/* Expressions.java{{IS_NOTE	Purpose:			Description:			History:		Thu Aug 30 11:09:33     2007, Created by tomyeh}}IS_NOTECopyright (C) 2007 Potix Corporation. All Rights Reserved.{{IS_RIGHT	This program is distributed under GPL Version 2.0 in the hope that	it will be useful, but WITHOUT ANY WARRANTY.}}IS_RIGHT*/package org.zkoss.xel;import java.util.Collection;import java.util.Collections;import org.zkoss.lang.Classes;import org.zkoss.util.logging.Log;/** * Utilities to use XEL. * * @author tomyeh * @since 3.0.0 */public class Expressions {	private static final Log log = Log.lookup(Expressions.class);	/** An empty function mapper, i.e., it has no function defined at all.	 * It is serializable.	 */	public static final FunctionMapper EMPTY_MAPPER = new EmptyMapper();	/** An empty variable resolver, i.e., it has no variable defined at all.	 * It is serializable.	 */	public static final VariableResolver EMPTY_RESOLVER = new EmptyResolver();	/** An dummy expression that does nothing.	 * It is usually used as a flag to denote exceptional cases.	 * It it serializable.	 */	public static final Expression DUMMY_EXPRESSION = new DummyExpr();	/** The implemetation of {@link ExpressionFactory}. */	private static Class _expfcls;	/** Instantiates an instance of {@link ExpressionFactory}.	 *	 * <p>The default class is {@link org.zkoss.xel.el.ELFactory}.	 * To override it, you can specify the class by calling	 * {@link #setExpressionFactoryClass}.	 *	 * <p>For the ZK user, you can override it with zk.xml	 * or org.zkoss.zk.ui.util.Configuration.	 *	 * @exception XelException if the specified class failed to load,	 * or instantiate.	 */	public static final ExpressionFactory newExpressionFactory()	throws XelException {		return newExpressionFactory(_expfcls);	}	/** Instantiates an instance of {@link ExpressionFactory}.	 *	 * <p>The default class is {@link org.zkoss.xel.el.ELFactory}.	 * To override it, you can specify the class by calling	 * {@link #setExpressionFactoryClass}.	 *	 * <p>For the ZK user, you can override it with zk.xml	 * or org.zkoss.zk.ui.util.Configuration.	 *	 * @param expfcls the class that implements {@link ExpressionFactory},	 * or null to use the default.	 * @exception XelException if the specified class failed to load,	 * or instantiate.	 */	public static final ExpressionFactory newExpressionFactory(Class expfcls) {		if (expfcls != null) {			try {				return (ExpressionFactory)expfcls.newInstance();			} catch (Throwable ex) {				throw XelException.Aide.wrap(ex, "Unable to instantiate "+expfcls);			}		}		return newDefautFactory();	}	private static final ExpressionFactory newDefautFactory() {		return new org.zkoss.xel.el.ELFactory();	}	/** Evaluates an expression.	 *	 * @param ctx the context information to evaluate the expression     * It can be null, in which case no functions are supported for this     * invocation.	 * @param expression the expression to be evaluated.	 * Note: the expression is enclosed	 * with ${ and }, regardingless what implemetnation is used.	 * @param expectedType the expected type of the result of the evaluation	 */	public static final Object evaluate(XelContext ctx,	String expression, Class expectedType)	throws XelException {		return newExpressionFactory().evaluate(ctx, expression, expectedType);	}	/** Sets the implementation of the expression factory that shall	 * be used by the whole system, or null to use the system default.	 *	 * <p>Default: null - it means {@link org.zkoss.xel.el.ELFactory}.	 *	 * <p>Note: you can only specify an implementation that is compatible	 * with JSP EL here, since all builtin pages depend on it.	 *	 * @param expfcls the implemtation class, or null to use the default.	 * Note: expfcls must implement {@link ExpressionFactory}.	 * If null, the system default is used.	 */	public static final void setExpressionFactoryClass(Class expfcls) {		if (expfcls != null && !ExpressionFactory.class.isAssignableFrom(expfcls))			throw new IllegalArgumentException(expfcls+" must implement "+ExpressionFactory.class);		_expfcls = expfcls;	}	/** Returns the implementation of the expression factory that	 * is used by the whole system, or null to use the system default.	 *	 * @see #setExpressionFactoryClass	 */	public static final Class getExpressionFactoryClass() {		return _expfcls;	}}/*package*/ class EmptyMapperimplements FunctionMapper, java.io.Serializable {	//-- FunctionMapper --//	public Function resolveFunction(String prefix, String name) {		return null;	}	public Collection getClassNames() {		return Collections.EMPTY_LIST;	}	public Class resolveClass(String name) {		return null;	}}/*package*/ class EmptyResolverimplements VariableResolver, java.io.Serializable {	public Object resolveVariable(String name) {		return null;	}}/*package*/ class DummyExpr implements Expression, java.io.Serializable {	public Object evaluate(XelContext ctx) throws XelException {		return null;	}};