How does the binding attribute work in JSF When and how should it be used

Question

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       lt h form gt       lt h commandLink binding    user link   value    user name     gt   lt  h form gt    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    lt 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  gt       lt input name  j id jsp 1847466274 1  type  hidden  value  j id jsp 1847466274 1  gt       lt a href     onclick       gt Name lt  a gt       lt input autocomplete  off  id  javax faces ViewState  name  javax faces ViewState          type  hidden  value  -908991273579182886 -7278326187282654551  gt   lt  form gt    Where is the binding stored here

User · Answer

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.

User · Answer

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 Java Threads at 100  CPU utilization using richfaces UIDataAdaptorBase and its internal HashMap  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    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   lt ui repeat gt    lt h dataTable gt   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  lt cc implementation gt  to the backing component  i e  the component identified by  lt cc interface componentType gt   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  quot true quot  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

[jsf] How does the 'binding' attribute work in JSF? When and how should it be used?

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