JavaServer Faces Expression Language - Sun Java Studio Creator 2

Introduction

JavaServer Faces provides an expression language (JSF EL) that is used in web application pages to access the JavaBeans components in the page bean and in other beans associated with the web application, such as the session bean and the application bean. The IDE in most cases takes care of specifying the correct expression for you, for example, when you bind a component's text property to a data provider or to a JavaBean property.

To bind any property of a component, you can add the component to a page and then right-click the component and choose Property Bindings. You can then use the Property Bindings dialog box to select a property of the component and choose which JavaBeans property the component property is to be bound to.

As an example of binding a component to a database table, the following code sample references a Static Text component. Here's how to produce the code sample:

Drag the Static Text component from the Basic category of the Palette to a page in the Visual Designer.
Open the Servers window and drag the Customer table from the Order database and drop it on the page.

The IDE automatically adds a data provider object for that database table to the page and binds the the text property to the CUSTOMER_TABLE.CUSTOMER_NUM field of the data provider. You see the text of the component change to 123.
Right-click the component and choose Bind to Data.
In the Bind to Data dialog box, choose the CUSTOMER_TABLE.NAME field of the data provider and click OK to change the binding of the text property to the correct field.
Click the JSP button above the page to see the resulting source code.

The resulting code in the JSP editor looks like this:

  <ui:staticText binding="#{Page1.staticText1}" 

   id="staticText1" 

   style="position: absolute; left: 216px; top: 192px" 

   text="#{Page1.customer_tblDataProvider.value['CUSTOMER_TBL.NAME']}"/>

The first line of code shows the name of the JavaServer Faces component, staticText. It uses the qualifier ui:, which identifies the XML namespace for the staticText component. The ui: qualifier is defined in the page header as xmlns:ui="http://www.sun.com/web/ui". This namespace points to a custom tag library for rendering UI components in the Basic, Composite, and Layout categories of the Palette.

There are two other qualifiers that you will see in JSP code that are defined on this same line :
- h: - Defined in the page header as xmlns:h="http://java.sun.com/jsf/html", this namespace points to a JavaServer Faces custom tag library for rendering JavaServer Faces Reference Implementation components that are primarily in the Standard category of the palette.
- f: - Defined in the page header as xmlns:f="http://java.sun.com/jsf/core", this namespace points to a JavaServer Faces custom tag library for representing event handlers, validators, and other actions.
The TLD documentation for these two qualifiers is located at:
http://java.sun.com/j2ee/javaserverfaces/1.1/docs/tlddocs/index.html.
The binding attribute connects this component to a specific JavaBeans object staticText1 in the Page1 page bean. The binding attribute and the attributes id, style, and text are all JavaServer Faces tag library attributes. The last three attributes, id, style, and text, are represented in the IDE as properties of the component and can be set in the component's Properties window.
The binding and text attributes use the JavaServer Faces expression language. You can use the JavaServer Faces expression language to set the value attribute in the component's Properties window.

As described in the sections that follow, the JavaServer Faces expression language syntax uses the delimiters #{}. A JavaServer Faces expression can be a value-binding expression (for binding UI components or their values to external data sources) or a method-binding expression (for referencing backing bean methods). It can also accept mixed literals and the evaluation syntax and operators of the 2.0 expression language.

JavaServer Faces EL Expression Syntax

JSF EL can be used to bind JavaBeans to component properties to simplify how the components access data from various sources. JSF EL expressions use the syntax #{expr};

The syntax of a value binding expression is identical to the syntax of an expression language expression defined in the JavaServer Pages Specification (version 2.0), sections 2.3 through 2.9, with the following exceptions:

The expression delimiters for a value binding expression are #{ and } instead
of ${ and }.
Value binding expressions do not support JSP expression language functions.

In addition to the differences in delimiters, the two expression types have the following semantic differences:

During rendering, value binding expressions are evaluated by the JavaServer Faces implementation (via calls to the getValue method) rather than by the compiled code for a page.
Value binding expressions can be evaluated programmatically, even when a page is not present.
Value binding expression evaluation leverages the facilities of the configured VariableResolver and PropertyResolver objects available through the Application object for the current web application, for which applications can provide plug-in replacement classes that provide additional capabilities.
If a value binding expression is used for the value property of an EditableValueHolder component (any input field component), the expression is used to modify the referenced value rather than to retrieve it during the Update Model Values phase of the request processing lifecycle.

