As to the cause, the <f:attribute> is specific to the component itself (populated during view build time), not to the iterated row (populated during view render time).

There are several ways to achieve the requirement.

1. Use <f:param> instead. It adds a request parameter.

   <h:commandLink action="#{bean.insert}" value="insert">
       <f:param name="id" value="#{item.id}" />
   </h:commandLink>
   

   If your bean is request scoped, let JSF set it by @ManagedProperty

   @ManagedProperty(value="#{param.id}")
   private Long id; // +setter
   

   Or if your bean has a broader scope or if you want more fine grained validation/conversion, use<f:viewParam> on the target view, see also f:viewParam vs @ManagedProperty:

   <f:viewParam name="id" value="#{bean.id}" required="true" />
   

   Either way, this has the advantage that the datamodel doesn't necessarily need to be preserved for the form submit (for the case that your bean is request scoped).

   ---

2. Use <f:setPropertyActionListener> instead. The advantage is that this removes the need for accessing the request parameter map when the bean has a broader scope than the request scope.

   <h:commandLink action="#{bean.insert}" value="insert">
       <f:setPropertyActionListener target="#{bean.id}" value="#{item.id}" />
   </h:commandLink>
   

   In combination with

   private Long id; // +setter
   

   It'll be just available by property id in action method. This only requires that the datamodel is preserved for the form submit request. Best is to put the bean in the view scope by @ViewScoped.

   ---

3. If your servletcontainer supports Servlet 3.0 / EL 2.2, then just pass it as method argument. This also requires that the datamodel is preserved for the form submit request. Best is to put the bean in the view scope by @ViewScoped.

   <h:commandLink action="#{bean.insert(item.id)}" value="insert" />
   

   In combination with:

   public void insert(Long id) {
       // ...
   }
   

   You can even pass the entire item object:

   <h:commandLink action="#{bean.insert(item)}" value="insert" />
   

   with:

   public void insert(Item item) {
       // ...
   }
   

   On Servlet 2.5 containers, this is also possible if you supply an EL implementation which supports this, like as JBoss EL. For configuration detail, see this answer.

   ---

4. Bind the datatable value to DataModel<E> instead which in turn wraps the items.

   <h:dataTable value="#{bean.model}" var="item">
   

   with

   private transient DataModel<Item> model;
   
   public DataModel<Item> getModel() {
       if (model == null) {
           model = new ListDataModel<Item>(items);
       }
       return model;
   }
   

   (making it transient and lazily instantiating it in the getter is mandatory when you're using this on a view or session scoped bean since DataModel doesn't implement Serializable)

   Then you'll be able to access the current row by DataModel#getRowData() without passing anything around (JSF determines the row based on the request parameter name of the clicked command link/button).

   public void insert() {
       Item item = model.getRowData();
       Long id = item.getId();
       // ...
   }
   

   This also requires that the datamodel is preserved for the form submit request. Best is to put the bean in the view scope by @ViewScoped.

   ---

5. You can use Application#evaluateExpressionGet() to programmatically evaluate the current #{item}.

   public void insert() {
       FacesContext context = FacesContext.getCurrentInstance();
       Item item = context.getApplication().evaluateExpressionGet(context, "#{item}", Item.class);
       Long id = item.getId();
       // ...
   }
   

   ---

Which way to choose depends on the functional requirements and whether the one or the other offers more advantages for other purposes. I personally would go ahead with #3 or, when you'd like to support servlet 2.5 containers as well, with #2.