Value and Method Expressions - The Java EE 6 Tutorial (original) (raw)

The EL defines two kinds of expressions: value expressions and method expressions. Value expressions can either yield a value or set a value. Method expressions reference methods that can be invoked and can return a value.

Value Expressions

Value expressions can be further categorized into rvalue and lvalue expressions. Rvalue expressions can read data but cannot write it. Lvalue expressions can both read and write data.

All expressions that are evaluated immediately use the ${} delimiters and are always rvalue expressions. Expressions whose evaluation can be deferred use the #{} delimiters and can act as both rvalue and lvalue expressions. Consider the following two value expressions:

${customer.name}

#{customer.name}

The former uses immediate evaluation syntax, whereas the latter uses deferred evaluation syntax. The first expression accesses the name property, gets its value, adds the value to the response, and gets rendered on the page. The same can happen with the second expression. However, the tag handler can defer the evaluation of this expression to a later time in the page lifecycle, if the technology using this tag allows.

In the case of JavaServer Faces technology, the latter tag’s expression is evaluated immediately during an initial request for the page. In this case, this expression acts as an rvalue expression. During a postback request, this expression can be used to set the value of the name property with user input. In this case, the expression acts as an lvalue expression.

Referencing Objects Using Value Expressions

Both rvalue and lvalue expressions can refer to the following objects and their properties or attributes:

JavaBeans components
Collections
Java SE enumerated types
Implicit objects

To refer to these objects, you write an expression using a variable that is the name of the object. The following expression references a managed bean called customer:

${customer}

The web container evaluates the variable that appears in an expression by looking up its value according to the behavior of PageContext.findAttribute(String), where the Stringargument is the name of the variable. For example, when evaluating the expression${customer}, the container will look for customer in the page, request, session, and application scopes and will return its value. If customer is not found, a null value is returned.

You can use a custom EL resolver to alter the way variables are resolved. For instance, you can provide an EL resolver that intercepts objects with the name customer, so that ${customer} returns a value in the EL resolver instead.

To reference an enum constant with an expression, use a String literal. For example, consider this Enum class:

public enum Suit {hearts, spades, diamonds, clubs}

To refer to the Suit constant Suit.hearts with an expression, use the Stringliteral "hearts". Depending on the context, the String literal is converted to the enumconstant automatically. For example, in the following expression in which mySuit is an instance of Suit, "hearts" is first converted to Suit.hearts before it is compared to the instance:

${mySuit == "hearts"}

Referring to Object Properties Using Value Expressions

To refer to properties of a bean or an enum instance, items of a collection, or attributes of an implicit object, you use the .or [] notation.

To reference the name property of the customer bean, use either the expression${customer.name} or the expression ${customer["name"]}. The part inside the brackets is aString literal that is the name of the property to reference.

You can use double or single quotes for the String literal. You can also combine the [] and . notations, as shown here:

${customer.address["street"]}

Properties of an enum constant can also be referenced in this way. However, as with JavaBeans component properties, the properties of an Enum class must follow JavaBeans component conventions. This means that a property must at least have an accessor method called get_Property_, where Property is the name of the property that can be referenced by an expression.

For example, consider an Enum class that encapsulates the names of the planets of our galaxy and includes a method to get the mass of a planet. You can use the following expression to reference the method getMassof the Enum class Planet:

${myPlanet.mass}

If you are accessing an item in an array or list, you must use either a literal value that can be converted to int or the [] notation with an int and without quotes. The following examples could resolve to the same item in a list or array, assuming that sockscan be converted to int:

${customer.orders[1]}
${customer.orders.socks}

In contrast, an item in a Map can be accessed using a string literal key; no coercion is required:

${customer.orders["socks"]}

An rvalue expression also refers directly to values that are not objects, such as the result of arithmetic operations and literal values, as shown by these examples:

${"literal"}
${customer.age + 20}
${true}
${57}

The EL 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

You can also write expressions that perform operations on an enum constant. For example, consider the following Enum class:

public enum Suit {club, diamond, heart, spade}

After declaring an enum constant called mySuit, you can write the following expression to test whether mySuit is spade:

${mySuit == "spade"}

When it resolves this expression, the EL resolving mechanism will invoke the valueOfmethod of the Enum class with the Suit class and the spade type, as shown here:

mySuit.valueOf(Suit.class, "spade"}

Where Value Expressions Can Be Used

Value expressions using the ${} delimiters can be used in

Static text
Any standard or custom tag attribute that can accept an expression

The value of an expression in static text is computed and inserted into the current output. Here is an example of an expression embedded in static text:

some:tag some text ${expr} some text

If the static text appears in a tag body, note that an expression will not be evaluated if the body is declared to be tagdependent.

