ProtocolResolver SPI allows for custom resource resolution

Issue: SPR-12857
Issue: SPR-13905

spring-core/src/main/java/org/springframework/core/io/DefaultResourceLoader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * 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.
@@ -18,6 +18,9 @@ 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;
@@ -42,6 +45,8 @@ public class DefaultResourceLoader implements ResourceLoader {
 
 	private ClassLoader classLoader;
 
+	private final Set<ProtocolResolver> protocolResolvers = new LinkedHashSet<ProtocolResolver>(4);
+
 
 	/**
 	 * Create a new DefaultResourceLoader.
@@ -84,10 +89,40 @@ public class DefaultResourceLoader implements ResourceLoader {
 		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);
 		}

spring-core/src/main/java/org/springframework/core/io/ProtocolResolver.java

@@ -0,0 +1,42 @@
+/*
+ * 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
+ *
+ *      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;
+
+/**
+ * A resolution strategy for protocol-specific resource handles.
+ *
+ * <p>Used as an SPI for {@link DefaultResourceLoader}, allowing for
+ * custom protocols to be handled without subclassing the loader
+ * implementation (or application context implementation).
+ *
+ * @author Juergen Hoeller
+ * @since 4.3
+ * @see DefaultResourceLoader#addProtocolResolver
+ */
+public interface ProtocolResolver {
+
+	/**
+	 * Resolve the given location against the given resource loader
+	 * if this implementation's protocol matches.
+	 * @param location the user-specified resource location
+	 * @param resourceLoader the associated resource loader
+	 * @return a corresponding {@code Resource} handle if the given location
+	 * matches this resolver's protocol, or {@code null} otherwise
+	 */
+	Resource resolve(String location, ResourceLoader resourceLoader);
+
+}