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Data binding to NetUI Implicit Objects


In NetUI, data binding allows JSP tags or other UI technologies to read and write data available in the web-tier environment. This document discusses both the implicit objects that are available via NetUI to the JSP author and the expression languages that are used to bind UI objects to those implicit objects.

NetUI tags support binding to both read-only and read-write data. Read-only data is usually bound to tag attributes that simply display information on a page. Read-write data is bound with the intention of being updated from a web browser. Often, read-write data is displayed within an HTML form tag and bound to HTML text boxes, radio buttons, select boxes, and other HTML widget types. Each of these types of binding use the syntax of the JSP 2.0 Expression Language (EL) to express a binding from JSP tag to JavaBean property, Map member, List item, array element, and so on. The JSP 2.0 Expression Language is used to bind read-only data to tag attributes. This language is documented in detail here. For example, this example binds a NetUI span tag to a value from a JSP's PageContext attribute map:

<netui:span value="${pageScope.fooAttribute}"/>

Here, the JSP container evaluates the expression and invokes the span tag's setValue attribute method to pass the result to the tag. The JSP 2.0 EL is also able to perform simple arithmetic and boolean operations in expressions.

When using the NetUI JSP tags, read-write data is bound to NetUI JSP tags differently. NetUI tags use a derivation of the JSP 2.0 EL to refer to implicit objects in a JSP, but unlike the JSP 2.0 EL, the syntax is slightly different. For example, when binding a NetUI textBox tag to data that is meant to be read and then updated during an HTML form POST, the textBox tag might look like:

<netui:textBox dataSource="actionForm.userName"/> 

This expression syntax is used on NetUI JSP tag attributes named dataSource. The expression syntax is necessary to allow the NetUI tags to know both the value of the expression and the expression text. The expression text is needed in order to write the tag's name in the HTML rendered to a web browser. For example, with a userName of "foo", the JSP tag above will render:

<input type="text" name="{actionForm.userName}" value="foo"/> 

The expression text is used by the <netui:textBox> tag to render the value of the HTML input's name attribute, and when the containing HTML form POSTs, this name is used to detect the presence of a NetUI expression that can then be used to update a property on a JavaBean or other data structure.

Best Practice
In order to prevent POSTing data into JSP implicit objects such as requestScope or sessionScope, only a few NetUI implicit objects should be used in read/write expressions. These include pageFlow, sharedFlow, and actionForm.

JSP Implicit Objects

A JSP 2.0+ container exposes a set of implicit objects for use by JSP authors. These implicit objects are documented here. These can be used on any of the NetUI JSP tag attributes that accept runtime expressions. For example, in a webapp called foo the following JSP snippet uses the pageContext implicit object as a JavaBean to build a fully-qualified image path:

<netui:image value="${pageContext.request.contextPath}/images/banner.png"/> 

This renders the following HTML markup:

<img source="/foo/images/banner.png"/> 

The JSP container also makes implicit objects available that provide access to the attribute maps for the page context, request, session, and servlet context. By adding attributes to the page context, request, and session, webapp developers can add their own implicit objects. In the following example, a JavaBean of type Widget is added to the request in a page flow action:

getRequest().setAttribute("widget", fooWidget);

Then, this JSP snippet uses the expression language to data bind to the Widget's density property:

The density is: ${widget.density}

This is effectively the same as writing code that does:

The density is: <%= ((Widget)request.getAttribute("widget")).getDensity() %>

NetUI Implicit Objects

In addition to the implicit objects that the JSP container provides, the NetUI runtime provides an additional set of objects that are available when using certain NetUI features. Not all of the implicit objects are always available -- for example, the actionForm implicit object is only available when used inside of a <netui:form tag for accessing the form's associated form bean.

A summary of the NetUI implicit objects is shown in the table below; details are available further down this document.

Implicit ObjectContextDescription
actionForm Within the <netui:form> tag Provides access to the properties of a JavaBean used as the form bean for an HTML form.
bundle Inside of a JSP where the <netui-data:declareBundle> tag is used to refer to a resource bundle or where a JSP is part of a page flow that has resource bundles declared with the @Jpf.MessageBundle annotation. Provides access to message strings contained in a Java properties file. Strings are referred to by name in the expression.
container The container implicit object is available inside of several NetUI JSP tags that "repeat" over data sets including the <netui-data:dataGrid, <netui-data:repeater, <netui-data:cellRepeater, <netui:select, <netui:checkBoxGroup, and <netui:radioButtonGroup, Provides access to the JavaBean properties exposed by the IDataAccessProvider interface. Implementations of this interface are made available to the PageContext during rendering so that tag bodies can access information about the current data item, the item's index, and so on.
pageFlow Available to any JSP that is part of a page flow. The pageFlow implicit object provides access to the current page flow controller instance as a JavaBean. This allows a page flow controller to expose properties to JSPs.
pageInput Available to any JSP that was reached by a page flow Forward object that had page inputs attached to the Forward. Page flows allow action outputs to be attached to Forward objects as a way to provide a data contract between a page flow action and a page. This ensures that all actions that forward to JSPs provide the JSP with the appropriate data and that all JSPs receive the correct data. This data contract is validated at both the action and at the JSP when using the <netui-data:declarePageInput>. tag.
sharedFlow Available to any JSP that is part of a page flow which references shared flows. The sharedFlow implicit object provides access to any JavaBean properties on Shared Flows associated with the current page flow. This allows Shared Flows to expose properties to JSPs.

