#################
navbar.ftl
#################
<#include "security.ftl">
<#import "login.ftl" as l>
<nav class="navbar navbar-expand-lg navbar-light bg-light">
.....
    <div class="navbar-text mr-3"><#if user??>${name}${name}<#else>Please, login</#if></div> <!-- OK -->
    <@l.logout /> <!-- Not OK -->
    <div class="navbar-text mr-3">
        <form action="/logout" method="post">
            <input type="hidden" name="_csrf" value="${_csrf.token}"/>
            <button  class="btn btn-primary" type="submit" ><#if user??>Sign Out<#else>Login</#if></button>
        </form>
    </div>
</nav>

#################
security.ftl
################
<#assign
    known = Session.SPRING_SECURITY_CONTEXT??
>

<#if known>
    <#assign
        user = Session.SPRING_SECURITY_CONTEXT.authentication.principal
        name = user.getUsername()
        isAdmin = user.isAdmin()
        currentUserId = user.getId()
    >
<#else>
    <#assign
        name = "unknown"
        isAdmin = false
        currentUserId = -1
    >
</#if>

################
login.ftl
################
<#macro login path isRegisterForm>
....
</#macro>
<#macro logout>
<form action="/logout" method="post">
    <input type="hidden" name="_csrf" value="${_csrf.token}"/>
    <button  class="btn btn-primary" type="submit" ><#if user??>Sign Out<#else>Login</#if></button>
</form>
</#macro>