code: @Override
      public List<ConfigMessage> init(Processor processor) {
          List<ConfigMessage> configMessages = super.init(processor);
          if (((ExpressionBasedAction) getAction()).getExpression() != null) {
              try {
                  expression = ExpressionHandlerFactory.getHandler(
                          ((ExpressionBasedAction) getAction()).getExpression());
  
                  expression.init(processor, getAction(), false);
  
                  if (!isUsesHeaders()) {
                      setUsesHeaders(expression.isUsesHeaders());
                  }
                  if (!isUsesContent()) {
                      setUsesContent(expression.isUsesContent());
                  }
              } catch (Throwable t) {
                  log.severe(processor.getDescription() + ":" + getAction().getDescription()
                       + ":Failed to compile expression");
                  ConfigMessage configMessage = new ConfigMessage();
                  configMessage.setSeverity(Severity.Error);
                  configMessage.setMessage(t.getMessage());
                  configMessage.setField("expression");
                  configMessage.setProcessor(processor.getDescription());
                  configMessage.setAction(getAction().getDescription());
                  configMessages.add(configMessage);
              }
          } else {
              String message = "Expression has not been defined";
              log.severe(processor.getDescription() + ":" + getAction().getDescription() + ":" + message);
              ConfigMessage configMessage = new ConfigMessage();
              configMessage.setSeverity(Severity.Error);
              configMessage.setMessage(message);
              configMessage.setField("expression");
              configMessage.setProcessor(processor.getDescription());
              configMessage.setAction(getAction().getDescription());
              configMessages.add(configMessage);
          }
  
          return configMessages;
      }
docs: This method initialises the process action handler.
  
  @param processor The processor