Batch SSE events writes when possible

Prior to this commit, the `SseEventBuilder` would be used to create SSE
events and write them to the connection using the `ResponseBodyEmitter`.
This would send each data item one by one, effectively writing and
flushing to the network for each. Since multiple data lines are prepared
by the `SseEventBuilder`, a typical write of an SSE event performs
multiple flushes operations.

This commit adds a method on `ResponseBodyEmitter` to perform batch
writes (given a `Set<DataWithMediaType>`) and only flush once all
elements of the set have been written.
This also applies in case of early writes, where now all buffered
elements are written then flushed altogether.

Fixes gh-30912

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitter.java

@@ -128,9 +128,7 @@ synchronized void initialize(Handler handler) throws IOException {
 		this.handler = handler;
 
 		try {
-			for (DataWithMediaType sendAttempt : this.earlySendAttempts) {
-				sendInternal(sendAttempt.getData(), sendAttempt.getMediaType());
-			}
+			sendInternal(this.earlySendAttempts);
 		}
 		finally {
 			this.earlySendAttempts.clear();
@@ -194,11 +192,7 @@ public void send(Object object) throws IOException {
 	 */
 	public synchronized void send(Object object, @Nullable MediaType mediaType) throws IOException {
 		Assert.state(!this.complete, () -> "ResponseBodyEmitter has already completed" +
-						(this.failure != null ? " with error: " + this.failure : ""));
-		sendInternal(object, mediaType);
-	}
-
-	private void sendInternal(Object object, @Nullable MediaType mediaType) throws IOException {
+				(this.failure != null ? " with error: " + this.failure : ""));
 		if (this.handler != null) {
 			try {
 				this.handler.send(object, mediaType);
@@ -217,6 +211,43 @@ private void sendInternal(Object object, @Nullable MediaType mediaType) throws I
 		}
 	}
 
+	/**
+	 * Write a set of data and MediaType pairs in a batch.
+	 * <p>Compared to {@link #send(Object, MediaType)}, this batches the write operations
+	 * and flushes to the network at the end.
+	 * @param items the object and media type pairs to write
+	 * @throws IOException raised when an I/O error occurs
+	 * @throws java.lang.IllegalStateException wraps any other errors
+	 * @since 6.0.12
+	 */
+	public synchronized void send(Set<DataWithMediaType> items) throws IOException {
+		Assert.state(!this.complete, () -> "ResponseBodyEmitter has already completed" +
+				(this.failure != null ? " with error: " + this.failure : ""));
+		sendInternal(items);
+	}
+
+	private void sendInternal(Set<DataWithMediaType> items) throws IOException {
+		if (items.isEmpty()) {
+			return;
+		}
+		if (this.handler != null) {
+			try {
+				this.handler.send(items);
+			}
+			catch (IOException ex) {
+				this.sendFailed = true;
+				throw ex;
+			}
+			catch (Throwable ex) {
+				this.sendFailed = true;
+				throw new IllegalStateException("Failed to send " + items, ex);
+			}
+		}
+		else {
+			this.earlySendAttempts.addAll(items);
+		}
+	}
+
 	/**
 	 * Complete request processing by performing a dispatch into the servlet
 	 * container, where Spring MVC is invoked once more, and completes the
@@ -302,8 +333,17 @@ public String toString() {
 	 */
 	interface Handler {
 
+		/**
+		 * Immediately write and flush the given data to the network.
+		 */
 		void send(Object data, @Nullable MediaType mediaType) throws IOException;
 
+		/**
+		 * Immediately write all data items then flush to the network.
+		 * @since 6.0.12
+		 */
+		void send(Set<DataWithMediaType> items) throws IOException;
+
 		void complete();
 
 		void completeWithError(Throwable failure);

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitterReturnValueHandler.java

