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 a String 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 getProperty, 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 getMass of 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 socks can 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 valueOf method of the Enum class with the Suit class and the spade type, as shown here:

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