ViewParam vs @ManagedProperty(value = "#{param.id}")
What is the difference between defining View Params like this:
<f:metadata>
<f:viewParam name="id" value="#{someBean.id}"/>
</f:metadata>
And defining the property in the ManagedBean like this:
@ManagedProperty(value = "#{param.id}")
private Integer id;
Answer
<f:viewParam>:
Sets the value during update model values phase only (since it extends
UIInput).The set value is not available during
@PostConstruct, so you need an additional<f:event type="preRenderView" listener="#{bean.init}" />inside the<f:metadata>to do initialization/preloading based on the set values. Since JSF 2.2 you could use<f:viewAction>for that instead.Allows for nested
<f:converter>and<f:validator>for more fine-grained conversion/validation. Even a<h:message>can be attached.Can be included as GET query string using
includeViewParamsattribute of<h:link>orincludeViewParams=truerequest parameter in any URL.Can be used on a
@RequestScopedbean, but it requires the bean to be@ViewScopedif you want the view parameters to survive any validation failures caused by forms enclosed in the view, otherwise you need to manually retain all request parameters for the subsequent requests by<f:param>in the command components.
Example:
<f:metadata>
<f:viewParam id="user_id" name="id" value="#{bean.user}"
required="true" requiredMessage="Invalid page access. Please use a link from within the system."
converter="userConverter" converterMessage="Unknown user ID."
/>
</f:metadata>
<h:message for="user_id" />
with
private User user;
and an @FacesConverter("userConverter"). Invoking page by http://example.com/context/user.xhtml?id=123 will pass the id parameter through the converter and set the User object as a bean property.
@ManagedProperty:
Sets the value immediately after bean's construction.
Set value is available during
@PostConstructwhich allows easy initialization/preloading of other properties based on the set value.Doesn't allow for declarative conversion/validation in view.
Managed property of
#{param}is not allowed on beans with a broader scope than request scope, so the bean is required to be@RequestScoped.If you rely a managed property of
#{param}being present in the subsequent POST requests, then you need to include it as<f:param>in theUICommandcomponents.
Example:
@ManagedProperty("#{param.id}")
private Long id;
private User user;
@EJB
private UserService userService;
@PostConstruct
public void init() {
user = userService.find(id);
}
But you have to manage validation yourself whenever user is null by fiddling with FacesContext#addMessage() or something.
You can use them both when both @PostConstruct and includeViewParams are mandatory. You only won't be able to apply fine-grained conversion/validation anymore.