NetUI Implicit Object Details


The actionForm implicit object is a convenient way to explicitly reference a JavaBean used for authoring HTML forms. This implicit object is available only inside of <netui:form> tags with action attributes that reference page flow actions accepting a JavaBean. The actionForm implicit object allows data binding to JavaBean properties, Map attributes, Lists, and arrays as with any other implicit object. This example shows a JSP that contains a form which POSTs to a page flow action that accepts a JavaBean NameForm.

The JavaBean:

public class NameForm {
    private String _name;
    public String getName() {
        return _name;

    public void setName(String name) {
    _name = name;

The JSP:

<netui:form action="submitNameform">
    <netui:textBox dataSource="actionForm.name"/><br/>
    <netui:button value="Submit"/>

The page flow action submitNameForm:

public Forward submitNameForm(NameForm form) {

Here, the dataSource's actionForm.name expression refers to the value of the NameForm's name property. The result is data bound to the textBox tag. When the form is submitted to the server, the request parameter {actionForm.name} is applied to the action form which is then passed to the submitNameForm action.


The bundle implicit object is useful for binding UI to localized message strings. The bundle implicit object is available in one of two situations:

The declareBundle JSP tag is used to make a specific resource bundle available to a JSP. For example, given the following resource bundle and JSP, a page can data bind to messages in the resource bundle using the JSP 2.0 EL.

The resource bundle, which is located in WEB-INF/classes/org/foo/messages.properties:

message1=This is the first message
message2=Another message

The JSP can declare this resource bundle to be available to the page using this JSP snippet:

<%@ taglib uri="http://beehive.apache.org/netui/tags-databinding-1.0" prefix="netui-data"%>                

<netui-data:declareBundle name="fooMessages" bundlePath="org/foo/messages"/>

Finally, messages in the JSP can be data bound with JSP literal text and tags:

<netui:span value="${bundle.fooMessages.message1}"/>


The expressions above contain a reference to the bundle implicit object. Then, the specific bundle name is referred to with fooMessages; this name must match the value of a name attribute of a declareBundle tag or the name of a bundle declared in a page flow controller. Finally, the expressions use message1 and message2 to refer to message keys in the messages.properties file.

Resource bundles can also be registered with the bundle implicit object by using the @Jpf.MessageBundle class-level annotation. This allows a page flow to also integrate with the implicit message resources object which is available via a Struts module. These resource bundles will be available to all JSPs that are part of a page flow without having to use the declareBundle JSP tag. Because a default resource bundle can be associated with a Struts module, the bundle name default is reserved for referencing this bundle. For example, the following page flow controller declares the resource bundle above as the default page flow bundle:

    forwards = {...}
    messageBundles = {
public class Controller 
    extends PageFlowController {

Message strings from this bundle can be referred to in JSPs with an expression like:


A resource bundle can also be registered with a specific name by adding an annotation like:

    forwards = {...}
    messageBundles = {
        @Jpf.MessageBundle(bundleName="jpfBundle", bundlePath="org.foo.messages")
public class Controller 
    extends PageFlowController {

Then, the same message strings from the previous two examples are available with an expression like


Each of these three ways to register a resource bundle (JSP tag, implicit page flow bundle, and explicit page flow bundle) can be used together in a single page flow.


The container implicit object is available to a JSP when inside of several NetUI tags that repeat over a Map, List, array, or various other kinds of data sets. Generally, JSP tags that can be bound to data sets iterate through them from start to end as in a Java for loop. Such JSP tags include:

The container implicit object provides access to the current item in the data set and to metadata about the current iteration. This access is based on the properties available on the IDataAccessProvider interface and includes:

Property Name Description
item Refers to the current data item. In an array of Widget beans, the JavaBean propeties of widget can be accessed with an expression like ${container.item.density}
index Refers to the current index of iteration. Tags are free to define their own rules for the the value of the index property, but in general, this is a zero-based index that increments each time the JSP tag renders its body / iterates to the next data item.
container Refers to an outer repeating container. This value is used when two repeating containers are nested and the inner container needs to access the current item in the outer container. For example, this might be used when rendering hierarchical data sets.

The following example uses the NetUI <netui-data:repeater> to iterate over an array of Widget beans displaying each Bean's name and density properties and their index in the array.

