There are lot of materials out there differentiating value attribute and binding attribute in JSF.
I'm interested in how both approaches differ from each other. Given:
public class User {
private String name;
private UICommand link;
// Getters and setters omitted.
}
<h:form>
<h:commandLink binding="#{user.link}" value="#{user.name}" />
</h:form>
It is pretty straight forward what happens when a value attribute is specified. The getter runs to return the name property value of the User bean. The value is printed to HTML output.
But I couldn't understand how binding works. How does the generated HTML maintain a binding with the link property of the User bean?
Below is the relevant part of the generated output after manual beautification and commenting (note that the id j_id_jsp_1847466274_1 was auto-generated and that there are two hidden input widgets).
I'm using Sun's JSF RI, version 1.2.
<form action="/TestJSF/main.jsf" enctype="application/x-www-form-urlencoded"
id="j_id_jsp_1847466274_1" method="post" name="j_id_jsp_1847466274_1">
<input name="j_id_jsp_1847466274_1" type="hidden" value="j_id_jsp_1847466274_1">
Name
<input autocomplete="off" id="javax.faces.ViewState" name="javax.faces.ViewState"
type="hidden" value="-908991273579182886:-7278326187282654551">
</form>
Where is the binding stored here?
How does it work?
When a JSF view (Facelets/JSP file) get built/restored, a JSF component tree will be produced. At that moment, the view build time, all binding attributes are evaluated (along with id attribtues and taghandlers like JSTL). When the JSF component needs to be created before being added to the component tree, JSF will check if the binding attribute returns a precreated component (i.e. non-null) and if so, then use it. If it's not precreated, then JSF will autocreate the component "the usual way" and invoke the setter behind binding attribute with the autocreated component instance as argument.
In effects, it binds a reference of the component instance in the component tree to a scoped variable. This information is in no way visible in the generated HTML representation of the component itself. This information is in no means relevant to the generated HTML output anyway. When the form is submitted and the view is restored, the JSF component tree is just rebuilt from scratch and all binding attributes will just be re-evaluated like described in above paragraph. After the component tree is recreated, JSF will restore the JSF view state into the component tree.
Component instances are request scoped!
Important to know and understand is that the concrete component instances are effectively request scoped. They're newly created on every request and their properties are filled with values from JSF view state during restore view phase. So, if you bind the component to a property of a backing bean, then the backing bean should absolutely not be in a broader scope than the request scope. See also JSF 2.0 specitication chapter 3.1.5:
3.1.5 Component Bindings
...
Component bindings are often used in conjunction with JavaBeans that are dynamically instantiated via the Managed
Bean Creation facility (see Section 5.8.1 “VariableResolver and the Default VariableResolver”). It is strongly
recommend that application developers place managed beans that are pointed at by component binding expressions in
“request” scope. This is because placing it in session or application scope would require thread-safety, since
UIComponent instances depends on running inside of a single thread. There are also potentially negative impacts on
memory management when placing a component binding in “session” scope.
Otherwise, component instances are shared among multiple requests, possibly resulting in "duplicate component ID" errors and "weird" behaviors because validators, converters and listeners declared in the view are re-attached to the existing component instance from previous request(s). The symptoms are clear: they are executed multiple times, one time more with each request within the same scope as the component is been bound to.
And, under heavy load (i.e. when multiple different HTTP requests (threads) access and manipulate the very same component instance at the same time), you may face sooner or later an application crash with e.g. Stuck thread at UIComponent.popComponentFromEL, or Threads stuck at 100% CPU utilization in HashMap during JSF saveState(), or even some "strange" IndexOutOfBoundsException or ConcurrentModificationException coming straight from JSF implementation source code while JSF is busy saving or restoring the view state (i.e. the stack trace indicates saveState() or restoreState() methods and like).
Also, as a single component basically references the rest of the entire component tree via getParent() and getChildren(), when binding a single component to a view or session scoped bean, you're essentially saving the entire JSF component tree in the HTTP session for nothing. This will get really costly in terms of available server memory when you have relatively a lot of components in the view.
