolean isResultValid( EnforcerRule arg0 )
       {
           return false;
       }
   }

3. Build and Install or Deploy your custom rule.
4. Add your custom-rule artifact as a dependency of the maven-enforcer-plugin in your build:

   <project>
     ...
     <build>
       <plugins>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-enforcer-plugin</artifactId>
           <version>1.4</version>
           <dependencies>
             <dependency>
               <groupId>custom-rule</groupId>
               <artifactId>custom-rule-sample</artifactId>
               <version>1.0</version>
             </dependency>
           </dependencies>
           ...
         </plugin>   
       </plugins>
     </build>
     ...
   </project>

5. Add your rule to the configuration section of the maven-enforcer-plugin. The name of your class will be the name of the rule, and you must add an implementation hint that contains the fully qualified class name:

           ...
           <configuration>
             <rules>
               <myCustomRule implementation="org.apache.maven.enforcer.rule.MyCustomRule">
                 <shouldIfail>true</shouldIfail>
               </myCustomRule>
             </rules>
           </configuration>
           ...

6. That's it. The full plugin config may look like this:

   <project>
     ...
     <build>
       <plugins>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-enforcer-plugin</artifactId>
           <version>1.4</version>
           <dependencies>
             <dependency>
               <groupId>custom-rule</groupId>
               <artifactId>custom-rule-sample</artifactId>
               <version>1.0</version>
             </dependency>
           </dependencies>
           <executions>
             <execution>
               <id>enforce</id>
               <configuration>
                 <rules>
                   <myCustomRule implementation="org.apache.maven.enforcer.rule.MyCustomRule">
                     <shouldIfail>false</shouldIfail>
                   </myCustomRule>
                 </rules>
               </configuration>
               <goals>
                 <goal>enforce</goal>
               </goals>
             </execution>
           </executions>
         </plugin>
       </plugins>
     </build>
     ...
   </project>