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~~ under the License.    
 
  ------
  Writing a custom rule
  ------
  Brian Fox
  ------
  Nov 2007
  ------

Writing a custom rule
 
  Custom rules are easy to make with the <<<maven-enforcer-rule-api>>>. These rules can then be invoked with the 
  {{{http://maven.apache.org/plugins/maven-enforcer-plugin/}maven-enforcer-plugin}}. 
  
  Note: The files shown below may be downloaded here: {{{./custom-rule.zip}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>${project.version}</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 {{{./apidocs/index.html}EnforcerRule}} interface.
   The rule can get access to components and the log via the {{{./apidocs/index.html}EnforcerRuleHelper}} interface.
   If the rule succeeds, it should just simply return. If the rule fails, it should throw an {{{./apidocs/index.html}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>1.0-beta-1</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>1.0-beta-1</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>
+---+