Thymeleaf - replace c:url and c:param

As part of my conversion from jsp’s to Thymeleaf I came across a block that was creating url’s using c:url and c:param for the url parameters.

So there is several things going on in the code

<fmt:message key="linkDesc" var="linkDescVar"/>
<c:set value="?${pageContext.request.queryString}" var="queryString"/>
<c:set var="baseUrl" value="/test/app"/>
<c:if test="${queryString == '?'}">
    <c:set value="" var="queryString"/>
</c:if>

<c:set var="testLink" value="${pageContext.request.requestURI}${queryString}"/>
<c:url value="${baseUrl}" context="/" var="baseUrlLink">
    <c:param name="link" value="${testLink}"></c:param>
    <c:param name="linkDesc" value="${linkDescVar}"></c:param>
</c:url>

So it’s forming a url link to test/app and adding parameters of where it’s currently at including the current query string. So if you go to say http://localhost:8080/pub/home?key=value it should form a link that would go to http://localhost:8080/test/app?link=home?key=value&linkDesc=testit

The linkDesc comes from the messages.properties file

So things are done this way because of how the jsp taglibs work. The Thymeleaf solution might not be a line by line replacement. But lets go through it line by line, look at the options in Thymeleaf then see about mushing it all together into something that works.

<fmt:message key="linkDesc" var="linkDescVar"/>

The normal replacement for fmt:message would be th:text but in this case it’s using it to get a value from the properties file and store it in a variable. Don’t think that’s a option for th:text

So a direct replacement I think would be

<div th:with="linkDescVar=#{linkDesc}"></div>

Which sets up the linkDescVar within the context of the div (reckon the within the div bit is something that’s worth noting as the fmt:message var is available to the whole page)

Another variant would be

<div th:with="linkDescVar=${#messages.msgOrNull('linkDesc')}"></div>

This uses the #messages utility so can be used directly in expressions, might be useful later as there may not be a need to create the variable anymore.

<c:set value="?${pageContext.request.queryString}" var="queryString"/>

The replacement for c:set I believe is th:with like above. The request object is available in Thymeleaf as request so this can be a fairly straight replacement

<div th:with="queryString=${'?' + #request.queryString}"></div>

But there’s some gotcha’s here, while c:set returned blank if ${pageContext.request.queryString} was blank Thymeleaf returns null so in the case where there are no query parameters you get ?null which is not what we want.

The code here

<c:if test="${queryString == '?'}">
    <c:set value="" var="queryString"/>
</c:if>

Handled the case of no query parameters by checking for just the ? and replacing it with blank.

This needs to be handled in the th:with in Thymeleaf I believe so this

<div th:with="queryString=${#request.queryString == null ? '' : '?' + #request.queryString}"></div>

an awful lot of question marks but this is using (if) ? (then) : (else) in the expression. If it’s null set queryString to blank else set it you a ? and concat on the query string.

<c:set var="testLink" value="${pageContext.request.requestURI}${queryString}"/>

Our friend th:with again for this job, which is just concatenating two values.

<div th:with="testLink=${#request.requestURI + queryString}"></div>

Last part

<c:url value="${baseUrl}" context="/" var="baseUrlLink">
    <c:param name="link" value="${testLink}"></c:param>
    <c:param name="linkDesc" value="${linkDescVar}"></c:param>
</c:url>

Yea th:with again, the go to variable assignment wizard.

So some Thymeleaf url magic beans to cover first, for forming url’s use @{...} which handles alot of the url dark arts, context root etc… within that to add parameters you use () so @{/test/app(key=value)} to get the context to be server root like context="/" you use a tilde ~ at the start of the url.

With that in mind

<div th:with="baseUrlLink=@{~/test/app(backLink=${testLink},linkDesc=${linkDescVar})}"></div>

Creates the link and assigns it to the baseUrlLink variable and uses the two other variables set up earlier testLink and linkDescVar

This gives an overall block like this

<div th:with="linkDescVar=#{linkDesc}">
    <div th:with="queryString=${#request.queryString == null ? '' : '?' + #request.queryString}">
        <div th:with="testLink=${#request.requestURI + queryString}">
            <div th:with="baseUrlLink=@{/test/app(backLink=${testLink},linkDesc=${linkDescVar})}">
                <a th:href="${baseUrlLink}">example of link</a>
            </div>
        </div>
    </div>
</div>

So seeing as these are all expressions you can collapse them down as much as you like for example you could move the linkDescVar inline.

<div th:with="queryString=${#request.queryString == null ? '' : '?' + #request.queryString}">
    <div th:with="testLink=${#request.requestURI + queryString}">
        <div th:with="baseUrlLink=@{/test/app(backLink=${testLink},linkDesc=${#messages.msgOrNull('linkDesc')})}">
            <a th:href="${baseUrlLink}">example of link</a>
        </div>
    </div>
</div>

In theory you can get the whole thing on one line if you wanted, that would be a complex looking line but you might prefer it that way.