Use HttpMessageNotWritableException instead of ISE

As a follow-up to the recent commit #37f9ce, this change replaces the
raised IllegalStateException with HttpMessageNotWritableException.

See gh-23205

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/AbstractMessageWriterResultHandler.java

@@ -33,6 +33,7 @@ import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.Hints;
 import org.springframework.http.MediaType;
 import org.springframework.http.codec.HttpMessageWriter;
+import org.springframework.http.converter.HttpMessageNotWritableException;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
@@ -163,7 +164,7 @@ public abstract class AbstractMessageWriterResultHandler extends HandlerResultHa
 
 		MediaType contentType = exchange.getResponse().getHeaders().getContentType();
 		if (contentType != null && contentType.equals(bestMediaType)) {
-			return Mono.error(new IllegalStateException(
+			return Mono.error(new HttpMessageNotWritableException(
 					"No Encoder for [" + elementType + "] with preset Content-Type '" + contentType + "'"));
 		}
 

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

@@ -51,6 +51,7 @@ import org.springframework.http.codec.HttpMessageWriter;
 import org.springframework.http.codec.ResourceHttpMessageWriter;
 import org.springframework.http.codec.json.Jackson2JsonEncoder;
 import org.springframework.http.codec.xml.Jaxb2XmlEncoder;
+import org.springframework.http.converter.HttpMessageNotWritableException;
 import org.springframework.mock.web.test.server.MockServerWebExchange;
 import org.springframework.util.ObjectUtils;
 import org.springframework.web.reactive.HandlerResult;
@@ -365,7 +366,7 @@ public class ResponseEntityResultHandlerTests {
 
 		StepVerifier.create(resultHandler.handleResult(exchange, result))
 				.consumeErrorWith(ex -> assertThat(ex)
-						.isInstanceOf(IllegalStateException.class)
+						.isInstanceOf(HttpMessageNotWritableException.class)
 						.hasMessageContaining("with preset Content-Type"))
 				.verify();
 	}

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

@@ -176,6 +176,9 @@ public abstract class AbstractMessageConverterMethodProcessor extends AbstractMe
 	 * @throws IOException thrown in case of I/O errors
 	 * @throws HttpMediaTypeNotAcceptableException thrown when the conditions indicated
 	 * by the {@code Accept} header on the request cannot be met by the message converters
+	 * @throws HttpMessageNotWritableException thrown if a given message cannot
+	 * be written by a converter, or if the content-type chosen by the server
+	 * has no compatible converter.
 	 */
 	@SuppressWarnings({"rawtypes", "unchecked"})
 	protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
@@ -306,7 +309,7 @@ public abstract class AbstractMessageConverterMethodProcessor extends AbstractMe
 
 		if (body != null) {
 			if (isContentTypePreset) {
-				throw new IllegalStateException(
+				throw new HttpMessageNotWritableException(
 						"No converter for [" + valueType + "] with preset Content-Type '" + contentType + "'");
 			}
 			throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);

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

@@ -47,6 +47,7 @@ import org.springframework.http.MediaType;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.HttpMessageNotWritableException;
 import org.springframework.mock.web.test.MockHttpServletRequest;
 import org.springframework.mock.web.test.MockHttpServletResponse;
 import org.springframework.web.HttpMediaTypeNotAcceptableException;
@@ -59,7 +60,7 @@ import static java.time.Instant.ofEpochMilli;
 import static java.time.format.DateTimeFormatter.RFC_1123_DATE_TIME;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
-import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyCollection;
 import static org.mockito.ArgumentMatchers.argThat;
@@ -325,10 +326,11 @@ public class HttpEntityMethodProcessorMockTests {
 		given(stringHttpMessageConverter.canWrite(String.class, null)).willReturn(true);
 		given(stringHttpMessageConverter.getSupportedMediaTypes()).willReturn(Collections.singletonList(TEXT_PLAIN));
 
-		assertThatIllegalStateException()
-				.isThrownBy(() ->
-						processor.handleReturnValue(returnValue, returnTypeResponseEntity, mavContainer, webRequest))
-				.withMessageContaining("with preset Content-Type");
+		assertThatThrownBy(() ->
+				processor.handleReturnValue(
+						returnValue, returnTypeResponseEntity, mavContainer, webRequest))
+				.isInstanceOf(HttpMessageNotWritableException.class)
+				.hasMessageContaining("with preset Content-Type");
 	}
 
 	@Test