/*
* Copyright 2002,2004 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.apache.commons.jexl;
import java.io.StringReader;
import org.apache.commons.jexl.parser.ASTExpressionExpression;
import org.apache.commons.jexl.parser.ASTReferenceExpression;
import org.apache.commons.jexl.parser.Parser;
import org.apache.commons.jexl.parser.SimpleNode;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
/**
* <p>
* Creates Expression objects. To create a JEXL Expression object, pass
* valid JEXL syntax to the static createExpression() method:
* </p>
*
* <pre>
* String jexl = "array[1]";
* Expression expression = ExpressionFactory.createExpression( jexl );
* </pre>
*
* <p>
* When an {@link Expression} object is created, the JEXL syntax is
* parsed and verified. If the supplied expression is neither an
* expression not a reference, an exception is thrown from createException().
* </p>
* @author <a href="mailto:geirm@apache.org">Geir Magnusson Jr.</a>
* @version $Id: ExpressionFactory.java,v 1.7 2004/08/23 13:50:00 dion Exp $
*/
public class ExpressionFactory
{
/**
* The Log to which all ExpressionFactory messages will be logged.
*/
protected static Log log =
LogFactory.getLog("org.apache.commons.jexl.ExpressionFactory");
/**
* The singleton ExpressionFactory also holds a single instance of {@link Parser}.
* When parsing expressions, ExpressionFactory synchronizes on Parser.
*/
protected static Parser parser = new Parser(new StringReader(";")); //$NON-NLS-1$
/**
* ExpressionFactory is a single and this is the private
* status instance fufilling that pattern.
*/
protected static ExpressionFactory ef = new ExpressionFactory();
/**
* Private constructor, the single instance is always obtained
* with a call to getInstance().
*/
private ExpressionFactory(){}
/**
* Returns the single instance of ExpressionFactory.
* @return the instance of ExpressionFactory.
*/
protected static ExpressionFactory getInstance()
{
return ef;
}
/**
* Creates an Expression from a String containing valid
* JEXL syntax. This method parses the expression which
* must contain either a reference or an expression.
* @param expression A String containing valid JEXL syntax
* @return An Expression object which can be evaluated with a JexlContext
* @throws Exception An exception can be thrown if there is a problem parsing
* his expression, or if the expression is neither an
* expression or a reference.
*/
public static Expression createExpression(String expression)
throws Exception
{
return getInstance().createNewExpression(expression);
}
/**
* Creates a new Expression based on the expression string.
*
* @param expression valid Jexl expression
* @return Expression
* @throws Exception for a variety of reasons - mostly malformed
* Jexl expression
*/
protected Expression createNewExpression(String expression)
throws Exception {
String expr = cleanExpression(expression);
// Parse the Expression
SimpleNode tree;
synchronized(parser)
{
log.debug( "Parsing expression: " + expr );
tree = parser.parse(new StringReader(expr));
}
// Must be a simple reference or expression, otherwise
// throw an exception.
SimpleNode node = (SimpleNode) tree.jjtGetChild(0);
if( (node instanceof ASTReferenceExpression) ||
(node instanceof ASTExpressionExpression) )
{
node = (SimpleNode) node.jjtGetChild(0);
Expression e = new ExpressionImpl(expression, node);
return e;
}
else
{
log.error( "Invalid Expression, node of type: " + node.getClass().getName() );
throw new Exception("Invalid Expression: neither Reference nor Expression");
}
}
/**
* Trims the expression and adds a semi-colon if missing.
* @param expression to clean
* @return trimmed expression ending in a semi-colon
*/
private String cleanExpression(String expression) {
String expr = expression.trim();
if (!expr.endsWith(";"))
{
expr += ";";
}
return expr;
}
}