Lvalue expressions can be used only in tag attributes that can accept lvalue expressions.

A tag attribute value using either an rvalue or lvalue expression can be set in the following ways:

With a single expression construct:
<some:tag value="${expr}"/>
<another:tag value="#{expr}"/>
These expressions are evaluated, and the result is converted to the attribute’s expected type.
With one or more expressions separated or surrounded by text:
<some:tag value="some${expr}${expr}text${expr}"/>
<another:tag value="some#{expr}#{expr}text#{expr}"/>
These kinds of expression, called composite expressions, are evaluated from left to right. Each expression embedded in the composite expression is converted to a String and then concatenated with any intervening text. The resulting String is then converted to the attribute’s expected type.
With text only:
<some:tag value="sometext"/>
This expression is called a literal expression. In this case, the attribute’s String value is converted to the attribute’s expected type. Literal value expressions have special syntax rules. See Literal Expressions for more information. When a tag attribute has an enum type, the expression that the attribute uses must be a literal expression. For example, the tag attribute can use the expression "hearts" to mean Suit.hearts. The literal is converted to Suit, and the attribute gets the value Suit.hearts.

All expressions used to set attribute values are evaluated in the context of an expected type. If the result of the expression evaluation does not match the expected type exactly, a type conversion will be performed. For example, the expression ${1.2E4} provided as the value of an attribute of type float will result in the following conversion:

Float.valueOf("1.2E4").floatValue()

See Section 1.18 of the JavaServer Pages 2.2 Expression Language specification (available fromhttp://jcp.org/aboutJava/communityprocess/final/jsr245/) for the complete type conversion rules.

Method Expressions

Another feature of the EL is its support of deferred method expressions. A method expression is used to invoke an arbitrary public method of a bean, which can return a result.

In JavaServer Faces technology, a component tag represents a component on a page. The component tag uses method expressions to invoke methods that perform some processing for the component. These methods are necessary for handling events that the components generate and for validating component data, as shown in this example:

<h:form> <h:inputText id="name" value="#{customer.name}" validator="#{customer.validateName}"/> <h:commandButton id="submit" action="#{customer.submit}" />

The h:inputText tag displays as a text field. The validator attribute of this h:inputText tag references a method, called validateName, in the bean, called customer.

Because a method can be invoked during different phases of the lifecycle, method expressions must always use the deferred evaluation syntax.

Like lvalue expressions, method expressions can use the . and the [] operators. For example, #{object.method} is equivalent to #{object["method"]}. The literal inside the [] is converted to String and is used to find the name of the method that matches it. Once the method is found, it is invoked, or information about the method is returned.

Method expressions can be used only in tag attributes and only in the following ways:

With a single expression construct, where bean refers to a JavaBeans component and method refers to a method of the JavaBeans component:
<some:tag value="#{_bean_._method_}"/>
The expression is evaluated to a method expression, which is passed to the tag handler. The method represented by the method expression can then be invoked later.
With text only:
<some:tag value="sometext"/>
Method expressions support literals primarily to support action attributes in JavaServer Faces technology. When the method referenced by this method expression is invoked, the method returns the String literal, which is then converted to the expected return type, as defined in the tag’s tag library descriptor.

Parameterized Method Calls

The EL offers support for parameterized method calls. Method calls can use parameters without having to use static EL functions.

Both the . and [] operators can be used for invoking method calls with parameters, as shown in the following expression syntax:

_expr-a_[_expr-b_](_parameters_)
expr-a.identifier-b(parameters)

In the first expression syntax, expr-a is evaluated to represent a bean object. The expression expr-b is evaluated and cast to a string that represents a method in the bean represented by expr-a. In the second expression syntax, _expr-a_is evaluated to represent a bean object, and identifier-b is a string that represents a method in the bean object. The parameters in parentheses are the arguments for the method invocation. Parameters can be zero or more values or expressions, separated by commas.

Parameters are supported for both value expressions and method expressions. In the following example, which is a modified tag from the guessnumber application, a random number is provided as an argument rather than from user input to the method call:

<h:inputText value="#{userNumberBean.userNumber('5')}">

The preceding example uses a value expression.

Consider the following example of a JavaServer Faces component tag that uses a method expression:

<h:commandButton action="#{trader.buy}" value="buy"/>

The EL expression trader.buy calls the trader bean’s buy method. You can modify the tag to pass on a parameter. Here is the revised tag where a parameter is passed:

<h:commandButton action="#{trader.buy('SOMESTOCK')}" value="buy"/>

In the preceding example, you are passing the string 'SOMESTOCK' (a stock symbol) as a parameter to the buy method.

For more information on the updated EL, see http://uel.java.net/.