ExpressionHandler.java

/*
 * Copyright 2007 Wilfred Springer
 *
 * 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 com.agilejava.docbkx.maven;

import java.lang.reflect.Method;

import javax.servlet.jsp.el.ELException;
import javax.servlet.jsp.el.ExpressionEvaluator;
import javax.servlet.jsp.el.FunctionMapper;
import javax.servlet.jsp.el.VariableResolver;

import org.apache.commons.el.ExpressionEvaluatorImpl;
import org.apache.maven.plugin.logging.Log;
import org.xml.sax.ContentHandler;
import org.xml.sax.SAXException;

/**
 * A {@link ProcessingInstructionHandler} that evaluates expressions passed as
 * data as JSP expression language expressions.
 * 
 * 
 * @author Wilfred Springer
 * 
 */
public class ExpressionHandler implements ProcessingInstructionHandler {

	/**
	 * The name of the processing instruction.
	 */
	private final String PI_NAME = "eval";

	/**
	 * The object responsible for resolving variables.
	 */
	private VariableResolver resolver;

	/**
	 * The object used for logging.
	 */
	private Log log;

	/**
	 * A simplified function mapper that basically does not support any
	 * functions at all.
	 */
	private FunctionMapper mapper = new FunctionMapper() {
		public Method resolveFunction(String prefix, String localName) {
			return null;
		}
	};

	/**
	 * Constructs a new instance.
	 * 
	 * @param resolver
	 *            The object used for resolving variables. (Not
	 *            <code>null</code>.)
	 * @param log
	 *            The object used for logging. (Not <code>null</code>.)
	 */
	public ExpressionHandler(VariableResolver resolver, Log log) {
		this.resolver = resolver;
		this.log = log;
	}

	// JavaDoc inherited
	public void handle(String data, ContentHandler handler) {
		ExpressionEvaluator evaluator = new ExpressionEvaluatorImpl();
		Object value;
		try {
			value = evaluator.evaluate(data, Object.class, resolver, mapper);
			if (value != null) {
				char[] result = value.toString().toCharArray();
				handler.characters(result, 0, result.length);
			} else {
				log.debug("Failed to resolve " + data);
			}
		} catch (ELException ele) {
			log.error("Failed to handle EL expression.", ele);
		} catch (SAXException saxe) {
			log.error("Failed to generate content.", saxe);
		}
	}

	// JavaDoc inherited
	public boolean matches(String target) {
		return PI_NAME.matches(target);
	}

}