What can , and be used for?

Question

Can anyone clarify how we can use in general, or a in real world example, this snippet?

Answer 1

Process GET parameters

The manages the setting, conversion and validation of GET parameters. It's like the , but then for GET parameters.

The following example

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 and in it like as with )
• 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.

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Performing business action on GET parameters

You can use the for this.

with

public void onload() {
    // ...
}

The is however new since JSF 2.2 (the already exists since JSF 2.0). If you can't upgrade, then your best bet is using instead.

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 this way is in essence a workaround/hack, that's exactly why the was introduced in JSF 2.2.

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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.

which generates with the above example basically the following link

with the original parameter value.

This approach only requires that next.xhtml has also a on the very same parameter, otherwise it won't be passed through.

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Use GET forms in JSF

The can also be used in combination with "plain HTML" GET forms.

    
    

...

    Query
    
    
    

...

     ...

With basically this @RequestScoped bean:

private String query;
private List results;

public void search() {
    results = service.search(query);
}

Note that the is for the , not the plain HTML ! 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).

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See also:

• ViewParam vs @ManagedProperty(value = "#{param.id}")
• Communication in JSF 2.0 - Processing GET request parameters