Creating Array using JSTL or EL

问题:

I'm working on a web application using Java and its frameworks(Spring 3.1.1). And I'm trying to avoid using scriptlets as much as possible, however I can't find a way other than this to define an array:

<%
    String[] alphabet = {"A", "B", "C", ... , "Z"};
    pageContext.setAttribute("alphabet", alphabet);      
%> 

After setting pageContext attribute, I can use it with ${alphabet}. But I want to know, is it possible to use plain JSTL/EL to create an array?

UPDATE: I'm using this array to create links. For example, if user clicks 'S', a list of employees whose first name starts with 'S' comes. So, instead of creating links one by one I'm iterating ${alphabet}.

回答1:

If you're already on EL 3.0 (Tomcat 8+, WildFly 8+, GlassFish 4+, Payara 4+, TomEE 7+, etc), which supports new operations on collection objects, you can use ${[...]} syntax to construct a list, and ${{...}} syntax to construct a set.

<c:set var="alphabet" value="${['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z']}" scope="application" />

If you're not on EL 3.0 yet, use the ${fn:split()} function trick on a single string which separates the individual characters by a common separator, such as comma.

<c:set var="alphabet" value="${fn:split('A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z', ',')}" scope="application" />

I do however agree that you're better off using normal Java code for this. Given that it's apparently static data, just create this listener class:

@WebListener
public class ApplicationData implements ServletContextListener {

    private static final String[] ALPHABET = { "A", "B", "C", ..., "Z" };

    @Override
    public void contextInitialized(ServletContextEvent event) {
        event.getServletContext().setAttribute("alphabet", ALPHABET);
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        // NOOP.
    }

}

It'll transparently auto-register itself on webapp's startup and put the desired data in application scope.

回答2:

If you want to iterate over tokens in string then simply use forTokens:

<c:set var="alphabet">A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z</c:set>

<c:forTokens items="${alphabet}" delims="," var="letter">
    ${letter}
</c:forTokens>

回答3:

If you use Java EE 7 / Expression Language 3.0 you can create a List literal

<c:set var="alphabet" value="${['A', 'B', 'C', ... , 'Z']}" />

which can then iterate over much like an Array.

回答4:

JSP's are not intended for this kind of stuffs. They are meant to consume, not create. If you want to create an array, then you probably need a Servlet here.

Add the logic of array creation (or even better, List creation), in a Servlet, and use it to pre-process the request to your JSP page. And then, you can use the List attribute set in the servlet in your JSP page.

回答5:

Not pure EL, but a pretty clean solution nevertheless:

<c:set var="alphabet" value='<%=new String[]{"A", "B"} %>'/>

回答6:

Without knowing which framework are you using, the best approach to work with JSPs without using is scriptlets is to back every JSP (view) with a Java bean (an object):

Backing bean:

public class MyBackingBean {

   private List<String> alphabet;

   public List<String> getAlphabet() {
      if (alphabet == null) {
         // Using lazy initialization here, this could be replaced by a
         // database lookup or anything similar
         alphabet= Arrays.asList(new String[]{ "A", "B", "C", ... });
      }
      return alphabet;
   }

}

Then instantiate the bean at the JSP this way:

<jsp:useBean id="backingBean" scope="page" class="com.example.MyBackingBean" />

After that, you could use the EL ${backingBean.alphabet} to access that list.

Note: if you need more complex processing then you will have to use Servlets or any of the features provided by any framework.