Using binding on a bean property is bad practice
Regardless, using binding this way, binding a whole component instance to a bean property, even on a request scoped bean, is in JSF 2.x a rather rare use case and generally not the best practice. It indicates a design smell. You normally declare components in the view side and bind their runtime attributes like value, and perhaps others like styleClass, disabled, rendered, etc, to normal bean properties. Then, you just manipulate exactly that bean property you want instead of grabbing the whole component and calling the setter method associated with the attribute.
In cases when a component needs to be "dynamically built" based on a static model, better is to use view build time tags like JSTL, if necessary in a tag file, instead of createComponent(), new SomeComponent(), getChildren().add() and what not. See also How to refactor snippet of old JSP to some JSF equivalent?
Or, if a component needs to be "dynamically rendered" based on a dynamic model, then just use an iterator component (<ui:repeat>, <h:dataTable>, etc). See also How to dynamically add JSF components.
Composite components is a completely different story. It's completely legit to bind components inside a <cc:implementation> to the backing component (i.e. the component identified by <cc:interface componentType>. See also a.o. Split java.util.Date over two h:inputText fields representing hour and minute with f:convertDateTime and How to implement a dynamic list with a JSF 2.0 Composite Component?
Only use binding in local scope
However, sometimes you'd like to know about the state of a different component from inside a particular component, more than often in use cases related to action/value dependent validation. For that, the binding attribute can be used, but not in combination with a bean property. You can just specify an in the local EL scope unique variable name in the binding attribute like so binding="#{foo}" and the component is during render response elsewhere in the same view directly as UIComponent reference available by #{foo}. Here are several related questions where such a solution is been used in the answer:
Validate input as required only if certain command button is pressed
How to render a component only if another component is not rendered?
JSF 2 dataTable row index without dataModel
Primefaces dependent selectOneMenu and required="true"
Validate a group of fields as required when at least one of them is filled
How to change css class for the inputfield and label when validation fails?
Getting JSF-defined component with Javascript
Use an EL expression to pass a component ID to a composite component in JSF
(and that's only from the last month...)
See also:
How to use component binding in JSF right ? (request-scoped component in session scoped bean)
View scope: java.io.NotSerializableException: javax.faces.component.html.HtmlInputText
Binding attribute causes duplicate component ID found in the view
each JSF component renders itself out to HTML and has complete control over what HTML it produces. There are many tricks that can be used by JSF, and exactly which of those tricks will be used depends on the JSF implementation you are using.
Ensure that every from input has a totaly unique name, so that when the form gets submitted back to to component tree that rendered it, it is easy to tell where each component can read its value form.
The JSF component can generate javascript that submitts back to the serer, the generated javascript knows where each component is bound too, because it was generated by the component.
For things like hlink you can include binding information in the url as query params or as part of the url itself or as matrx parameters. for examples.
http:..../somelink?componentId=123 would allow jsf to look in the component tree to see that link 123 was clicked. or it could e htp:..../jsf;LinkId=123
The easiest way to answer this question is to create a JSF page with only one link, then examine the html output it produces. That way you will know exactly how this happens using the version of JSF that you are using.
I have 6 columns in my dataTable and I am particularly interested in two InputText: I must prevent the possibility of writing in both inputText on the same row. Either is written into one or the other, but not both.
The solution I found is to implement a Validator and browse the dataTable or specifically visiting the tree of components with UIComponent#visitTree() on UIData as advocated by BalusC here. My question is: how do I know on which row I am? How can I retrieve the value of a InputText specifying the row ?
My goal is to validate two InputText relative to another. When one has a value, the other must be null. And reciprocally.
If you have another solution, I'm interested.
Thanks for your help.
I have developed a JSF custom component, based on UISelectOne component and I want implement a custom renderer for it. To specify the list of items I use a child UISelectItems component (through f:selectItems tag) to indicate a list of plain Java objects (not SelectItem objects).
