/* $Id: FinderFromClass.java,v 1.5 2004/05/10 06:34:01 skitching Exp $
 *
 * Copyright 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.digester.plugins.strategies;

import java.util.Properties;
import org.apache.commons.digester.Digester;
import org.apache.commons.digester.plugins.RuleFinder;
import org.apache.commons.digester.plugins.RuleLoader;
import org.apache.commons.digester.plugins.PluginException;

/**
 * A rule-finding algorithm which expects the caller to specify a classname and
 * methodname as plugin properties.
 *
 * @since 1.6
 */

public class FinderFromClass extends RuleFinder {
    public static String DFLT_RULECLASS_ATTR = "ruleclass";
    public static String DFLT_METHOD_ATTR = "method";
    public static String DFLT_METHOD_NAME = "addRules";
    
    private String ruleClassAttr;
    private String methodAttr;
    private String dfltMethodName;
    
    /**
     * See {@link #findLoader}.
     */
    public FinderFromClass() {
        this(DFLT_RULECLASS_ATTR, DFLT_METHOD_ATTR, DFLT_METHOD_NAME);
    }

    /**
     * Create a rule-finder which invokes a user-specified method on a
     * user-specified class whenever dynamic rules for a plugin need to be
     * loaded. See the findRules method for more info.
     *
     * @param ruleClassAttr must be non-null.
     * @param methodAttr may be null.
     * @param dfltMethodName may be null.
     */
    public FinderFromClass(String ruleClassAttr, String methodAttr, 
                String dfltMethodName) {
        this.ruleClassAttr = ruleClassAttr;
        this.methodAttr = methodAttr;
        this.dfltMethodName = dfltMethodName;
    }
    
    /**
     * If there exists a property with the name matching constructor param
     * ruleClassAttr, then load the specified class, locate the appropriate 
     * rules-adding method on that class, and return an object encapsulating 
     * that info.
     * <p>
     * If there is no matching property provided, then just return null.
     * <p>
     * The returned object (when non-null) will invoke the target method
     * on the selected class whenever its addRules method is invoked. The
     * target method is expected to have the following prototype:
     * <code> public static void xxxxx(Digester d, String patternPrefix); </code>
     * <p>
     * The target method can be specified in several ways. If this object's
     * constructor was passed a non-null methodAttr parameter, and the
     * properties defines a value with that key, then that is taken as the
     * target method name. If there is no matching property, or the constructor
     * was passed null for methodAttr, then the dfltMethodName passed to the
     * constructor is used as the name of the method on the target class. And
     * if that was null, then DFLT_METHOD_NAME will be used.
     * <p>
     * When the user explicitly declares a plugin in the input xml, the
     * xml attributes on the declaration tag are passed here as properties,
     * so the user can select any class in the classpath (and any method on
     * that class provided it has the correct prototype) as the source of
     * dynamic rules for the plugged-in class.
     */
    public RuleLoader findLoader(Digester digester, Class pluginClass, 
                        Properties p) throws PluginException {

        String ruleClassName = p.getProperty(ruleClassAttr);
        if (ruleClassName == null) {
            // nope, user hasn't requested dynamic rules to be loaded
            // from a specific class.
            return null;
        }
        
        // ok, we are in business
        String methodName = null;
        if (methodAttr != null) { 
            methodName = p.getProperty(methodAttr);
        }
        if (methodName == null) {
            methodName = dfltMethodName;
        }
        if (methodName == null) {
            methodName = DFLT_METHOD_NAME;
        }
        
        Class ruleClass;
        try {
            // load the plugin class object
            ruleClass = 
                digester.getClassLoader().loadClass(ruleClassName);
        } catch(ClassNotFoundException cnfe) {
            throw new PluginException(
                "Unable to load class " + ruleClassName, cnfe);
        }

        return new LoaderFromClass(ruleClass, methodName);
    }
}