http://example.com/some/page?p1=11

http://example.com/some/page?p1=11&p2=32

<a th:href="@{?(p2=32)}">Click here</a>

<div th:with="currentUrl=(${#httpServletRequest.requestURI + '?' + #strings.defaultString(#httpServletRequest.queryString, '')})">
   <a th:href="@{${currentUrl}(myparam=test)}">click here</a>
</div>

http://localhost:8080/some-page?param1=1&myparam=test

http://localhost:8080/some-page?&myparam=test

http://localhost:8080/some-page?param1=1&myparam=test&myparam=test

<div th:with="currentUrl=(${@currentUrlWithoutParam.apply('myparam')})">
    <a th:href="@{${currentUrl}(myparam=test)}">click here</a>
</div>

@Bean
public Function<String, String> currentUrlWithoutParam() {
    return param ->   ServletUriComponentsBuilder.fromCurrentRequest().replaceQueryParam(param).toUriString();
}

<span th:with="urlBuilder=${T(org.springframework.web.servlet.support.ServletUriComponentsBuilder).fromCurrentRequest()}"
      th:text="${urlBuilder.replaceQueryParam('p2', '32').toUriString()}">
</span>

http://example.com/some/page?p1=11&p2=32

<span th:with="urlBuilder=${T(org.springframework.web.servlet.support.ServletUriComponentsBuilder)}">
    <span th:text="${urlBuilder.fromCurrentRequest().replaceQueryParam('p2', 'whatever').toUriString()}"></span>
    <span th:text="${urlBuilder.fromCurrentRequest().replaceQueryParam('p3', 'whatever').toUriString()}"></span>
    <span th:text="${urlBuilder.fromCurrentRequest().replaceQueryParam('p4', 'whatever').toUriString()}"></span>
</span>

http://example.com/some/page?p2=whatever
http://example.com/some/page?p3=whatever
http://example.com/some/page?p4=whatever

<ul class="pagination">
                <li th:if="${currentPage > 1}"><a th:href="'/search?key=' + ${param.key[0]} + '&page=' +   ${currentPage-1}">Previous</a></li>

                      <li th:each="i : ${#numbers.sequence( 1, total+1)}" th:class="${i==currentPage}?active:''">
                            <a th:href="'/search?key=' + ${param.key[0]} + '&page=' +   ${i}" th:inline="text">
                            [[${i}]] <span  class="sr-only">(current)</span>
                            </a>
                      </li>
                      <li><a th:if="${total + 1 > currentPage}" th:href="'/search?key=' + ${param.key[0]} + '&page=' +   ${currentPage+1}">Next</a></li>
            </ul>

th:href="@{/your/link?parameter=__${appendParameter}__}"