Improve handling of empty response with Mono<T>

Issue: SPR-17560

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

@@ -119,7 +119,10 @@ public class EncoderHttpMessageWriter<T> implements HttpMessageWriter<T> {
 		if (inputStream instanceof Mono) {
 			HttpHeaders headers = message.getHeaders();
 			return Mono.from(body)
-					.defaultIfEmpty(message.bufferFactory().wrap(new byte[0]))
+					.switchIfEmpty(Mono.defer(() -> {
+						headers.setContentLength(0);
+						return message.setComplete().then(Mono.empty());
+					}))
 					.flatMap(buffer -> {
 						headers.setContentLength(buffer.readableByteCount());
 						return message.writeWith(Mono.just(buffer));

spring-web/src/test/java/org/springframework/http/codec/EncoderHttpMessageWriterTests.java

@@ -34,23 +34,17 @@ import reactor.test.StepVerifier;
 import org.springframework.core.codec.Encoder;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
-import org.springframework.core.io.buffer.support.DataBufferTestUtils;
 import org.springframework.http.MediaType;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpResponse;
 import org.springframework.util.MimeType;
 import org.springframework.util.MimeTypeUtils;
 
-import static java.nio.charset.StandardCharsets.ISO_8859_1;
-import static java.nio.charset.StandardCharsets.UTF_8;
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertTrue;
+import static java.nio.charset.StandardCharsets.*;
+import static org.junit.Assert.*;
 import static org.mockito.ArgumentMatchers.any;
-import static org.mockito.Mockito.when;
-import static org.springframework.core.ResolvableType.forClass;
-import static org.springframework.http.MediaType.TEXT_HTML;
-import static org.springframework.http.MediaType.TEXT_PLAIN;
-import static org.springframework.http.MediaType.TEXT_XML;
+import static org.mockito.Mockito.*;
+import static org.springframework.core.ResolvableType.*;
+import static org.springframework.http.MediaType.*;
 
 /**
  * Unit tests for {@link EncoderHttpMessageWriter}.
@@ -174,7 +168,7 @@ public class EncoderHttpMessageWriterTests {
 	public void emptyBodyWritten() {
 		HttpMessageWriter<String> writer = getWriter(MimeTypeUtils.TEXT_PLAIN);
 		writer.write(Mono.empty(), forClass(String.class), TEXT_PLAIN, this.response, NO_HINTS).block();
-		StepVerifier.create(this.response.getBody()).expectNextCount(1).verifyComplete();
+		StepVerifier.create(this.response.getBody()).expectComplete();
 		assertEquals(0, this.response.getHeaders().getContentLength());
 	}
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java

@@ -149,6 +149,19 @@ public class RequestMappingMessageConversionIntegrationTests extends AbstractReq
 		assertEquals(expected, responseEntity.getBody());
 	}
 
+	@Test // SPR-17506
+	public void personResponseBodyWithEmptyMono() throws Exception {
+		ResponseEntity<Person> responseEntity = performGet("/person-response/mono-empty", JSON, Person.class);
+		assertEquals(0, responseEntity.getHeaders().getContentLength());
+		assertNull(responseEntity.getBody());
+
+		// As we're on the same connection, the 2nd request proves server response handling
+		// did complete after the 1st request..
+		responseEntity = performGet("/person-response/mono-empty", JSON, Person.class);
+		assertEquals(0, responseEntity.getHeaders().getContentLength());
+		assertNull(responseEntity.getBody());
+	}
+
 	@Test
 	public void personResponseBodyWithMonoDeclaredAsObject() throws Exception {
 		Person expected = new Person("Robert");
@@ -495,6 +508,11 @@ public class RequestMappingMessageConversionIntegrationTests extends AbstractReq
 			return Mono.just(new Person("Robert"));
 		}
 
+		@GetMapping("/mono-empty")
+		public Mono<Person> getMonoEmpty() {
+			return Mono.empty();
+		}
+
 		@GetMapping("/mono-declared-as-object")
 		public Object getMonoDeclaredAsObject() {
 			return Mono.just(new Person("Robert"));