However, when I want to implement Renderer.encodeEnd and get the value of the child UISelectItems component, instead of getting a list of SelectItems (as indicated in the documentation) I get my original list of plain Java objects.
I imagined that internally, UISelectItems will still be working with SelectItem object, however the custom tag will provide a mean to convert the list of Java objects into a list of SelectItem and I will only see, internally, SelectItem objects. Which is not the case.
Of course, I can just consider my own plain Java objects and proceed with the implementation of the encodeEn method. But I want the component to fully benefit from the fact that through f:selectItems you can specify any list of Java objects as long as you mention also itemValue and itemLabel attributes.
Thanks.
I am new bee to jsf , I am using prime faces, I did not understand how converter works, in case of single select menu.
My confusion is, is it called for converting from request parameter to object in formBean or is it called for rendering my list?
In my list If I specify
<f:selectItems
value="#{granteeSelectionManager.getGrantProgramDTOs()}"
var="grantProgramDTO" itemLabel="#{grantProgramDTO.name}"
itemValue="#{grantProgramDTO.id}" />
how to render my list and specify a converter, the converter is beign called for every item in the list?
Please help me understand if it is called for updating selection in my managed bean or for rendering or both ?
It's used for both cases.
When the list is rendered, the converter's getAsString() is used to convert the Java object behind <f:selectItem(s) itemValue> to a String which in turn is rendered as <option value> (which in turn is used as HTTP request parameter). This is indeed done on a per-item basis.
When the form is submitted, the converter's getAsObject() is used to convert the submitted value (the <option value> which appears as HTTP request parameter) back to the concrete Java object so that it can be set in the model (the backing bean) via <x:selectOneMenu value>.
In your particular case you seem to use object's own id property as item value. In such case a converter is completely unnecessary. You only need to make sure that <x:selectOneMenu value> is bound to a property of exactly the same type as <f:selectItem(s) itemValue>, which is probably Integer or Long.
If you however want to get and set a concrete Java object as value like so
<h:selectOneMenu value="#{bean.grantProgramDTO}">
<f:selectItems ... itemValue="#{grantProgramDTO}" />
</h:selectOneMenu>
then you definitely need a converter for the simple reason that Java objects can't be represented in HTML output and HTTP request parameters without converting them to their unique String representation first. In Java perspective, HTML output is basically one large String and HTTP request parameters are per definition Strings.
See also:
Our selectOneMenu wiki page
Why selectOneMenu Send ItemLabel to the converter?
Is a custom JSF converter needed for this simple class?
Conversion Error setting value for 'null Converter'
Strategy for mapping entity relationships and converting entities
How create a custom coverter in JSF 2?
I know the basic Concept of Model Driven process.
How ever i would like to know whether the same ModelDriven Interface process can be used to display bulk data on the action tagged response page ?
To explain this with example
Request page is normal has an argument which is action controlled.
On validate of request page an Intermediate process returns with the Array List Object
Response success page has an Array List of buckets 100
Each bucket of the Array List holds a JavaBean Object of 10 values.
Question : By ModelDriven Interface process need to display data by looping thru Array List buckets and casting of the Java Bean on the Response page to extract the original data ??
with regards
karthik
I am not able to understand your question properly.
Request page is normal has an argument which is action controlled.
not sure what the above line actually mean
ModelDriven Actions provide a model object to be pushed onto the ValueStack in addition to the Action itself.
It provides more convenient than object back java bean.When you have implemented the model driven interface all that mean that framework will place that object on the top of valuestack.
That means say i have an object person with properties name and age,in normal approach i have to use java bean type accessor to access name property like
<s:textfield name="name" value="%{person.name}"/>
but when i have model driven interface all i need to do is
<s:textfield name="name" value="%{name}"/>
since now person is already on top of value stack so i need not to go traverse inside the person object