base/src/main/java/org/apache/commons/chain2/CatalogFactory.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.commons.chain2;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

import org.apache.commons.chain2.impl.CatalogFactoryBase;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * <p>A {@link CatalogFactory} is a class used to store and retrieve
 * {@link Catalog}s.  The factory allows for a default {@link Catalog}
 * as well as {@link Catalog}s stored with a name key.  Follows the
 * Factory pattern (see GoF).</p>
 *
 * <p>The base <code>CatalogFactory</code> implementation also implements
 * a resolution mechanism which allows lookup of a command based on a single
 * String which encodes both the catalog and command names.</p>
 *
 * @param <K> the type of keys maintained by the context associated with this command
 * @param <V> the type of mapped values
 * @param <C> Type of the context associated with this command
 *
 * @version $Id$
 */
public abstract class CatalogFactory<K, V, C extends Map<K, V>> {

    /**
     * <p>Values passed to the <code>getCommand(String)</code> method should
     * use this as the delimiter between the "catalog" name and the "command"
     * name.</p>
     */
    public static final String DELIMITER = ":";

    // --------------------------------------------------------- Public Methods

    /**
     * <p>Gets the default instance of Catalog associated with the factory
     * (if any); otherwise, return <code>null</code>.</p>
     *
     * @return the default Catalog instance
     */
    public abstract Catalog<K, V, C> getCatalog();

    /**
     * <p>Sets the default instance of Catalog associated with the factory.</p>
     *
     * @param catalog the default Catalog instance
     */
    public abstract void setCatalog(Catalog<K, V, C> catalog);

    /**
     * <p>Retrieves a Catalog instance by name (if any); otherwise
     * return <code>null</code>.</p>
     *
     * @param name the name of the Catalog to retrieve
     * @return the specified Catalog
     */
    public abstract Catalog<K, V, C> getCatalog(String name);

    /**
     * <p>Adds a named instance of Catalog to the factory (for subsequent
     * retrieval later).</p>
     *
     * @param name the name of the Catalog to add
     * @param catalog the Catalog to add
     */
    public abstract void addCatalog(String name, Catalog<K, V, C> catalog);

    /**
     * <p>Return an <code>Iterator</code> over the set of named
     * {@link Catalog}s known to this {@link CatalogFactory}.
     * If there are no known catalogs, an empty Iterator is returned.</p>
     * @return An Iterator of the names of the Catalogs known by this factory.
     */
    public abstract Iterator<String> getNames();

    /**
     * <p>Return a <code>Command</code> based on the given commandID.</p>
     *
     * <p>At this time, the structure of commandID is relatively simple:  if the
     * commandID contains a DELIMITER, treat the segment of the commandID
     * up to (but not including) the DELIMITER as the name of a catalog, and the
     * segment following the DELIMITER as a command name within that catalog.
     * If the commandID contains no DELIMITER, treat the commandID as the name
     * of a command in the default catalog.</p>
     *
     * <p>To preserve the possibility of future extensions to this lookup
     * mechanism, the DELIMITER string should be considered reserved, and
     * should not be used in command names.  commandID values which contain
     * more than one DELIMITER will cause an
     * <code>IllegalArgumentException</code> to be thrown.</p>
     *
     * @param <CMD> the expected {@link Command} type to be returned
     * @param commandID the identifier of the command to return
     * @return the command located with commandID, or <code>null</code>
     *  if either the command name or the catalog name cannot be resolved
     * @throws IllegalArgumentException if the commandID contains more than
     *  one DELIMITER
     *
     * @since Chain 1.1
     */
    public <CMD extends Command<K, V, C>> CMD getCommand(String commandID) {
        String commandName = commandID;
        String catalogName = null;
        Catalog<K, V, C> catalog;

        if (commandID != null) {
            int splitPos = commandID.indexOf(DELIMITER);
            if (splitPos != -1) {
                catalogName = commandID.substring(0, splitPos);
                commandName = commandID.substring(splitPos + DELIMITER.length());
                if (commandName.contains(DELIMITER)) {
                    throw new IllegalArgumentException("commandID [" +
                                                       commandID +
                                                       "] has too many delimiters (reserved for future use)");
                }
            }
        }

        if (catalogName != null) {
            catalog = this.getCatalog(catalogName);
            if (catalog == null) {
                Log log = LogFactory.getLog(CatalogFactory.class);
                log.warn("No catalog found for name: " + catalogName + ".");
                return null;
            }
        } else {
            catalog = this.getCatalog();
            if (catalog == null) {
                Log log = LogFactory.getLog(CatalogFactory.class);
                log.warn("No default catalog found.");
                return null;
            }
        }

        return catalog.<CMD>getCommand(commandName);
    }

    // ------------------------------------------------------- Static Variables

    /**
     * <p>The set of registered {@link CatalogFactory} instances,
     * keyed by the relevant class loader.</p>
     */
    private static final Map<ClassLoader, CatalogFactory<?, ?, ? extends Map<?, ?>>> factories =
            new HashMap<ClassLoader, CatalogFactory<?, ?, ? extends Map<?, ?>>>();

    // -------------------------------------------------------- Static Methods

    /**
     * <p>Return the singleton {@link CatalogFactory} instance
     * for the relevant <code>ClassLoader</code>.  For applications
     * that use a thread context class loader (such as web applications
     * running inside a servet container), this will return a separate
     * instance for each application, even if this class is loaded from
     * a shared parent class loader.</p>
     *
     * @param <K> Context key type
     * @param <V> Context value type
     * @param <C> Type of the context associated with this command
     * @return the per-application singleton instance of {@link CatalogFactory}
     */
    public static <K, V, C extends Map<K, V>> CatalogFactory<K, V, C> getInstance() {
        CatalogFactory<?, ?, ? extends Map<?, ?>> factory;
        ClassLoader cl = getClassLoader();
        synchronized (factories) {
            factory = factories.get(cl);
            if (factory == null) {
                factory = new CatalogFactoryBase<K, V, C>();
                factories.put(cl, factory);
            }
        }

        /* This should always convert cleanly because we are using the
         * base most generic for definition. */
        @SuppressWarnings("unchecked")
        CatalogFactory<K, V, C> result = (CatalogFactory<K, V, C>) factory;

        return result;
    }

    /**
     * <p>Clear all references to registered catalogs, as well as to the
     * relevant class loader.  This method should be called, for example,
     * when a web application utilizing this class is removed from
     * service, to allow for garbage collection.</p>
     */
    public static void clear() {
        synchronized (factories) {
            factories.remove(getClassLoader());
        }
    }

    // ------------------------------------------------------- Private Methods

    /**
     * <p>Return the relevant <code>ClassLoader</code> to use as a Map key
     * for this request.  If there is a thread context class loader, return
     * that; otherwise, return the class loader that loaded this class.</p>
     */
    private static ClassLoader getClassLoader() {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        if (cl == null) {
            cl = CatalogFactory.class.getClassLoader();
        }
        return cl;
    }

    /**
     * Check to see if we have an implementation of a valid configuration
     * parsing class loaded at runtime. If not, we throw an
     * IllegalStateException.
     */
    public static void checkForValidConfigurationModule() {
        try {
            ClassLoader cl = getClassLoader();
            cl.loadClass("org.apache.commons.chain2.config.ConfigParser");
        } catch (ClassNotFoundException e) {
            String msg = "Couldn't not find a configuration implementation. " +
                    "Load a chain configuration module such as xml-configuration " +
                    "into the classpath and try again.";
            throw new IllegalStateException(msg, e);
        }
    }
}