EL Resolvers (The Java EE 5 Tutorial)

The Java EE 5 Tutorial

EL Resolvers

At the center of the EL machinery is the extensible ELResolver class. A class that implements ELResolver defines how to resolve expressions referring to a particular type of object or property. In terms of the following expression, a BeanELResolver instance is called the first time to find the base object, employee, which is a JavaBeans component. Once the resolver finds the object, it is called again to resolve the property, lName of the employee object.

${employee.lName}

The unified EL includes a set of standard resolver implementations. Table 5–4 lists these standard resolvers and includes example expressions that they can resolve.

Table 5–4 Standard EL Resolvers


Resolver	Example Expression	Description
`ArrayELResolver`	`${myArray[1]}`	Returns the value at index 1 in the array called `myArray`
`BeanELResolver`	`${employee.lName}`	Returns the value of the `lName` property of the `employee` bean
`ListELResolver`	`${myList[5]}`	Returns the value at index 5 of `myList` list
`MapELResolver`	`${myMap.someKey}`	Returns the value stored at the key, `someKey`, in the `Map`, `myMap`
`ResourceBundleELResolver`	`${myRB.myKey}`	Returns the message at `myKey` in the resource bundle called `myRB`

Depending on the technology using the unified EL, other resolvers might be available. In addition, application developers can add their own implementations of ELResolver to support resolution of expressions not already supported by the unified EL by registering them with an application.

All of the standard and custom resolvers available to a particular application are collected in a chain in a particular order. This chain of resolvers is represented by a CompositeELResolver instance. When an expression is encountered, the CompositeELResolver instance iterates over the list of resolvers and consults each resolver until it finds one that can handle the expression.

If an application is using JSP technology, the chain of resolvers includes the ImplicitObjectELResolver and the ScopedAttributeELResolver. These are described in the following section.

See section JSP 2.9 of the JavaServer Pages 2.1 specification to find out the order in which resolvers are chained together in a CompositeELResolver instance.

To learn how to create a custom EL resolver, see The Unified Expression Language .