@@ -20,6 +20,7 @@
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Set;
 import java.util.function.Consumer;
 
 import jakarta.servlet.ServletRequest;
@@ -202,14 +203,22 @@ public HttpMessageConvertingHandler(ServerHttpResponse outputMessage, DeferredRe
 		@Override
 		public void send(Object data, @Nullable MediaType mediaType) throws IOException {
 			sendInternal(data, mediaType);
+			this.outputMessage.flush();
+		}
+
+		@Override
+		public void send(Set<ResponseBodyEmitter.DataWithMediaType> items) throws IOException {
+			for (ResponseBodyEmitter.DataWithMediaType item : items) {
+				sendInternal(item.getData(), item.getMediaType());
+			}
+			this.outputMessage.flush();
 		}
 
 		@SuppressWarnings("unchecked")
 		private <T> void sendInternal(T data, @Nullable MediaType mediaType) throws IOException {
 			for (HttpMessageConverter<?> converter : ResponseBodyEmitterReturnValueHandler.this.sseMessageConverters) {
 				if (converter.canWrite(data.getClass(), mediaType)) {
 					((HttpMessageConverter<T>) converter).write(data, mediaType, this.outputMessage);
-					this.outputMessage.flush();
 					return;
 				}
 			}

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/SseEmitter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -123,9 +123,7 @@ public void send(Object object, @Nullable MediaType mediaType) throws IOExceptio
 	public void send(SseEventBuilder builder) throws IOException {
 		Set<DataWithMediaType> dataToSend = builder.build();
 		synchronized (this) {
-			for (DataWithMediaType entry : dataToSend) {
-				super.send(entry.getData(), entry.getMediaType());
-			}
+			super.send(dataToSend);
 		}
 	}
 

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ReactiveTypeHandlerTests.java

@@ -365,6 +365,11 @@ public void send(Object data, MediaType mediaType) throws IOException {
 			this.values.add(data);
 		}
 
+		@Override
+		public void send(Set<ResponseBodyEmitter.DataWithMediaType> items) throws IOException {
+			items.forEach(item -> this.values.add(item.getData()));
+		}
+
 		@Override
 		public void complete() {
 		}

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitterTests.java

@@ -30,9 +30,9 @@
 import static org.assertj.core.api.Assertions.assertThatIOException;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anySet;
 import static org.mockito.BDDMockito.willThrow;
 import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
 
@@ -52,56 +52,54 @@ public class ResponseBodyEmitterTests {
 
 
 	@Test
-	public void sendBeforeHandlerInitialized() throws Exception {
+	void sendBeforeHandlerInitialized() throws Exception {
 		this.emitter.send("foo", MediaType.TEXT_PLAIN);
 		this.emitter.send("bar", MediaType.TEXT_PLAIN);
 		this.emitter.complete();
 		verifyNoMoreInteractions(this.handler);
 
 		this.emitter.initialize(this.handler);
-		verify(this.handler).send("foo", MediaType.TEXT_PLAIN);
-		verify(this.handler).send("bar", MediaType.TEXT_PLAIN);
+		verify(this.handler).send(anySet());
 		verify(this.handler).complete();
 		verifyNoMoreInteractions(this.handler);
 	}
 
 	@Test
-	public void sendDuplicateBeforeHandlerInitialized() throws Exception {
+	void sendDuplicateBeforeHandlerInitialized() throws Exception {
 		this.emitter.send("foo", MediaType.TEXT_PLAIN);
 		this.emitter.send("foo", MediaType.TEXT_PLAIN);
 		this.emitter.complete();
 		verifyNoMoreInteractions(this.handler);
 
 		this.emitter.initialize(this.handler);
-		verify(this.handler, times(2)).send("foo", MediaType.TEXT_PLAIN);
+		verify(this.handler).send(anySet());
 		verify(this.handler).complete();
 		verifyNoMoreInteractions(this.handler);
 	}
 
