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Expression Objects
An expression is written according to the rules of grammar defined in the Expression Language specification. So when an Expression Language evaluates and runs expressions, it interprets the commands within them and converts them into the language spoken by the underlying environment. There are two types of expressions, value expressions and method expressions. Value expressions retrieve the value of a variable within a model object or set the value into a variable in a model object. Method expressions invoke a method within a model object. All expression objects extend from the core class Expression-hence, all expressions are serializable. Furthermore the Expression base class gets the reference to the ELContext object, which is used as the context for evaluating the expression.
Creating Expressions
Expressions are created through the ExpressionFactory class. This class provides two methods, one to create ValueExpressions and the other to create MethodExpressions. To create an expression through the ExpressionFactory, you need the following:
- ELContext
- String representing the expression
- The type of expression
Resolving Model Objects
One of the most helpful things about the Expression Language how it can help you resolve complex data types like arrays, vectors, and JavaBeans. To facilitate this, the EL API provides a variety of resolver classes, all of which derive from the ELResolver base class:
- ArrayELResolver: This class manipulates arrays It's used to handle objects that are of type Java arrays. You can construct ArrayELResolver in read-only mode when you're not required to update the array with the calling JSP.
- ListELResolver: This class manipulates Lists.
- BeanELResolver: This class provides access to JavaBeans. It's used to manipulate any JavaBean that conforms to the JavaBean Specification. You can instantiate BeanELResolver in read-only mode to avoid accidentally writing to the JavaBean.
- CompositeELResolver: This class resolves complex objects using a list of child ELResolvers. It also uses ELContext to help keep track of the resolved properties, combining the results once all the individual elements in the complex expression have been resolved.
EL Functions
Often, JSP pages need to perform repetitive actions on data elements before they are rendered to the page (converting to upper case, parsing a string, etc). To avoid repeating these operations every time, the EL API provides a convenient way to define these repetitive actions as a function. All custom functions are mapped with a FunctionMapper and are available to the ELContext. Whenever you need a particular function, you simply invoke the function using the function name.
${fn:formatDate(Hotel.checkinDate)}
The ELContext translates the function to the actual function registered with the FunctionMapper, executes the function, and returns the result to the caller.
EL Variables
Like custom functions, JSP pages sometimes need access to variables that may not be defined in any of the model elements present in the scope of the JSP page. For instance, a counter within a page that counts the number of occurrences of an object or a temporary variable that holds the values extracted from a model element. All EL Variables are mapped in a VariableMapper object, which is accessible to the ELContext. EL Variables do not need be defined as regular Java data types—they can be EL expressions. When an EL variable (that is also an expression) is invoked in the JSP page, ELResolver resolves the expression and returns the value of that expression as the value of the variable.
The Future
As of this release, the EL API is defined as part of the JSP2.1 Specification. Even though references are made to the EL API in the JSP2.1 specification, details of the API are defined as a separate document. Having a separate document gives Sun Microsystems the flexibility to move the EL API into a separate JSR if required in the future. The JSP specification and Java Server Faces specifications will refer to the EL Specification without including them directly into their respective specifications. As the Expression Language specification gains traction in the coming months, more frameworks and technologies may begin to implement them. For now, bundling the EL specification as part of the JSP 2.1 specification gives the Expression Language good exposure and allows it to evolve as an independent specification in the future.
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