/*
 * Copyright 2002-2016 the original author or authors.
 *
 * 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
 *
 *      https://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.springframework.core.io;

import java.net.MalformedURLException;
import java.net.URL;
import java.util.Collection;
import java.util.LinkedHashSet;
import java.util.Set;

import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;

/**
 * Default implementation of the {@link ResourceLoader} interface.
 * Used by {@link ResourceEditor}, and serves as base class for
 * {@link org.springframework.context.support.AbstractApplicationContext}.
 * Can also be used standalone.
 *
 * <p>Will return a {@link UrlResource} if the location value is a URL,
 * and a {@link ClassPathResource} if it is a non-URL path or a
 * "classpath:" pseudo-URL.
 *
 * @author Juergen Hoeller
 * @since 10.03.2004
 * @see FileSystemResourceLoader
 * @see org.springframework.context.support.ClassPathXmlApplicationContext
 */
public class DefaultResourceLoader implements ResourceLoader {

        private ClassLoader classLoader;

        private final Set<ProtocolResolver> protocolResolvers = new LinkedHashSet<ProtocolResolver>(4);


        /**
         * Create a new DefaultResourceLoader.
         * <p>ClassLoader access will happen using the thread context class loader
         * at the time of this ResourceLoader's initialization.
         * @see java.lang.Thread#getContextClassLoader()
         */
        public DefaultResourceLoader() {
                this.classLoader = ClassUtils.getDefaultClassLoader();
        }

        /**
         * Create a new DefaultResourceLoader.
         * @param classLoader the ClassLoader to load class path resources with, or {@code null}
         * for using the thread context class loader at the time of actual resource access
         */
        public DefaultResourceLoader(ClassLoader classLoader) {
                this.classLoader = classLoader;
        }


        /**
         * Specify the ClassLoader to load class path resources with, or {@code null}
         * for using the thread context class loader at the time of actual resource access.
         * <p>The default is that ClassLoader access will happen using the thread context
         * class loader at the time of this ResourceLoader's initialization.
         */
        public void setClassLoader(ClassLoader classLoader) {
                this.classLoader = classLoader;
        }

        /**
         * Return the ClassLoader to load class path resources with.
         * <p>Will get passed to ClassPathResource's constructor for all
         * ClassPathResource objects created by this resource loader.
         * @see ClassPathResource
         */
        @Override
        public ClassLoader getClassLoader() {
                return (this.classLoader != null ? this.classLoader : ClassUtils.getDefaultClassLoader());
        }

        /**
         * Register the given resolver with this resource loader, allowing for
         * additional protocols to be handled.
         * <p>Any such resolver will be invoked ahead of this loader's standard
         * resolution rules. It may therefore also override any default rules.
         * @since 4.3
         * @see #getProtocolResolvers()
         */
        public void addProtocolResolver(ProtocolResolver resolver) {
                Assert.notNull(resolver, "ProtocolResolver must not be null");
                this.protocolResolvers.add(resolver);
        }

        /**
         * Return the collection of currently registered protocol resolvers,
         * allowing for introspection as well as modification.
         * @since 4.3
         */
        public Collection<ProtocolResolver> getProtocolResolvers() {
                return this.protocolResolvers;
        }


        @Override
        public Resource getResource(String location) {
                Assert.notNull(location, "Location must not be null");

                for (ProtocolResolver protocolResolver : this.protocolResolvers) {
                        Resource resource = protocolResolver.resolve(location, this);
                        if (resource != null) {
                                return resource;
                        }
                }

                if (location.startsWith("/")) {
                        return getResourceByPath(location);
                }
                else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
                        return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
                }
                else {
                        try {
                                // Try to parse the location as a URL...
                                URL url = new URL(location);
                                return new UrlResource(url);
                        }
                        catch (MalformedURLException ex) {
                                // No URL -> resolve as resource path.
                                return getResourceByPath(location);
                        }
                }
        }

        /**
         * Return a Resource handle for the resource at the given path.
         * <p>The default implementation supports class path locations. This should
         * be appropriate for standalone implementations but can be overridden,
         * e.g. for implementations targeted at a Servlet container.
         * @param path the path to the resource
         * @return the corresponding Resource handle
         * @see ClassPathResource
         * @see org.springframework.context.support.FileSystemXmlApplicationContext#getResourceByPath
         * @see org.springframework.web.context.support.XmlWebApplicationContext#getResourceByPath
         */
        protected Resource getResourceByPath(String path) {
                return new ClassPathContextResource(path, getClassLoader());
        }


        /**
         * ClassPathResource that explicitly expresses a context-relative path
         * through implementing the ContextResource interface.
         */
        protected static class ClassPathContextResource extends ClassPathResource implements ContextResource {

                public ClassPathContextResource(String path, ClassLoader classLoader) {
                        super(path, classLoader);
                }

                @Override
                public String getPathWithinContext() {
                        return getPath();
                }

                @Override
                public Resource createRelative(String relativePath) {
                        String pathToUse = StringUtils.applyRelativePath(getPath(), relativePath);
                        return new ClassPathContextResource(pathToUse, getClassLoader());
                }
        }

}