I would like some technique/pattern advice on retaining each single bean I bind to my Composite Components.
I am adding composite components to a form pragmatically based on user actions, and when the user is finished I want to harvest the single bean behind each composite component they added. Ex: If user selects & adds 4 composite components, that's one bean for each, so when the user is finished I want the 4 beans with the user's entered values.
This seems a bit hairy in the JSF world, but I continue to dig through stackoverflow and experiment. I'm relatively new to JSF details, but having fun.
I've got the composite components loading, each being given a bean to be used as "cc.attrs.bean" and it properly adds the control to the form. Here is what I am currently doing and what I expected:
Load Composite Component
Instantiate its Bean
Save Bean reference in a separate list in a ViewScoped bean (my hook to the bean for later)
Give Bean to Composite Component (as an attribute)
Add Composite Component to form
...User interacts with form and Composite Components adding/editing values...
User finally pushes "Done" (now I need the modified beans).
Thought I could get all the user's values from the "separate list in a ViewScoped bean" from #3 above.
My preliminary experiments tell me that if I instantiate the bean, save the bean reference in a separate ViewScoped list then give the bean to the Composite Component, the bean I saved won't have the Composite Component's values. Between all the build/render phases it seems to lose the connection between the bean I saved and the bean the Composite Component is bound to.
I don't know if I should be following this path, or if I should use a FacesComponent event technique to intercept the bean attribute being passed along, or if I should be using filters, or maybe even magical pixie dust etc...
This seemed promising: I already wrap each of my user selectable Composite Components in a single common Wrapper Composite Component (lets me put a nice PrimeFaces collapsible panel frame around them). For example, I put "Composite Component A" into "Wrapper", then I add "Wrapper" to the form. If I passed the single bean as an attribute to both those Composite Components, I was hoping that the FacesComponent event "init" technique on the Wrapper could nicely capture the "real bound bean" in my separate list in the ViewScoped bean. In my attempts on this today I'm having trouble finding the right event type and getting access to the bean... and getting lots of strange errors (probably due to my lack of detailed understanding of the lifecycle).
Stack: Eclipse Mars, JSF 2.2, Mojarra 2.2, Tomcat 8.0
Related
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.
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.
In JSF, I wonder how a ManagedBean understands that it should be available to an xhtml or facelet ?
To bind component values and objects to managed bean properties or to reference managed bean methods from component tags, page authors should use the Expression Language syntax.
When user make request to the page that contains EL, that refers to specific manage bean, this bean will be instantiated by JSF framework (except managed beans that have attribute eager="true", that means that managed bean is created before it is requested for the first time).
So the fact is that managed bean will be instantiated by JSF container and works as a model for the appropriate UI Component so the Facelet will know about the Bean, not vice versa
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.
Yet another problem among others with view scoped backing beans in JSF. I created an inner class in my view scoped backing bean. That inner class is converter. I need the inner class because I have to access some fields from my backing bean (list of select-one items in this case). Suddenly I figure out that my backing bean's #PostConstruct method is called after every request. After some inspection I realized that converter attribute is the problem and after some google search a find on (as always) BalusC's blog reason for this.
So, my question is how to make my converter to work fine, and also have my list of data which is necessary for conversion?
Decouple the converter into a stand alone class and use Application#evaluateExpressionGet() to access the view scoped bean instance inside the converter method.
ViewBean viewBean = context.getApplication().evaluateExpressionGet(context, "#{viewBean}", ViewBean.class);
This is however a bit nasty design. If I understand your concrete functional requirement of converting the selected item based on the list of available items right, an alternative is to use a generic converter which converts based on the physical <f:selectItem>/<f:selectItems> components. The JSF utility library OmniFaces has two converters for exactly this purpose, the SelectItemsConverter and SelectItemsIndexConverter.