org.jbpm.jpdl.el.ExpressionEvaluator Maven / Gradle / Ivy
/*
* Copyright 2004 The Apache Software Foundation
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.jbpm.jpdl.el;
/**
* 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
*/
public abstract class ExpressionEvaluator {
/**
* 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.
*
* @param expression The expression to be evaluated.
* @param expectedType The expected type of the result of the evaluation
* @param 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().
* @return The Expression object encapsulating the arguments.
*
* @exception ELException Thrown if parsing errors were found.
*/
public abstract Expression parseExpression( String expression,
Class expectedType,
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.
*
* @param expression The expression to be evaluated.
* @param expectedType The expected type of the result of the evaluation
* @param vResolver A VariableResolver instance that can be used at
* runtime to resolve the name of implicit objects into Objects.
* @param fMapper A FunctionMapper to resolve functions found in
* the expression. It can be null, in which case no functions
* are supported for this invocation.
* @return The result of the expression evaluation.
*
* @exception ELException Thrown if the expression evaluation failed.
*/
public abstract Object evaluate( String expression,
Class expectedType,
VariableResolver vResolver,
FunctionMapper fMapper )
throws ELException;
}