Process GET parameters
The <f:viewParam> manages the setting, conversion and validation of GET parameters. It's like the <h:inputText>, but then for GET parameters.
The following example
<f:metadata>
<f:viewParam name="id" value="#{bean.id}" />
</f:metadata>
does basically the following:
- Get the request parameter value by name
id.
- Convert and validate it if necessary (you can use
required, validator and converter attributes and nest a <f:converter> and <f:validator> in it like as with <h:inputText>)
- If conversion and validation succeeds, then set it as a bean property represented by
#{bean.id} value, or if the value attribute is absent, then set it as request attribtue on name id so that it's available by #{id} in the view.
So when you open the page as foo.xhtml?id=10 then the parameter value 10 get set in the bean this way, right before the view is rendered.
As to validation, the following example sets the param to required="true" and allows only values between 10 and 20. Any validation failure will result in a message being displayed.
<f:metadata>
<f:viewParam id="id" name="id" value="#{bean.id}" required="true">
<f:validateLongRange minimum="10" maximum="20" />
</f:viewParam>
</f:metadata>
<h:message for="id" />
Performing business action on GET parameters
You can use the <f:viewAction> for this.
<f:metadata>
<f:viewParam id="id" name="id" value="#{bean.id}" required="true">
<f:validateLongRange minimum="10" maximum="20" />
</f:viewParam>
<f:viewAction action="#{bean.onload}" />
</f:metadata>
<h:message for="id" />
with
public void onload() {
// ...
}
The <f:viewAction> is however new since JSF 2.2 (the <f:viewParam> already exists since JSF 2.0). If you can't upgrade, then your best bet is using <f:event> instead.
<f:event type="preRenderView" listener="#{bean.onload}" />
This is however invoked on every request. You need to explicitly check if the request isn't a postback:
public void onload() {
if (!FacesContext.getCurrentInstance().isPostback()) {
// ...
}
}
When you would like to skip "Conversion/Validation failed" cases as well, then do as follows:
public void onload() {
FacesContext facesContext = FacesContext.getCurrentInstance();
if (!facesContext.isPostback() && !facesContext.isValidationFailed()) {
// ...
}
}
Using <f:event> this way is in essence a workaround/hack, that's exactly why the <f:viewAction> was introduced in JSF 2.2.
Pass view parameters to next view
You can "pass-through" the view parameters in navigation links by setting includeViewParams attribute to true or by adding includeViewParams=true request parameter.
<h:link outcome="next" includeViewParams="true">
<!-- Or -->
<h:link outcome="next?includeViewParams=true">
which generates with the above <f:metadata> example basically the following link
<a href="next.xhtml?id=10">
with the original parameter value.
This approach only requires that next.xhtml has also a <f:viewParam> on the very same parameter, otherwise it won't be passed through.
Use GET forms in JSF
The <f:viewParam> can also be used in combination with "plain HTML" GET forms.
<f:metadata>
<f:viewParam id="query" name="query" value="#{bean.query}" />
<f:viewAction action="#{bean.search}" />
</f:metadata>
...
<form>
<label for="query">Query</label>
<input type="text" name="query" value="#{empty bean.query ? param.query : bean.query}" />
<input type="submit" value="Search" />
<h:message for="query" />
</form>
...
<h:dataTable value="#{bean.results}" var="result" rendered="#{not empty bean.results}">
...
</h:dataTable>
With basically this @RequestScoped bean:
private String query;
private List<Result> results;
public void search() {
results = service.search(query);
}
Note that the <h:message> is for the <f:viewParam>, not the plain HTML <input type="text">! Also note that the input value displays #{param.query} when #{bean.query} is empty, because the submitted value would otherwise not show up at all when there's a validation or conversion error. Please note that this construct is invalid for JSF input components (it is doing that "under the covers" already).
See also:
