Doubt with Java EL code snippet

Question

I’m studying a code and I don’t understand this part of EL:

<c:choose>
    <c:when test="${not empty pedidoMesa}">
        Pedido: ${pedidoMesa.id} ${pedidoMesa.nomeCliente}  
    </c:when>
    <c:otherwise>
       Mesa
    </c:otherwise>
</c:choose>

The one that was referenced pedidoMesa, to appear the id and the nomeCliente? I turned the code around and found nothing to link it to Model.

Answer 1

Briefly:

<c:choose> in expression language is like a switch/case in your Java code.

The <when test="{condição}"> is like a case "condição":.

And the <c:otherwise> is equivalent to default within a structure switch/case.

---

Note that the core JSTL does not have a condition <c:else>, that is, if you need to test a condition and from it take an action in case it is false, you would have to do something like this:

<c:if test="${not empty pedidoMesa}">
  Pedido: ${pedidoMesa.id} ${pedidoMesa.nomeCliente}  
</c:if>

<c:if test="${empty pedidoMesa}">
  Mesa
</c:if>

There is no problem writing the code like this, but some developers prefer to use the <c:choose> (which is equivalent to switch/case) and make use of <c:otherwise> to treat the contrary condition within the same structure. The above code could be written like this:

<c:choose>
  <c:when teste="${not empty pedidoMesa}">
    Pedido: ${pedidoMesa.id} ${pedidoMesa.nomeCliente}  
  </c:when>
  <c:otherwise>
    Mesa
  </c:otherwise>
</c:choose>

When you use ${pedidoMesa}, the first attribute with name will be returned pedidoMesa which is not null, no matter in what context the request was created/defined. However in the following order: PageContext, HttpServletContext, HttpSession and ServletContext.