taglibs-rdc/src/main/java/org/apache/taglibs/rdc/core/ExpandTag.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.taglibs.rdc.core;

import java.io.IOException;
import java.io.StringWriter;
import java.text.MessageFormat;

import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspTagException;
import javax.servlet.jsp.tagext.JspFragment;
import javax.servlet.jsp.tagext.SimpleTagSupport;
import javax.servlet.jsp.el.VariableResolver;
import org.apache.commons.el.ExpressionEvaluatorImpl;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * <p>Implements the tag 
 * &lt;rdc:expand&gt;Hello from #{model.name}&lt;/rdc:expand&gt;.
 * This tag can be used to evaluate text containing expressions
 * that use the custom syntax "#{...}". It is useful in
 * authoring prompt templates  that refer to run-time values.</p>
 * 
 * @author <a href="mailto:tvraman@almaden.ibm.com">T. V. Raman</a>
 * @author Rahul Akolkar
 * @version 1.0
 */

public class ExpandTag
    extends SimpleTagSupport {
    
    // Error messages (to be i18n'zed)
    private static final String ERR_NO_BODY = "'rdc:expand' used without " +
        "a body";
    private static final String ERR_BAD_EL_EXPR = "<!-- Error evaluating " +
        " EL expression: \"{0}\" -->\n";
    
    // Logging
    private static Log log = LogFactory.getLog(ExpandTag.class);
    
    /* 
     * Rahul - 9/13/04
     * Do not use getJspContext().getExpressionEvaluator() --
     * which offers no guarantee whether multiple expressions
     * occuring in String "expression" will be evaluated
     */    
    private static final ExpressionEvaluatorImpl exprEvaluator = 
        new ExpressionEvaluatorImpl();
    
    /**
     * Captures the result of invoking body,
     * Replaces occurrences of #{ with ${,
     * and then evaluates the result using el.
     * The result of el evaluation is written to the JSP output stream.
     * in case of an el evaluation  error,
     * we produce appropriate error messages into the JSP output stream.
     * @exception java.io.IOException
     * @exception JspException
     * @exception JspTagException   
     */
    public void doTag()
        throws IOException, JspException, JspTagException   {
        JspFragment body = getJspBody();
        if (body == null) {
            throw new JspTagException(ERR_NO_BODY);
        }
        StringWriter bodyExpansion = new StringWriter();
        body.invoke(bodyExpansion);
        String expansion = bodyExpansion.getBuffer().toString();
        String expression = expansion.replaceAll("#\\{", "\\$\\{");
        
        PageContext pageContext = (PageContext) getJspContext();        
        VariableResolver varResolver = getJspContext().getVariableResolver();
        JspWriter out = pageContext.getOut();
        try {
            // Rahul - 9/13/04 - EL functions are not supported.
            // Since this is in the body of a <vxml:prompt> element, the 
            // result needs to be a String
            String result = (String) exprEvaluator.evaluate(expression, 
                java.lang.String.class, varResolver, null);
            out.write(result);           
        } catch (javax.servlet.jsp.el.ELException e) {
            MessageFormat msgFormat = new MessageFormat(ERR_BAD_EL_EXPR);
            String errMsg = msgFormat.format(new Object[] {expression});
            // Log error and send a comment to client
            log.error(errMsg);
            out.write(errMsg);
        } // end of try-catch
    }
}