code: public void execute( EnforcerRuleHelper helper )
          throws EnforcerRuleException
      {
  
          if ( !allowNulls && files.length == 0 )
          {
              throw new EnforcerRuleException( "The file list is empty and Null files are disabled." );
          }
  
          List<File> failures = new ArrayList<File>();
          for ( File file : files )
          {
              if ( !allowNulls && file == null )
              {
                  failures.add( file );
              }
              else if ( !checkFile( file ) )
              {
                  failures.add( file );
              }
          }
  
          // if anything was found, log it with the optional message.
          if ( !failures.isEmpty() )
          {
              String message = getMessage();
              
              StringBuilder buf = new StringBuilder();
              if ( message != null )
              {
                  buf.append( message + "\n" );
              }
              buf.append( getErrorMsg() );
  
              for ( File file : failures )
              {
                  if ( file != null )
                  {
                      buf.append( file.getAbsolutePath() + "\n" );
                  }
                  else
                  {
                      buf.append( "(an empty filename was given and allowNulls is false)\n" );
                  }
              }
  
              throw new EnforcerRuleException( buf.toString() );
          }
      }
docs: /*
  (non-Javadoc)
  
  @see org.apache.maven.enforcer.rule.api.EnforcerRule#execute(org.apache.maven.enforcer.rule.api.EnforcerRuleHelper)