 	@Test
-	public void sendBeforeHandlerInitializedWithError() throws Exception {
+	void sendBeforeHandlerInitializedWithError() throws Exception {
 		IllegalStateException ex = new IllegalStateException();
 		this.emitter.send("foo", MediaType.TEXT_PLAIN);
 		this.emitter.send("bar", MediaType.TEXT_PLAIN);
 		this.emitter.completeWithError(ex);
 		verifyNoMoreInteractions(this.handler);
 
 		this.emitter.initialize(this.handler);
-		verify(this.handler).send("foo", MediaType.TEXT_PLAIN);
-		verify(this.handler).send("bar", MediaType.TEXT_PLAIN);
+		verify(this.handler).send(anySet());
 		verify(this.handler).completeWithError(ex);
 		verifyNoMoreInteractions(this.handler);
 	}
 
 	@Test
-	public void sendFailsAfterComplete() throws Exception {
+	void sendFailsAfterComplete() throws Exception {
 		this.emitter.complete();
 		assertThatIllegalStateException().isThrownBy(() ->
 				this.emitter.send("foo"));
 	}
 
 	@Test
-	public void sendAfterHandlerInitialized() throws Exception {
+	void sendAfterHandlerInitialized() throws Exception {
 		this.emitter.initialize(this.handler);
 		verify(this.handler).onTimeout(any());
 		verify(this.handler).onError(any());
@@ -119,7 +117,7 @@ public void sendAfterHandlerInitialized() throws Exception {
 	}
 
 	@Test
-	public void sendAfterHandlerInitializedWithError() throws Exception {
+	void sendAfterHandlerInitializedWithError() throws Exception {
 		this.emitter.initialize(this.handler);
 		verify(this.handler).onTimeout(any());
 		verify(this.handler).onError(any());
@@ -138,7 +136,7 @@ public void sendAfterHandlerInitializedWithError() throws Exception {
 	}
 
 	@Test
-	public void sendWithError() throws Exception {
+	void sendWithError() throws Exception {
 		this.emitter.initialize(this.handler);
 		verify(this.handler).onTimeout(any());
 		verify(this.handler).onError(any());
@@ -154,7 +152,7 @@ public void sendWithError() throws Exception {
 	}
 
 	@Test
-	public void onTimeoutBeforeHandlerInitialized() throws Exception  {
+	void onTimeoutBeforeHandlerInitialized() throws Exception {
 		Runnable runnable = mock();
 		this.emitter.onTimeout(runnable);
 		this.emitter.initialize(this.handler);
@@ -169,7 +167,7 @@ public void onTimeoutBeforeHandlerInitialized() throws Exception  {
 	}
 
 	@Test
-	public void onTimeoutAfterHandlerInitialized() throws Exception  {
+	void onTimeoutAfterHandlerInitialized() throws Exception {
 		this.emitter.initialize(this.handler);
 
 		ArgumentCaptor<Runnable> captor = ArgumentCaptor.forClass(Runnable.class);
@@ -185,7 +183,7 @@ public void onTimeoutAfterHandlerInitialized() throws Exception  {
 	}
 
 	@Test
-	public void onCompletionBeforeHandlerInitialized() throws Exception  {
+	void onCompletionBeforeHandlerInitialized() throws Exception {
 		Runnable runnable = mock();
 		this.emitter.onCompletion(runnable);
 		this.emitter.initialize(this.handler);
@@ -200,7 +198,7 @@ public void onCompletionBeforeHandlerInitialized() throws Exception  {
 	}
 
 	@Test
-	public void onCompletionAfterHandlerInitialized() throws Exception  {
+	void onCompletionAfterHandlerInitialized() throws Exception {
 		this.emitter.initialize(this.handler);
 
 		ArgumentCaptor<Runnable> captor = ArgumentCaptor.forClass(Runnable.class);

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/SseEmitterTests.java

@@ -20,12 +20,14 @@
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Set;
 import java.util.function.Consumer;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.MediaType;
+import org.springframework.lang.Nullable;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.springframework.web.servlet.mvc.method.annotation.SseEmitter.event;
@@ -60,6 +62,7 @@ public void send() throws Exception {
 		this.handler.assertObject(0, "data:", TEXT_PLAIN_UTF8);
 		this.handler.assertObject(1, "foo");
 		this.handler.assertObject(2, "\n\n", TEXT_PLAIN_UTF8);
+		this.handler.assertWriteCount(1);
 	}
 
