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javax.servlet.jsp.el

Class ExpressionEvaluator

public abstract class ExpressionEvaluator
extends java.lang.Object
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:

Since:
2.0

Method Summary

abstract Object
evaluate(String expression, Class expectedType, VariableResolver vResolver, FunctionMapper fMapper)
Evaluates an expression.
abstract Expression
parseExpression(String expression, Class expectedType, FunctionMapper fMapper)
Prepare an expression for later evaluation.

Method Details

evaluate

public abstract 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.
Parameters:
expression - The expression to be evaluated.
expectedType - The expected type of the result of the evaluation
vResolver - A VariableResolver instance that can be used at runtime to resolve the name of implicit objects into Objects.
fMapper - A FunctionMapper to resolve functions found in the expression. It can be null, in which case no functions are supported for this invocation.
Returns:
The result of the expression evaluation.
Throws:
ELException - Thrown if the expression evaluation failed.

parseExpression

public abstract 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.
Parameters:
expression - The expression to be evaluated.
expectedType - The expected type of the result of the evaluation
fMapper - A FunctionMapper to resolve functions found in the expression. It can be null, in which case no functions are supported for this invocation. The ExpressionEvaluator must not hold on to the FunctionMapper reference after returning from parseExpression(). The Expression object returned must invoke the same functions regardless of whether the mappings in the provided FunctionMapper instance change between calling ExpressionEvaluator.parseExpression() and Expression.evaluate().
Returns:
The Expression object encapsulating the arguments.
Throws:
ELException - Thrown if parsing errors were found.

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