Examples of valid value binding expressions include:

   #{Page1.name}

   #{Foo.bar}

   #{Foo[bar]}

   #{Foo[“bar”]}

   #{Foo[3]}

   #{Foo[3].bar}

   #{Foo.bar[3]}

   #{Customer.status == ‘VIP’}

   #{(Page1.City.farenheitTemp - 32) * 5 / 9}

   Reporting Period: #{Report.fromDate} to #{Report.toDate}

For value binding expressions where the setValue method is going to be called (for example, for text property bindings for input fields during Update Model Values), the syntax of a value binding expression is limited to one of the following forms, where expr-a is a general expression that evaluates to some object, and value-b is an identifier:

   #{expr-a.value-b}

   #{expr-a[value-b]]

   #{value-b}

Get Value Semantics

When the getValue method of a ValueBinding instance is called (for example, when an expression on a JSP tag attribute is being evaluated during the rendering of the page), and the expression is evaluated, and the result of that evaluation is returned, evaluation takes as follows:

The expression language unifies the treatment of the . and [] operators. expr-a.expr-b is equivalent to a["expr-b"]; that is, the expression expr-b is used to construct a literal whose value is the identifier, and then the [] operator is used with that value.
The left-most identifier in an expression is evaluated by the VariableResolver instance that is acquired from the Application instance for this web application. If the value on the left side of the . or [] operator is a RowSet, the object on the right side is treated as a column name. See the next section for a more complete evaluation description of these operators.
Each occurrence of the . or [...] operators in an expression is evaluated by the PropertyResolver instance that is acquired from the Application instance for this web application.
Properties of variables are accessed by using the . operator and can be nested arbitrarily.

Set Value Semantics

When the setValue method of a ValueBinding is called (for example, for text property bindings for input fields during Update Model Values), the syntax of the value binding restriction is restricted as described in the previous section. The implementation must perform the following processing to evaluate an expression of the form #{expra.value-b} or #{expr-a[value-b]}:

Evaluate expr-a into value-a.
If value-a is null, throw PropertyNotFoundException.
If value-b is null, throw PropertyNotFoundException.
If value-a is a Map, call value-a.put(value-b, new-value).
If value-a is a List or an array:
- Coerce value-b to int, throwing ReferenceSyntaxException on an error.
- Attempt to execute value-a.set(value-b, new-value) or Array.set(value-b, new-value) as appropriate.
- If IndexOutOfBoundsException or ArrayIndexOutOfBoundsException is thrown, throw PropertyNotFoundException.
- If a different exception was thrown, throw EvaluationException.
Otherwise (value-a is a JavaBeans object):
- Coerce value-b to String.
- If value-b is a writeable property of value-a (as per the JavaBeans Specification), call the setter method (passing new-value). Throw ReferenceSyntaxException if an exception is thrown.
- Otherwise, throw PropertyNotFoundException.

If the entire expression consists of a single identifier, the following rules apply:

If the identifier matches the name of one of the implicit objects described below,
throw ReferenceSyntaxException.
Otherwise, if the identifier matches the key of an attribute in request scope,
session scope, or application scope, the corresponding attribute value will be
replaced by new-value.
Otherwise, a new request scope attribute will be created, whose key is the
identifier and whose value is new-value.

Implicit Objects

The expression language defines a set of implicit objects:

facesContext - The FacesContext instance for the current request.

param - Maps a request parameter name to a single value.
paramValues - Maps a request parameter name to an array of values.
header - Maps a request header name to a single value.
headerValues - Maps a request header name to an array of values.
cookie - Maps a cookie name to a single cookie.
initParam - Maps a context initialization parameter name to a single value.

Objects that allow access to various scoped variables:

requestScope - Maps request-scoped variable names to their values.
sessionScope - Maps session-scoped variable names to their values.
applicationScope - Maps application-scoped variable names to their values.

When an expression references one of these objects by name, the appropriate object is returned. An implicit object takes precedence over an attribute that has the same name. For example, #{facesContext} returns the FacesContext object, even if there is an existing facesContext attribute containing some other value.

Literals

The expression language defines the following literals:

Boolean: true and false
Integer: as in Java
Floating point: as in Java
String: with single and double quotes; " is escaped as \", ' is escaped as \', and \ is escaped as \\.
Null: null

Operators

In addition to the . and [] operators discussed above in Get Value Semantics and the section after that one, the expression language provides the following operators:

Arithmetic: +, - (binary), *, /, div, %, mod, - (unary)
Logical: and, &&, or, ||, not, !
Relational: ==, eq, !=, ne, <, lt, >, gt, <=, ge, >=, le. Comparisons can be made against other values, or against boolean, string, integer, or floating point literals.
Empty: The empty operator is a prefix operation that can be used to determine whether a value is null or empty.
Conditional: A ? B : C. Evaluate B or C, depending on the result of the evaluation of A.

The precedence of operators highest to lowest, left to right is as follows:

[] .
() (changes precedence of operators)
- (unary) not ! empty
* / div % mod
+ - (binary)
< > <= >= lt gt le ge
== != eq ne
&& and
|| or
? :

Reserved Words

The following words are reserved for the expression language and must not be used as identifiers:

`and`	`false`	`le`	`not`
`div`	`ge`	`lt`	`null`
`empty`	`gt`	`mod`	`or`
`eq`	`instanceof`	`ne`	`true`

See Also: About the JSP Editor; Adding Components to a Page; About Binding Components to Data; Binding Component Properties; About Pages