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Spring Web MVC
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/*
 * Copyright 2002-2019 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.web.servlet.function;

import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.net.URI;
import java.net.URL;
import java.util.EnumSet;
import java.util.Set;

import org.springframework.core.io.Resource;
import org.springframework.http.HttpMethod;
import org.springframework.http.HttpStatus;
import org.springframework.lang.Nullable;

/**
 * Resource-based implementation of {@link HandlerFunction}.
 *
 * @author Arjen Poutsma
 * @since 5.2
 */
class ResourceHandlerFunction implements HandlerFunction {

	private static final Set SUPPORTED_METHODS =
			EnumSet.of(HttpMethod.GET, HttpMethod.HEAD, HttpMethod.OPTIONS);


	private final Resource resource;


	public ResourceHandlerFunction(Resource resource) {
		this.resource = resource;
	}


	@Override
	public ServerResponse handle(ServerRequest request) {
		HttpMethod method = request.method();
		if (method != null) {
			switch (method) {
				case GET:
					return EntityResponse.fromObject(this.resource).build();
				case HEAD:
					Resource headResource = new HeadMethodResource(this.resource);
					return EntityResponse.fromObject(headResource).build();
				case OPTIONS:
					return ServerResponse.ok()
							.allow(SUPPORTED_METHODS).build();
			}
		}
		return ServerResponse.status(HttpStatus.METHOD_NOT_ALLOWED)
				.allow(SUPPORTED_METHODS).build();
	}


	private static class HeadMethodResource implements Resource {

		private static final byte[] EMPTY = new byte[0];

		private final Resource delegate;

		public HeadMethodResource(Resource delegate) {
			this.delegate = delegate;
		}

		@Override
		public InputStream getInputStream() throws IOException {
			return new ByteArrayInputStream(EMPTY);
		}

		// delegation

		@Override
		public boolean exists() {
			return this.delegate.exists();
		}

		@Override
		public URL getURL() throws IOException {
			return this.delegate.getURL();
		}

		@Override
		public URI getURI() throws IOException {
			return this.delegate.getURI();
		}

		@Override
		public File getFile() throws IOException {
			return this.delegate.getFile();
		}

		@Override
		public long contentLength() throws IOException {
			return this.delegate.contentLength();
		}

		@Override
		public long lastModified() throws IOException {
			return this.delegate.lastModified();
		}

		@Override
		public Resource createRelative(String relativePath) throws IOException {
			return this.delegate.createRelative(relativePath);
		}

		@Override
		@Nullable
		public String getFilename() {
			return this.delegate.getFilename();
		}

		@Override
		public String getDescription() {
			return this.delegate.getDescription();
		}
	}

}