java.lang.Object
   javax.servlet.jsp.el.ExpressionEvaluator

The abstract base class for an expression-language evaluator. Classes that implement an expression language expose their functionality via this abstract class.

An instance of the ExpressionEvaluator can be obtained via the JspContext / PageContext

The parseExpression() and evaluate() methods must be thread-safe. That is, multiple threads may call these methods on the same ExpressionEvaluator object simultaneously. Implementations should synchronize access if they depend on transient state. Implementations should not, however, assume that only one object of each ExpressionEvaluator type will be instantiated; global caching should therefore be static.

Only a single EL expression, starting with '${' and ending with '}', can be parsed or evaluated at a time. EL expressions cannot be mixed with static text. For example, attempting to parse or evaluate "abc${1+1}def${1+1}ghi" or even "${1+1}${1+1}" will cause an ELException to be thrown.

The following are examples of syntactically legal EL expressions:

• ${person.lastName}
• ${8 * 8}
• ${my:reverse('hello')}

since: 2.0 -

 abstract public Object evaluate(String expression,
    Class expectedType,
    VariableResolver vResolver,
    FunctionMapper fMapper) throws ELException
Evaluates an expression. This method may perform some syntactic
validation and, if so, it should raise an ELParseException error if
it encounters syntactic errors. EL evaluation errors should cause
an ELException to be raised.

 abstract public Expression parseExpression(String expression,
    Class expectedType,
    FunctionMapper fMapper) throws ELException
Prepare an expression for later evaluation. This method should perform
syntactic validation of the expression; if in doing so it detects
errors, it should raise an ELParseException.