# ResolverFactory.java

    /*
     * Copyright (c) 2018, WSO2 Inc. (http://www.wso2.org) All Rights Reserved.
     *
     * 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.synapse.commons.resolvers;

    import org.apache.commons.logging.Log;
    import org.apache.commons.logging.LogFactory;
    import java.util.ServiceLoader;
    import java.util.HashMap;
    import java.util.Map;
    import java.util.regex.Matcher;
    import java.util.regex.Pattern;

    /**
     *  Resolver Factory which can be used to register resolvers and retrieve a resolver for a given type.
     */
    public class ResolverFactory {

        private static final int RESOLVER_INDEX = 2;
        private static ResolverFactory resolverFactory = new ResolverFactory();
        private final Pattern rePattern = Pattern.compile("(\\$)([a-zA-Z0-9]+):([_a-zA-Z0-9]+)");
        private static final Log log = LogFactory.getLog(ResolverFactory.class);
        private static final String SYSTEM_VARIABLE_PREFIX = "$SYSTEM";

        private Map<String, Class<? extends Resolver>> resolverMap = new HashMap<>();

        /**
         * This function return an object of Resolver factory
         * @return resolverFactory
         */
        public static ResolverFactory getInstance() {
            return resolverFactory;
        }

        private ResolverFactory() {
            registerResolvers();
            registerExterns();
        }

        /**
         * This function returns a resolver based on the variable passed
         * @param input variable which is used decode and retrieve the resolver
         * @return resolver object
         */
        public Resolver getResolver(String input) {

            if (input == null) {
                return null;
            }

            if (input.startsWith(SYSTEM_VARIABLE_PREFIX)) {
                Matcher matcher = rePattern.matcher(input);
                Resolver resolverObject = null;
                if (matcher.find()) {
                    Class<? extends Resolver> resolverClass = resolverMap.get(matcher.group(RESOLVER_INDEX).toLowerCase());
                    if (resolverClass != null) {
                        try {
                            resolverObject = resolverClass.newInstance();
                            resolverObject.setVariable(matcher.group(3));
                            return resolverObject;
                        } catch (IllegalAccessException | InstantiationException e) {
                            throw new ResolverException("Resolver could not be found");
                        }
                    } else {
                        throw new ResolverException("Resolver could not be found");
                    }
                }
            }

            Resolver resolver = new DefaultResolver();
            resolver.setVariable(input);
            return resolver;

        }

        private void registerResolvers() {
            resolverMap.put("system", SystemResolver.class);
        }

        private void registerExterns() {
            ServiceLoader<Resolver> loaders = ServiceLoader.load(Resolver.class);
            for (Resolver resolver : loaders) {
                String className = resolver.getClass().getName();
                String[] packageList = className.split(".");
                className = packageList[packageList.length - 1];
                if (resolverMap.get(className.toLowerCase()) == null) {
                    resolverMap.put(className.toLowerCase(), resolver.getClass());
                    if (log.isDebugEnabled()) {
                        log.debug("Added Resolver " + className + " to resolver factory ");
                    }
                }
                else {
                    if (log.isDebugEnabled()) {
                        log.debug("Failed to Resolver " + className + " to resolver factory. Already exist");
                    }
                }
            }
        }
    }