    001    /* $Id: FinderFromClass.java 729125 2008-12-23 21:21:07Z rahul $
    002     *
    003     * Licensed to the Apache Software Foundation (ASF) under one or more
    004     * contributor license agreements.  See the NOTICE file distributed with
    005     * this work for additional information regarding copyright ownership.
    006     * The ASF licenses this file to You under the Apache License, Version 2.0
    007     * (the "License"); you may not use this file except in compliance with
    008     * the License.  You may obtain a copy of the License at
    009     *
    010     *      http://www.apache.org/licenses/LICENSE-2.0
    011     *
    012     * Unless required by applicable law or agreed to in writing, software
    013     * distributed under the License is distributed on an "AS IS" BASIS,
    014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    015     * See the License for the specific language governing permissions and
    016     * limitations under the License.
    017     */
    018
    019    package org.apache.commons.digester.plugins.strategies;
    020
    021    import java.util.Properties;
    022    import org.apache.commons.digester.Digester;
    023    import org.apache.commons.digester.plugins.RuleFinder;
    024    import org.apache.commons.digester.plugins.RuleLoader;
    025    import org.apache.commons.digester.plugins.PluginException;
    026
    027    /**
    028     * A rule-finding algorithm which expects the caller to specify a classname and
    029     * methodname as plugin properties.
    030     *
    031     * @since 1.6
    032     */
    033
    034    public class FinderFromClass extends RuleFinder {
    035        public static String DFLT_RULECLASS_ATTR = "ruleclass";
    036        public static String DFLT_METHOD_ATTR = "method";
    037        public static String DFLT_METHOD_NAME = "addRules";
    038
    039        private String ruleClassAttr;
    040        private String methodAttr;
    041        private String dfltMethodName;
    042
    043        /**
    044         * See {@link #findLoader}.
    045         */
    046        public FinderFromClass() {
    047            this(DFLT_RULECLASS_ATTR, DFLT_METHOD_ATTR, DFLT_METHOD_NAME);
    048        }
    049
    050        /**
    051         * Create a rule-finder which invokes a user-specified method on a
    052         * user-specified class whenever dynamic rules for a plugin need to be
    053         * loaded. See the findRules method for more info.
    054         *
    055         * @param ruleClassAttr must be non-null.
    056         * @param methodAttr may be null.
    057         * @param dfltMethodName may be null.
    058         */
    059        public FinderFromClass(String ruleClassAttr, String methodAttr,
    060                    String dfltMethodName) {
    061            this.ruleClassAttr = ruleClassAttr;
    062            this.methodAttr = methodAttr;
    063            this.dfltMethodName = dfltMethodName;
    064        }
    065
    066        /**
    067         * If there exists a property with the name matching constructor param
    068         * ruleClassAttr, then load the specified class, locate the appropriate
    069         * rules-adding method on that class, and return an object encapsulating
    070         * that info.
    071         * <p>
    072         * If there is no matching property provided, then just return null.
    073         * <p>
    074         * The returned object (when non-null) will invoke the target method
    075         * on the selected class whenever its addRules method is invoked. The
    076         * target method is expected to have the following prototype:
    077         * <code> public static void xxxxx(Digester d, String patternPrefix); </code>
    078         * <p>
    079         * The target method can be specified in several ways. If this object's
    080         * constructor was passed a non-null methodAttr parameter, and the
    081         * properties defines a value with that key, then that is taken as the
    082         * target method name. If there is no matching property, or the constructor
    083         * was passed null for methodAttr, then the dfltMethodName passed to the
    084         * constructor is used as the name of the method on the target class. And
    085         * if that was null, then DFLT_METHOD_NAME will be used.
    086         * <p>
    087         * When the user explicitly declares a plugin in the input xml, the
    088         * xml attributes on the declaration tag are passed here as properties,
    089         * so the user can select any class in the classpath (and any method on
    090         * that class provided it has the correct prototype) as the source of
    091         * dynamic rules for the plugged-in class.
    092         */
    093        public RuleLoader findLoader(Digester digester, Class<?> pluginClass,
    094                            Properties p) throws PluginException {
    095
    096            String ruleClassName = p.getProperty(ruleClassAttr);
    097            if (ruleClassName == null) {
    098                // nope, user hasn't requested dynamic rules to be loaded
    099                // from a specific class.
    100                return null;
    101            }
    102
    103            // ok, we are in business
    104            String methodName = null;
    105            if (methodAttr != null) {
    106                methodName = p.getProperty(methodAttr);
    107            }
    108            if (methodName == null) {
    109                methodName = dfltMethodName;
    110            }
    111            if (methodName == null) {
    112                methodName = DFLT_METHOD_NAME;
    113            }
    114
    115            Class<?> ruleClass;
    116            try {
    117                // load the plugin class object
    118                ruleClass =
    119                    digester.getClassLoader().loadClass(ruleClassName);
    120            } catch(ClassNotFoundException cnfe) {
    121                throw new PluginException(
    122                    "Unable to load class " + ruleClassName, cnfe);
    123            }
    124
    125            return new LoaderFromClass(ruleClass, methodName);
    126        }
    127    }
    128