Introduce PartEvent

This commit introduces the PartEvent API. PartEvents are either
- FormPartEvents, representing a form field, or
- FilePartEvents, representing a file upload.

The PartEventHttpMessageReader is a HttpMessageReader that splits
multipart data into a stream of PartEvents. Form fields generate one
FormPartEvent; file uploads produce at least one FilePartEvent. The last
element that makes up a particular part will have isLast set to true.

The PartEventHttpMessageWriter is a HttpMessageWriter that writes a
Publisher<PartEvent> to a outgoing HTTP message. This writer is
particularly useful for relaying a multipart request on the server.

Closes spring-projectsgh-28006

spring-web/src/main/java/org/springframework/http/codec/multipart/DefaultPartEvents.java

@@ -0,0 +1,176 @@
+/*
+ * Copyright 2002-2022 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.http.codec.multipart;
+
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DefaultDataBufferFactory;
+import org.springframework.http.HttpHeaders;
+import org.springframework.util.Assert;
+
+/**
+ * Default implementations of {@link PartEvent} and subtypes.
+ *
+ * @author Arjen Poutsma
+ * @since 6.0
+ */
+abstract class DefaultPartEvents {
+
+	public static FormPartEvent form(HttpHeaders headers) {
+		Assert.notNull(headers, "Headers must not be null");
+		return new DefaultFormFieldPartEvent(headers);
+	}
+
+	public static FormPartEvent form(HttpHeaders headers, String value) {
+		Assert.notNull(headers, "Headers must not be null");
+		Assert.notNull(value, "Value must not be null");
+		return new DefaultFormFieldPartEvent(headers, value);
+	}
+
+	public static FilePartEvent file(HttpHeaders headers, DataBuffer dataBuffer, boolean isLast) {
+		Assert.notNull(headers, "Headers must not be null");
+		Assert.notNull(dataBuffer, "DataBuffer must not be null");
+		return new DefaultFilePartEvent(headers, dataBuffer, isLast);
+	}
+
+	public static FilePartEvent file(HttpHeaders headers) {
+		Assert.notNull(headers, "Headers must not be null");
+		return new DefaultFilePartEvent(headers);
+	}
+
+	public static PartEvent create(HttpHeaders headers, DataBuffer dataBuffer, boolean isLast) {
+		Assert.notNull(headers, "Headers must not be null");
+		Assert.notNull(dataBuffer, "DataBuffer must not be null");
+		if (headers.getContentDisposition().getFilename() != null) {
+			return file(headers, dataBuffer, isLast);
+		}
+		else {
+			return new DefaultPartEvent(headers, dataBuffer, isLast);
+		}
+	}
+
+	public static PartEvent create(HttpHeaders headers) {
+		Assert.notNull(headers, "Headers must not be null");
+		if (headers.getContentDisposition().getFilename() != null) {
+			return file(headers);
+		}
+		else {
+			return new DefaultPartEvent(headers);
+		}
+	}
+
+
+
+	private static abstract class AbstractPartEvent implements PartEvent {
+
+		private final HttpHeaders headers;
+
+
+		protected AbstractPartEvent(HttpHeaders headers) {
+			this.headers = HttpHeaders.readOnlyHttpHeaders(headers);
+		}
+
+		@Override
+		public HttpHeaders headers() {
+			return this.headers;
+		}
+	}
+
+
+	/**
+	 * Default implementation of {@link PartEvent}.
+	 */
+	private static class DefaultPartEvent extends AbstractPartEvent {
+
+		private static final DataBuffer EMPTY = DefaultDataBufferFactory.sharedInstance.allocateBuffer(0);
+
+		private final DataBuffer content;
+
+		private final boolean last;
+
+
+		public DefaultPartEvent(HttpHeaders headers) {
+			this(headers, EMPTY, true);
+		}
+
+		public DefaultPartEvent(HttpHeaders headers, DataBuffer content, boolean last) {
+			super(headers);
+			this.content = content;
+			this.last = last;
+		}
+
+		@Override
+		public DataBuffer content() {
+			return this.content;
+		}
+
+		@Override
+		public boolean isLast() {
+			return this.last;
+		}
+
+	}
+
+	/**
+	 * Default implementation of {@link FormPartEvent}.
+	 */
+	private static final class DefaultFormFieldPartEvent extends AbstractPartEvent implements FormPartEvent {
+
+		private static final String EMPTY = "";
+
+		private final String value;
+
+
+		public DefaultFormFieldPartEvent(HttpHeaders headers) {
+			this(headers, EMPTY);
+		}
+
+		public DefaultFormFieldPartEvent(HttpHeaders headers, String value) {
+			super(headers);
+			this.value = value;
+		}
+
+		@Override
+		public String value() {
+			return this.value;
+		}
+
+		@Override
+		public DataBuffer content() {
+			byte[] bytes = this.value.getBytes(MultipartUtils.charset(headers()));
+			return DefaultDataBufferFactory.sharedInstance.wrap(bytes);
+		}
+
+		@Override
+		public boolean isLast() {
+			return true;
+		}
+	}
+
+	/**
+	 * Default implementation of {@link FilePartEvent}.
+	 */
+	private static class DefaultFilePartEvent extends DefaultPartEvent implements FilePartEvent {
+
+		public DefaultFilePartEvent(HttpHeaders headers) {
+			super(headers);
+		}
+
+		public DefaultFilePartEvent(HttpHeaders headers, DataBuffer content, boolean last) {
+			super(headers, content, last);
+		}
+	}
+}

spring-web/src/main/java/org/springframework/http/codec/multipart/DefaultPartHttpMessageReader.java

@@ -32,7 +32,6 @@
 import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.DecodingException;
 import org.springframework.core.io.buffer.DataBufferLimitException;
-import org.springframework.http.HttpMessage;
 import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpInputMessage;
 import org.springframework.http.codec.HttpMessageReader;
@@ -218,7 +217,7 @@ public Mono<Part> readMono(ResolvableType elementType, ReactiveHttpInputMessage
 	@Override
 	public Flux<Part> read(ResolvableType elementType, ReactiveHttpInputMessage message, Map<String, Object> hints) {
 		return Flux.defer(() -> {
-			byte[] boundary = boundary(message);
+			byte[] boundary = MultipartUtils.boundary(message, this.headersCharset);
 			if (boundary == null) {
 				return Flux.error(new DecodingException("No multipart boundary found in Content-Type: \"" +
 						message.getHeaders().getContentType() + "\""));
@@ -231,20 +230,4 @@ public Flux<Part> read(ResolvableType elementType, ReactiveHttpInputMessage mess
 		});
 	}
 
-	@Nullable
-	private byte[] boundary(HttpMessage message) {
-		MediaType contentType = message.getHeaders().getContentType();
-		if (contentType != null) {
-			String boundary = contentType.getParameter("boundary");
-			if (boundary != null) {
-				int len = boundary.length();
-				if (len > 2 && boundary.charAt(0) == '"' && boundary.charAt(len - 1) == '"') {
-					boundary = boundary.substring(1, len - 1);
-				}
-				return boundary.getBytes(this.headersCharset);
-			}
-		}
-		return null;
-	}
-
 }