Add MultipartBodyBuilder

This commit introduces the MultipartBodyBuilder, a builder for multipart
form bodies.

Issue: SPR-16134

spring-web/src/main/java/org/springframework/http/client/MultipartBodyBuilder.java

@@ -0,0 +1,162 @@
+/*
+ * 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.http.client;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+
+import org.springframework.http.HttpEntity;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.MediaType;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.MultiValueMap;
+
+/**
+ * A mutable builder for multipart form bodies. For example:
+ * <pre class="code">
+ *
+ * MultipartBodyBuilder builder = new MultipartBodyBuilder();
+ * MultiValueMap&lt;String, String&gt; form = new LinkedMultiValueMap&lt;&gt;();
+ * form.add("form field", "form value");
+ * builder.part("form", form).header("Foo", "Bar");
+ *
+ * Resource image = new ClassPathResource("image.jpg");
+ * builder.part("image", image).header("Baz", "Qux");
+ *
+ * MultiValueMap<String, HttpEntity<?>> multipartBody = builder.build();
+ * // use multipartBody with RestTemplate or WebClient
+ * </pre>
+
+ * @author Arjen Poutsma
+ * @since 5.0.2
+ * @see <a href="https://tools.ietf.org/html/rfc7578">RFC 7578</a>
+ */
+public final class MultipartBodyBuilder {
+
+	private final LinkedMultiValueMap<String, DefaultPartBuilder> parts = new LinkedMultiValueMap<>();
+
+
+	/**
+	 * Creates a new, empty instance of the {@code MultipartBodyBuilder}.
+	 */
+	public MultipartBodyBuilder() {
+	}
+
+
+	/**
+	 * Builds the multipart body.
+	 * @return the built body
+	 */
+	public MultiValueMap<String, HttpEntity<?>> build() {
+		MultiValueMap<String, HttpEntity<?>> result = new LinkedMultiValueMap<>(this.parts.size());
+		for (Map.Entry<String, List<DefaultPartBuilder>> entry : this.parts.entrySet()) {
+			for (DefaultPartBuilder builder : entry.getValue()) {
+				HttpEntity<?> entity = builder.build();
+				result.add(entry.getKey(), entity);
+			}
+		}
+		return result;
+	}
+
+	/**
+	 * Adds a part to this builder, allowing for further header customization with the returned
+	 * {@link PartBuilder}.
+	 * @param name the name of the part to add (may not be empty)
+	 * @param part the part to add
+	 * @return a builder that allows for further header customization
+	 */
+	public PartBuilder part(String name, Object part) {
+		return part(name, part, null);
+	}
+
+	/**
+	 * Adds a part to this builder, allowing for further header customization with the returned
+	 * {@link PartBuilder}.
+	 * @param name the name of the part to add (may not be empty)
+	 * @param part the part to add
+	 * @param contentType the {@code Content-Type} header for the part (may be {@code null})
+	 * @return a builder that allows for further header customization
+	 */
+	public PartBuilder part(String name, Object part, @Nullable MediaType contentType) {
+		Assert.hasLength(name, "'name' must not be empty");
+		Assert.notNull(part, "'part' must not be null");
+
+		Object partBody;
+		HttpHeaders partHeaders = new HttpHeaders();
+
+		if (part instanceof HttpEntity) {
+			HttpEntity<?> other = (HttpEntity<?>) part;
+			partBody = other.getBody();
+			partHeaders.addAll(other.getHeaders());
+		}
+		else {
+			partBody = part;
+		}
+
+		if (contentType != null) {
+			partHeaders.setContentType(contentType);
+		}
+		DefaultPartBuilder builder = new DefaultPartBuilder(partBody, partHeaders);
+		this.parts.add(name, builder);
+		return builder;
+	}
+
+
+	/**
+	 * Builder interface that allows for customization of part headers.
+	 */
+	public interface PartBuilder {
+
+		/**
+		 * Add the given part-specific header values under the given name.
+		 * @param headerName the part header name
+		 * @param headerValues the part header value(s)
+		 * @return this builder
+		 * @see HttpHeaders#add(String, String)
+		 */
+		PartBuilder header(String headerName, String... headerValues);
+	}
+
+
+	private static class DefaultPartBuilder implements PartBuilder {
+
+		@Nullable
+		private final Object body;
+
+		private final HttpHeaders headers;
+
+
+		public DefaultPartBuilder(@Nullable Object body, HttpHeaders headers) {
+			this.body = body;
+			this.headers = headers;
+		}
+
+		@Override
+		public PartBuilder header(String headerName, String... headerValues) {
+			this.headers.addAll(headerName, Arrays.asList(headerValues));
+			return this;
+		}
+
+		public HttpEntity<?> build() {
+			return new HttpEntity<>(this.body, this.headers);
+		}
+	}
+
+}