Primefaces automatically pass parameter from selectOneMenu without using command button - ajax

Question

I would like to call an action from a backing bean immediately after a value in a PF selectOneMenu is selected without using any button or link. I already looked at this question:
When to use valueChangeListener or f:ajax listener?
While very useful, I am not able to apply the recommended business logic that would allow the action to be called dynamically based on ONLY the selected value from the menu. I know how to use ajax to render changed values between two menus (where the value of one menu is changed). The options discussed on the above questions do not seem to work in my case which leads me to look deeper into the EL and backing bean methods to understand these concepts better.
I tried two ways to allow dynamic processing of my menu paramteres. The first was to use the valueChangeListener ayttribute in the EL:
<p:selectOneMenu id="Ctl" value="#{actionBean.department}"
valueChangeListener="#{actionBean.ShowListDeptAct(event)}" onchange="submit()">
<f:selectItem itemLabel="Dept A" itemValue="Dept A"/>
<f:selectItem itemLabel="Dept B" itemValue="Debt B"/>
<f:selectItem itemLabel="Dept C" itemValue="Dept C"/>
</p:selectOneMenu>
The bean method:
public void ShowListDeptAct(ValueChangeEvent event) {
department = FacesContext.getCurrentInstance().getExternalContext().getRequestParameterMap().get("department");
listDeptActions();
}
public String listDeptActions() {
//department = getDepartment();
deptActions = facade.findByDept(department);
return "DeptAction";
This only results in refreshing the page and the bean method does not even to be functioning.
The second way was to use the p ajax tag:
<p:selectOneMenu id="Ctl" value="#{actionBean.department}">
<f:selectItem itemLabel="Dept A" itemValue="Dept A"/>
<f:selectItem itemLabel="Dept B" itemValue="Debt B"/>
<f:selectItem itemLabel="Dept C" itemValue="Dept C"/>
<p:ajax listener="#{actionBean.ShowListDeptAct}" oncomplete="submit()"/>
</p:selectOneMenu>
bean method:
public void ShowListDeptAct(AjaxBehaviorEvent event) {
department = FacesContext.getCurrentInstance().getExternalContext().getRequestParameterMap().get("department");
listDeptActions();
}
public String listDeptActions() {
//department = getDepartment();
deptActions = facade.findByDept(department);
return "DeptAction";
}
This resulted in exactly the same behavior which makes me think my problem might be a mistake in how I"m writing the listener.
The EL is supposed to pass the name of the selected department to the following backing bean methods which will display a dataTable of the data for the selected department. The backing bean methods:
After reading the PF User Guide and some questions here, I'm neither sure about how to pass the correct EL nor the right backing bean methods to achieve this. Would appreciate some guidance. Thank you!

Answer 1

When you work with Ajax, it is very important to always keep in mind which part of the view needs to be submitted to the server and which part needs to be updated as the response. This is the number one error source when dealing with Ajax-Requests.
For the <p:ajax>-Tag, this is specified with the attributes process and update, in pure JSF <f:ajax> the attributes execute and render are used.
This is very helpful to read to understand how these work:
Understanding PrimeFaces process/update and JSF f:ajax execute/render attributes
In your first attempt you try to get the parameters from the request parameter map, which is not a good idea, because the key of each request parameter will be the id of the component that is sent. You would have to know the components' id to get the value. It is not recommended to work with component ids on the server side, if possible, as they may change when enclosing naming containers change and are therefore unreliable. Also this heavily interferes with separation of view and program logic.
The best attempt is to bind the values of components to variables on server side beans, as you already did, an work with them.
In your second attempt, the default value of process in the <p:ajax>-Tag is null, so there are no values of components sent to the server. If you write
and bind the value of the <p:selectOneMenu> to a variable on the server, you can use the value of the department in the doAction()-Method.