/*
 * Copyright 2002-2017 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
 *
 *      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.springframework.core.io;

import java.net.MalformedURLException;
import java.net.URL;
import java.util.Collection;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ResourceUtils;
import org.springframework.util.StringUtils;

Default implementation of the ResourceLoader interface. Used by ResourceEditor, and serves as base class for AbstractApplicationContext. Can also be used standalone. 
Will return a UrlResource if the location value is a URL, and a ClassPathResource if it is a non-URL path or a "classpath:" pseudo-URL. 
Author:
Juergen Hoeller
See Also:
FileSystemResourceLoader
ClassPathXmlApplicationContext
Since:
10.03.2004/**
 * 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 {

	@Nullable
	private ClassLoader classLoader;

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

	private final Map<Class<?>, Map<Resource, ?>> resourceCaches = new ConcurrentHashMap<>(4);


	
Create a new DefaultResourceLoader.
ClassLoader access will happen using the thread context class loader
at the time of this ResourceLoader's initialization.
See Also:
getContextClassLoader.getContextClassLoader()/**
	 * 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.
Params:
classLoader – the ClassLoader to load class path resources with, or null for using the thread context class loader at the time of actual resource access/**
	 * 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(@Nullable ClassLoader classLoader) {
		this.classLoader = classLoader;
	}


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

	
Return the ClassLoader to load class path resources with.
Will get passed to ClassPathResource's constructor for all
ClassPathResource objects created by this resource loader.
See Also:
ClassPathResource/**
	 * 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
	@Nullable
	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.
Any such resolver will be invoked ahead of this loader's standard
resolution rules. It may therefore also override any default rules.
See Also:
getProtocolResolvers()
Since:
4.3/**
	 * 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/**
	 * 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;
	}

	
Obtain a cache for the given value type, keyed by Resource. 
Params:
valueType – the value type, e.g. an ASM MetadataReader
Returns:
the cache Map, shared at the ResourceLoader level
Since:
5.0/**
	 * Obtain a cache for the given value type, keyed by {@link Resource}.
	 * @param valueType the value type, e.g. an ASM {@code MetadataReader}
	 * @return the cache {@link Map}, shared at the {@code ResourceLoader} level
	 * @since 5.0
	 */
	@SuppressWarnings("unchecked")
	public <T> Map<Resource, T> getResourceCache(Class<T> valueType) {
		return (Map<Resource, T>) this.resourceCaches.computeIfAbsent(valueType, key -> new ConcurrentHashMap<>());
	}

	
Clear all resource caches in this resource loader.
See Also:
getResourceCache
Since:
5.0/**
	 * Clear all resource caches in this resource loader.
	 * @since 5.0
	 * @see #getResourceCache
	 */
	public void clearResourceCaches() {
		this.resourceCaches.clear();
	}


	@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 (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : 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.
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.
Params:
path – the path to the resource
See Also:
ClassPathResource
getResourceByPath.getResourceByPath
getResourceByPath.getResourceByPath
Returns:
the corresponding Resource handle/**
	 * 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.
/**
	 * 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, @Nullable 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());
		}
	}

}