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Features
Here are some of the features that make EL a powerful tool:
- Simple syntax and better readability.
- Support for scoped variables.
- Support for relational, logical, arithmetic, and conditional operators.
- Support for functions through static methods in a Java class.
- Automatic type conversions when data types are convertible.
- A pluggable API for resolving the variable references to their corresponding java objects.
- Ability to defer evaluation of expressions if they refer to methods or variables of an object.
Expressions
The most important element of the Expression language is an expression. An expression can be defined as a set of Java commands or operations you can combine to form a logical unit. The evaluation of the expression leads to a value, which is used by the EL API for further processing or rendering on the screen. EL expressions can be defined either as an eval-expression, a literal-expression, or a composite expression.
- Eval Expressions: Eval-expressions are created using the constructs ${expression} and #{expression}. Both these constructs perform the same operations in EL—the difference lies in where they are used. This distinction is decided upon by an application's underlying technology. In Web-tier applications, the ${expression} construct is used for immediate evaluation of the expression and #{expression} means the expression evaluation will be deferred.
Eval-expressions are evaluated as either value or method expressions. Value expressions return a value after the expression is evaluated. The returned value can be assigned to a variable in EL or rendered to the screen. Method expressions point to some method in a Java class. When the method expression is evaluated, the EL API verifies if the method conforms to the method signature defined by the method expression and then invokes the method.
- Literal Expressions: Literal-expressions evaluate the expression to the string representation of the expression. They literally convert expressions into strings and display them to the screen.
- Composite Expressions: These expressions group multiple EL expressions into one expression and evaluate them according to the rules of the EL API. They can consist of multiple eval-expressions along with multiple literal-expressions. All the eval-expressions are evaluated from left to right, coerced to strings, and then combined with the literal-expressions to get the final value.
The Expression Language API
The Expression Language API is defined as part of the jaxax.el package. Though this language is extensively used in JSPs, it was included as a separate package in order to provide the flexibility to evolve independent of the needs of JSP developers. The EL specification is presently shared by the JSP2.1 and the JSF 1.2 technologies.
This API contains classes and interfaces for interacting with the Expression Language engine. These classes can be logically divided to support the following functions:
- Expression Language Context (ELContext)
- Expression Objects
- Expression Creation
- Object Resolution
- EL Functions
- EL Variables
Expression Language Context
The ELContext provides access to the environment in which you wish to use the Expression language. It provides the required support to interact with the underlying technologies and converts the expressions to a format which the underlying environment understands. In the case of JSP, the factory method JspContext.getELContext is used to get access to the underlying JSP environment.
Having a generic type of control through an ELContext class allows the EL specification to support other technologies and environments. The EL API supports two commonly used technologies, JSP and Java Server Faces, through the JSPContext and FacesContext methods.
The ELContext performs an important function for evaluating any wxpression defined in the client application—as per the rules defined in the EL Specification. To do so, ELContext holds references to the ELResolver, FunctionMapper, and VariableMapper. At times, multiple resolvers like ArrayELResolver, BeanELResolver, etc. might be required for evaluating expressions in an application. To support multiple resolvers, the ELContext is able to hold a list of resolvers used in the application. The ELContext also stores state information about an expression when it is evaluated. The state information may be the status of a property and/or whether the appropriate resolver has resolved the property already. Since the ELContext stores the state during expression evaluation, it is not thread-safe and hence should be used by different threads.
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