 	@Test
@@ -69,12 +72,14 @@ public void sendWithMediaType() throws Exception {
 		this.handler.assertObject(0, "data:", TEXT_PLAIN_UTF8);
 		this.handler.assertObject(1, "foo", MediaType.TEXT_PLAIN);
 		this.handler.assertObject(2, "\n\n", TEXT_PLAIN_UTF8);
+		this.handler.assertWriteCount(1);
 	}
 
 	@Test
 	public void sendEventEmpty() throws Exception {
 		this.emitter.send(event());
 		this.handler.assertSentObjectCount(0);
+		this.handler.assertWriteCount(0);
 	}
 
 	@Test
@@ -84,6 +89,7 @@ public void sendEventWithDataLine() throws Exception {
 		this.handler.assertObject(0, "data:", TEXT_PLAIN_UTF8);
 		this.handler.assertObject(1, "foo");
 		this.handler.assertObject(2, "\n\n", TEXT_PLAIN_UTF8);
+		this.handler.assertWriteCount(1);
 	}
 
 	@Test
@@ -95,6 +101,7 @@ public void sendEventWithTwoDataLines() throws Exception {
 		this.handler.assertObject(2, "\ndata:", TEXT_PLAIN_UTF8);
 		this.handler.assertObject(3, "bar");
 		this.handler.assertObject(4, "\n\n", TEXT_PLAIN_UTF8);
+		this.handler.assertWriteCount(1);
 	}
 
 	@Test
@@ -104,6 +111,7 @@ public void sendEventFull() throws Exception {
 		this.handler.assertObject(0, ":blah\nevent:test\nretry:5000\nid:1\ndata:", TEXT_PLAIN_UTF8);
 		this.handler.assertObject(1, "foo");
 		this.handler.assertObject(2, "\n\n", TEXT_PLAIN_UTF8);
+		this.handler.assertWriteCount(1);
 	}
 
 	@Test
@@ -115,14 +123,17 @@ public void sendEventFullWithTwoDataLinesInTheMiddle() throws Exception {
 		this.handler.assertObject(2, "\ndata:", TEXT_PLAIN_UTF8);
 		this.handler.assertObject(3, "bar");
 		this.handler.assertObject(4, "\nevent:test\nretry:5000\nid:1\n\n", TEXT_PLAIN_UTF8);
+		this.handler.assertWriteCount(1);
 	}
 
 
 	private static class TestHandler implements ResponseBodyEmitter.Handler {
 
-		private List<Object> objects = new ArrayList<>();
+		private final List<Object> objects = new ArrayList<>();
 
-		private List<MediaType> mediaTypes = new ArrayList<>();
+		private final List<MediaType> mediaTypes = new ArrayList<>();
+
+		private int writeCount;
 
 
 		public void assertSentObjectCount(int size) {
@@ -139,10 +150,24 @@ public void assertObject(int index, Object object, MediaType mediaType) {
 			assertThat(this.mediaTypes.get(index)).isEqualTo(mediaType);
 		}
 
+		public void assertWriteCount(int writeCount) {
+			assertThat(this.writeCount).isEqualTo(writeCount);
+		}
+
 		@Override
-		public void send(Object data, MediaType mediaType) throws IOException {
+		public void send(Object data, @Nullable MediaType mediaType) throws IOException {
 			this.objects.add(data);
 			this.mediaTypes.add(mediaType);
+			this.writeCount++;
+		}
+
+		@Override
+		public void send(Set<ResponseBodyEmitter.DataWithMediaType> items) throws IOException {
+			for (ResponseBodyEmitter.DataWithMediaType item : items) {
+				this.objects.add(item.getData());
+				this.mediaTypes.add(item.getMediaType());
+			}
+			this.writeCount++;
 		}
 
 		@Override