code: public static Query parameterExists(final String name)
     {
        Assert.notNull(name, "Parameter name pattern must not be null.");
  
        return new Query() {
           final ParameterizedPatternParser pattern = new RegexParameterizedPatternParser("{" + name + "}");
           final String parameterName = name;
  
           @Override
           public boolean evaluateHttp(final HttpServletRewrite event, final EvaluationContext context)
           {
              QueryStringBuilder queryString = QueryStringBuilder.createFromEncoded(event.getAddress().getQuery())
                       .decode();
  
              for (String name : queryString.getParameterNames()) {
                 String[] parameterValues = queryString.getParameterValues(name);
                 if (parameterName.equals(name))
                 {
                    if (parameterValues == null || (parameterValues.length == 0))
                    {
                       return pattern.parse("").matches();
                    }
                    else
                    {
                       for (String value : parameterValues) {
  
                          ParameterizedPatternResult parseResult = pattern.parse(value);
                          if (parseResult.matches())
                          {
                             return parseResult.submit(event, context);
                          }
                       }
                    }
                 }
              }
  
              return false;
           }
  
           @Override
           public String toString()
           {
              return "Query.parameterExists(\"" + name + "\")";
           }
  
           @Override
           public Set<String> getRequiredParameterNames()
           {
              return pattern.getRequiredParameterNames();
           }
  
           @Override
           public void setParameterStore(ParameterStore store)
           {
              pattern.setParameterStore(store);
           }
        };
     }
docs: Create a {@link Condition} asserting the existence of specific parameter names within the current
  {@link Address#getQuery()}
  <p>
  The given parameter name is automatically parameterized:
  <p>
  <code>
  Query.paramterExists("param") <br>
  ...<br/>
  Forward.to("/{param}/page.jsp")
  </code>
  <p>
  
  @param pattern name of the {@link Parameter} matching query parameter names within the current
  {@link Address#getQuery()}.
  
  @see ConfigurationRuleParameterBuilder#where(String)