How do you get the length of a list in the JSF exp

问题:

How would I get the length of an ArrayList using a JSF EL expression?

#{MyBean.somelist.length}

does not work.

回答1:

Yes, since some genius in the Java API creation committee decided that, even though certain classes have size() members or length attributes, they won't implement getSize() or getLength() which JSF and most other standards require, you can't do what you want.

There's a couple ways to do this.

One: add a function to your Bean that returns the length:

In class MyBean:
public int getSomelistLength() { return this.somelist.length; }

In your JSF page:
#{MyBean.somelistLength}

Two: If you're using Facelets (Oh, God, why aren't you using Facelets!), you can add the fn namespace and use the length function

In JSF page:
#{ fn:length(MyBean.somelist) }

回答2:

You mean size() don't you?

#{MyBean.somelist.size()}

works for me (using JBoss Seam which has the Jboss EL extensions)

回答3:

Note: This solution is better for older versions of JSTL. For versions greater then 1.1 I recommend using fn:length(MyBean.somelist) as suggested by Bill James.

---

This article has some more detailed information, including another possible solution;

The problem is that we are trying to invoke the list's size method (which is a valid LinkedList method), but it's not a JavaBeans-compliant getter method, so the expression list.size-1 cannot be evaluated.

There are two ways to address this dilemma. First, you can use the RT Core library, like this:

<c_rt:out value='<%= list[list.size()-1] %>'/>

Second, if you want to avoid Java code in your JSP pages, you can implement a simple wrapper class that contains a list and provides access to the list's size property with a JavaBeans-compliant getter method. That bean is listed in Listing 2.25.

The problem with c_rt method is that you need to get the variable from request manually, because it doesn't recognize it otherwise. At this point you are putting in a lot of code for what should be built in functionality. This is a GIANT flaw in the EL.

I ended up using the "wrapper" method, here is the class for it;

public class CollectionWrapper {

    Collection collection;

    public CollectionWrapper(Collection collection) {
        this.collection = collection;
    }

    public Collection getCollection() {
        return collection;
    }

    public int getSize() {
        return collection.size();
    }
}

A third option that no one has mentioned yet is to put your list size into the model (assuming you are using MVC) as a separate attribute. So in your model you would have "someList" and then "someListSize". That may be simplest way to solve this issue.

回答4:

<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn"%>

<h:outputText value="Table Size = #{fn:length(SystemBean.list)}"/>

On screen it displays the Table size

Example: Table Size = 5

回答5:

You can eventually extend the EL language by using the EL Functor, which will allow you to call any Java beans methods, even with parameters...

回答6:

After 7 years... the facelets solution still works fine for me as a jsf user

include the namespace as xmlns:fn="http://java.sun.com/jsp/jstl/functions"

and use the EL as #{fn:length(myBean.someList)} for example if using in jsf ui:fragment below example works fine

<ui:fragment rendered="#{fn:length(myBean.someList) gt 0}">
    <!-- Do something here-->
</ui:fragment>

回答7:

You can get the length using the following EL:

#{Bean.list.size()}