JSTL - Using forEach to iterate over a user-define

Question

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javax.servlet.ServletException: javax.servlet.jsp.JspTagException: Don't know how to iterate over supplied “items” in <forEach> (3 answers)

Closed 2 years ago.

What methods do I need to add to a custom Java class so that I can iterate over the items in one of its members? I couldn't find any specifications about how the JSTL forEach tag actually works so I'm not sure how to implement this.

For example, if I made a generic "ProjectSet" class and I woud like to use the following markup in the JSP view:

<c:forEach items="${projectset}" var="project">
...
</c:forEach>

Basic class file:

public class ProjectSet {
    private ArrayList<Project> projects;
    public ProjectSet() {
        this.projects = new ArrayList<Project>();
    }
    // .. iteration methods ??
}

Is there any interface that I must implement like PHP's ArrayAccess or Iterator in order for this to work?

Edit: Without directly accessing the ArrayList itself, because I will likely be using some sort of Set class using generics, and the JSP view shouldn't have to know about the inner workings of the class.

Answer 1

The Iterable interface provides this functionality:

public class ProjectSet implements Iterable<Project> {
    private ArrayList<Project> projects;
    public ProjectSet() {
        this.projects = new ArrayList<Project>();
    }

    // .. iteration methods ??
   @Override
   public Iterator<Project> iterator() {
       return projects.iterator();
   }
}

You can exchange the iterator logic as you need.

Answer 2

Unfortunately, the forEach tag does not support Iterable as the items source. It does support:

• Arrays
• Collection
• Iterator
• Enumeration
• Map
• Comma-separated list of values given in a String

(relevant source code: ForEachSupport)

Of these, it's probably best to use an Iterator as the forEach source when you wish to fetch items from your own class.

Answer 3

Yes, you should implement java.util.Iterable