Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Value and Method Expressions

The EL defines two kinds of expressions: value expressions and method expressions. Value expressions can be evaluated to yield a value, and method expressions are used to reference a method.

The following topics are addressed here:

- Value Expressions

- Method Expressions

- Lambda Expressions

### Value Expressions

Value expressions can be further categorized into rvalue and lvalue expressions. An lvalue expression can specify a target, such as an object, a bean property, or elements of a collection, that can be assigned a value. An rvalue expression cannot specify such a target.

All expressions that are evaluated immediately use the `${}` delimiters, and although the expression can be an lvalue expression, no assignments will ever happen. Expressions whose evaluation can be deferred use the `#{}` delimiters and can act as both rvalue and lvalue expressions; if the expression is an lvalue expression, it can be assigned a new value. 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, and passes the value to the tag handler. With the second expression, the tag handler can defer the expression evaluation 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. During a postback request, this expression can be used to set the value of the `name` property with user input.

#### Referencing Objects

A top-level identifier (such as `customer` in the expression `customer.name`) can refer to the following objects:

- Lambda parameters

- EL variables

- Managed beans

- Implicit objects

- Classes of static fields and methods

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}

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. (JavaServer Faces technology uses an EL resolver to handle managed beans.)

An `enum` constant is a special case of a static field, and you can reference such a constant directly. For example, consider this `enum` class:

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

In the following expression, in which `mySuit` is an instance of `Suit`, you can compare `suit.hearts` to the instance:

    ${mySuit == suit.hearts}

#### Referencing Object Properties or Collection Elements

To refer to properties of a bean, static fields or methods of a class, or items of a collection, you use the `.` or `[]` notation. The same syntax can be used for attributes of an implicit object, because attributes are placed in a map.

To reference the `name` property of the `customer` bean, use either the expression `${customer.name}` or the expression `${customer["name"]}`. Here, the part inside the brackets is a `String` literal that is the name of the property to reference. The `[]` syntax is more general than the `.` syntax, because the part inside the brackets can be any `String` expression, not just literals.

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

    ${customer.address["street"]}

You can reference a static field or method using the syntax classname.field, as in the following example:

    Boolean.FALSE

The classname is the name of the class without the package name. By default, all the `java.lang` packages are imported. You can import other packages, classes, and static fields as needed.

If you are accessing an item in an array or list, you must use the `[]` notation and specify an index in the array or list. The index is an expression that can be converted to `int`. The following example references the first of the customer orders, assuming that `customer.orders` is a `List`:

    ${customer.orders[1]}

If you are accessing an item in a `Map`, you must specify the key for the `Map`. If the key is a `String` literal, the dot (`.)` notation can be used. Assuming that `customer.orders` is a `Map` with a `String` key, the following examples reference the item with the key `"socks"`:

    ${customer.orders["socks"]}

    ${customer.orders.socks}

#### Referencing Literals

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`

Here are some examples:

- `${"literal"}`

- `${true}`

- `${57}`

#### Parameterized Method Calls

The EL offers support for parameterized method calls.

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 of 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 in which 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.

#### Where Value Expressions Can Be Used

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

- In static text

- In 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
    </some:tag>

A tag attribute 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"/>

  The attribute’s `String` value is converted to the attribute’s expected type.

You can use the string concatenation operator += to create a single expression from what would otherwise be a composite expression. For example, you could change the composite expression

    <some:tag value="sometext ${expr} moretext"/>

to

    <some:tag value="${sometext += expr += moretext}"/>

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()

### Method Expressions

Another feature of the EL is its support of deferred method expressions. A method expression is used to refer to a public method of a bean and has the same syntax as an lvalue expression.

In JavaServer Faces technology, a component tag represents a component on a page. The component tag uses method expressions to specify methods that can be invoked to 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}" />
    </h:form>

The `h:inputText` tag displays as a 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.

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.

### Lambda Expressions

A lambda expression is a value expression with parameters. The syntax is similar to that of the lambda expression in the Java programming language, except that in the EL, the body of the lambda expression is an EL expression.

For basic information on lambda expressions, see `http://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html`.

Note: Lambda expressions are part of Java SE 8, but you can use them in EL expressions with Java SE 7, the Java version associated with the Java EE 7 platform.

A lambda expression uses the arrow token (`→)` operator. The identifiers to the left of the operator are called lambda parameters. The body, to the right of the operator, must be an EL expression. The lambda parameters are enclosed in parentheses; the parentheses can be omitted if there is only one parameter. Here are some examples:

    x -> x+1
    (x, y) -> x + y
    () -> 64

A lambda expression behaves like a function. It can be invoked immediately. For example, the following invocation evaluates to 7:

    ((x, y) -> x + y)(3, 4)

You can use a lambda expression in conjunction with the assignment and semicolon operators. For example, the following code assigns the previous lambda expression to a variable and then invokes it. The result is again 7:

    v = (x, y) -> x + y; v(3, 4)

A lambda expression can also be passed as an argument to a method and be invoked in the method. It can also be nested in another lambda expression.

Copyright © 2017, Oracle and/or its affiliates. All rights reserved.