<%@ taglib uri="http://beehive.apache.org/netui/tags-databinding-1.0" prefix="netui-data"%>                

<netui-data:repeater dataSource="requestScope.widgetBeanArray">

Notice in this example how the repeater tag has a dataSource attribute that references the data set to iterate through. The dataSource attribute requires the use of the NetUI Expression Language because the repeater can be used to render editing UI for data sets. For example, in the case of rendering a shopping cart, the repeater can be used to render each item in the cart with a <netui:textBox> for editing the quantity of each item. An example of this can be found in the Beehive sample webapp called netui-samples under the ui/repeaterediting/ directory.


The pageFlow implicit object is used to refer to the current page flow controller as a JavaBean. If there is no page flow present, the pageFlow implicit object will not be available for data binding. For example, if a page flow controller exposes a username property as:

    forwards={@Jpf.Forward(name="index", path="index.jsp")}
public class Controller {
    private String _username = null;
    public String getUsername() {
        return _username;

    public Forward begin() {
        _username = "Foo Bar";
        return new Forward("index");

The username property can be data bound in index.jsp as:

Best Practice
Because mutable JavaBean properties can be updated via an HTML form POST, JavaBean properties exposed by a page flow should usually be read-only unless the page flow itself is being used as a form bean.


The pageInput implicit object is used to refer to a Map of objects that are passed via a Forward from a page flow action to a JSP. Use of Page Inputs consists of two parts -- the first are called action outputs and the second are called page inputs. Action outputs are passed from page flow actions to pages via the action's Forward object. page flow actions use Java annotations to declare a validatable data contract that ensures that an action passes the correct data via a Forward. At the page, this data is called a page input and can again be checked to ensure that the page receives the data necessary to render successfully.

This example shows a page flow action that passes an action output of type Widget to a JSP which data binds to the density property on the Widget.

The page flow controller:

public class Controller
    extends PageFlowController {

                               actionOutputs={@Jpf.ActionOutput(name="theWidget", type=Widget.class, required=true)}
    protected Forward begin() {
        Widget widget = new Widget();
        Forward f = new Forward("success");
        f.addActionOutput("theWidget", widget);
        return f;

Notice here that the action has added an action output to the Forward via the addActionOutput method call. The NetUI runtime will then validate the data contract declared in the annotations against the returned forward object. If validation fails, a runtime error will be displayed in the browser.

The JSP:

<%@ taglib uri="http://beehive.apache.org/netui/tags-databinding-1.0" prefix="netui-data"%>                
<netui-data:declarePageInput name="theWidget" type="org.foo.Widget"/>


Notice here that the page has used a declarePageInput tag to ensure that the Widget entering the page is both present in the set of page inputs and is non-null. Given this information, the JSP then refers to properties on the Widget via the expression language.

Action outputs and page inputs can be used with or without validation; to disable action output validation, simply remove any action output annotations from a page flow action. To disable page input validation in a JSP, remove any declarePageInput tags from the JSP. The APIs to add action outputs to Forward objects and to refer to them via the pageInput implicit object will continue to work without any data contract validation.


The sharedFlow implicit object is used to refer to properties of shared flow objects that are associated with the current page flow. If there is no page flow present, the sharedFlow implicit object will not be available for data binding. In order for a shared flow to be available for data binding, it must be registered with a page flow by type; additionally, it is registered with a name that will uniquely identify it in the set of shared flows associated with a page flow. More information on shared flows can be found here.

The following example shows a shared flow, a page flow that uses the Shared Flow, and a JSP that uses the JSP 2.0 EL to data bind to a JavaBean propety of the shared flow.

The Shared Flow:

package org.foo;

import org.apache.beehive.netui.pageflow.SharedFlowController;

public class SharedFlow 
   extends SharedFlowController {

    private String _sharedMessage = null;
    public String getSharedMessage() {
        return _sharedMessage;

The page flow controller:

    sharedFlowRefs={@Jpf.SharedFlowRef(name="aSharedFlow", type=org.foo.SharedFlow.class)}
public class Controller
   extends PageFlowController {


Above, the page flow controller adds an explicit reference to the shared flow controller org.foo.SharedFlow defined above.

The JSP:


In a JSP whose current page flow controller is the Controller class defined above, the JSP has access to all of the shared flows associated to the page flow via its @Jpf.SharedFlowRef annotation. The shared flow can then be referenced by the name attribute of the SharedFlowRef annotation. In this case, the name aSharedFlow is used in the JSP 2.0 expression to refer to the sharedFlow's sharedMessage property.

Expression Language Details

Due to limitations in the JSP 2.0 Expression Language specification, expressions can not be used to reference read-write data because the JSP tag itself can never obtain both the expression text and the value of the expression. In NetUI, both the value and expression text are required to bind to editable data because the expression text is written into the JSP as the HTML name attribute of HTML form input fields.