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')}
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