code: public void execute()
          throws MojoExecutionException, MojoFailureException
      {
          validateOutputFile();
          Properties projProperties = new Properties();
          projProperties.putAll( getProject().getProperties() );
  
          Properties systemProperties = System.getProperties();
  
          // allow system properties to over write key/value found in maven properties
          Enumeration<?> enumeration = systemProperties.keys();
          while ( enumeration.hasMoreElements() )
          {
              String key = (String) enumeration.nextElement();
              String value = systemProperties.getProperty( key );
              if ( projProperties.get( key ) != null )
              {
                  projProperties.put( key, value );
              }
  
          }
  
          writeProperties( projProperties, getOutputFile() );
      }
docs: {@inheritDoc}