Add body conversion capabilities in RestClient::exchange

This commit introduces a ConvertibleClientHttpResponse type that
extends ClientHttpResponse, and that can convert the body to a desired
type. Before this commit, it was not easy to use the configured HTTP
message converters in combination with RestClient::exchange.

Closes gh-31597

spring-web/src/main/java/org/springframework/web/client/DefaultRestClient.java

@@ -17,6 +17,8 @@
 package org.springframework.web.client;
 
 import java.io.IOException;
+import java.io.InputStream;
+import java.io.UncheckedIOException;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.net.URI;
@@ -185,6 +187,61 @@ final class DefaultRestClient implements RestClient {
 		return new DefaultRestClientBuilder(this.builder);
 	}
 
+	@SuppressWarnings({"rawtypes", "unchecked"})
+	private <T> T readWithMessageConverters(ClientHttpResponse clientResponse, Runnable callback, Type bodyType, Class<T> bodyClass) {
+		MediaType contentType = getContentType(clientResponse);
+
+		try (clientResponse) {
+			callback.run();
+
+			for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
+				if (messageConverter instanceof GenericHttpMessageConverter genericHttpMessageConverter) {
+					if (genericHttpMessageConverter.canRead(bodyType, null, contentType)) {
+						if (logger.isDebugEnabled()) {
+							logger.debug("Reading to [" + ResolvableType.forType(bodyType) + "]");
+						}
+						return (T) genericHttpMessageConverter.read(bodyType, null, clientResponse);
+					}
+				}
+				if (messageConverter.canRead(bodyClass, contentType)) {
+					if (logger.isDebugEnabled()) {
+						logger.debug("Reading to [" + bodyClass.getName() + "] as \"" + contentType + "\"");
+					}
+					return (T) messageConverter.read((Class)bodyClass, clientResponse);
+				}
+			}
+			throw new UnknownContentTypeException(bodyType, contentType,
+					clientResponse.getStatusCode(), clientResponse.getStatusText(),
+					clientResponse.getHeaders(), RestClientUtils.getBody(clientResponse));
+		}
+		catch (UncheckedIOException | IOException | HttpMessageNotReadableException ex) {
+			throw new RestClientException("Error while extracting response for type [" +
+					ResolvableType.forType(bodyType) + "] and content type [" + contentType + "]", ex);
+		}
+	}
+
+	private static MediaType getContentType(ClientHttpResponse clientResponse) {
+		MediaType contentType = clientResponse.getHeaders().getContentType();
+		if (contentType == null) {
+			contentType = MediaType.APPLICATION_OCTET_STREAM;
+		}
+		return contentType;
+	}
+
+	@SuppressWarnings("unchecked")
+	private static <T> Class<T> bodyClass(Type type) {
+		if (type instanceof Class<?> clazz) {
+			return (Class<T>) clazz;
+		}
+		if (type instanceof ParameterizedType parameterizedType &&
+				parameterizedType.getRawType() instanceof Class<?> rawType) {
+			return (Class<T>) rawType;
+		}
+		return (Class<T>) Object.class;
+	}
+
+
+
 
 	private class DefaultRequestBodyUriSpec implements RequestBodyUriSpec {
 
@@ -409,7 +466,8 @@ final class DefaultRestClient implements RestClient {
 				}
 				clientResponse = clientRequest.execute();
 				observationContext.setResponse(clientResponse);
-				return exchangeFunction.exchange(clientRequest, clientResponse);
+				ConvertibleClientHttpResponse convertibleWrapper = new DefaultConvertibleClientHttpResponse(clientResponse);
+				return exchangeFunction.exchange(clientRequest, convertibleWrapper);
 			}
 			catch (IOException ex) {
 				ResourceAccessException resourceAccessException = createResourceAccessException(uri, this.httpMethod, ex);
@@ -542,14 +600,14 @@ final class DefaultRestClient implements RestClient {
 
 		@Override
 		public <T> T body(Class<T> bodyType) {
-			return readWithMessageConverters(bodyType, bodyType);
+			return readBody(bodyType, bodyType);
 		}
 
 		@Override
 		public <T> T body(ParameterizedTypeReference<T> bodyType) {
 			Type type = bodyType.getType();
 			Class<T> bodyClass = bodyClass(type);
-			return readWithMessageConverters(type, bodyClass);
+			return readBody(type, bodyClass);
 		}
 
 		@Override
@@ -565,7 +623,7 @@ final class DefaultRestClient implements RestClient {
 		}
 
 		private <T> ResponseEntity<T> toEntityInternal(Type bodyType, Class<T> bodyClass) {
-			T body = readWithMessageConverters(bodyType, bodyClass);
+			T body = readBody(bodyType, bodyClass);
 			try {
 				return ResponseEntity.status(this.clientResponse.getStatusCode())
 						.headers(this.clientResponse.getHeaders())
@@ -579,77 +637,96 @@ final class DefaultRestClient implements RestClient {
 		@Override
 		public ResponseEntity<Void> toBodilessEntity() {
 			try (this.clientResponse) {
-				applyStatusHandlers(this.clientRequest, this.clientResponse);
+				applyStatusHandlers();
 				return ResponseEntity.status(this.clientResponse.getStatusCode())
 						.headers(this.clientResponse.getHeaders())
 						.build();
 			}
+			catch (UncheckedIOException ex) {
+				throw new ResourceAccessException("Could not retrieve response status code: " + ex.getMessage(), ex.getCause());
+			}
 			catch (IOException ex) {
 				throw new ResourceAccessException("Could not retrieve response status code: " + ex.getMessage(), ex);
 			}
 		}
 
-		@SuppressWarnings("unchecked")
-		private static <T> Class<T> bodyClass(Type type) {
-			if (type instanceof Class<?> clazz) {
-				return (Class<T>) clazz;
-			}
-			if (type instanceof ParameterizedType parameterizedType &&
-					parameterizedType.getRawType() instanceof Class<?> rawType) {
-				return (Class<T>) rawType;
-			}
-			return (Class<T>) Object.class;
-		}
 
-		@SuppressWarnings({"rawtypes", "unchecked"})
-		private <T> T readWithMessageConverters(Type bodyType, Class<T> bodyClass) {
-			MediaType contentType = getContentType();
+		private <T> T readBody(Type bodyType, Class<T> bodyClass) {
+			return DefaultRestClient.this.readWithMessageConverters(this.clientResponse, this::applyStatusHandlers,
+					bodyType, bodyClass);
 
-			try (this.clientResponse) {
-				applyStatusHandlers(this.clientRequest, this.clientResponse);
+		}
 
-				for (HttpMessageConverter<?> messageConverter : DefaultRestClient.this.messageConverters) {
-					if (messageConverter instanceof GenericHttpMessageConverter genericHttpMessageConverter) {
-						if (genericHttpMessageConverter.canRead(bodyType, null, contentType)) {
-							if (logger.isDebugEnabled()) {
-								logger.debug("Reading to [" + ResolvableType.forType(bodyType) + "]");
+		private void applyStatusHandlers() {
+			try {
+				ClientHttpResponse response = this.clientResponse;
+				if (response instanceof DefaultConvertibleClientHttpResponse convertibleResponse) {
+					response = convertibleResponse.delegate;
 				}
-							return (T) genericHttpMessageConverter.read(bodyType, null, this.clientResponse);
+				for (StatusHandler handler : this.statusHandlers) {
+					if (handler.test(response)) {
+						handler.handle(this.clientRequest, response);
+						return;
 					}
 				}
-					if (messageConverter.canRead(bodyClass, contentType)) {
-						if (logger.isDebugEnabled()) {
-							logger.debug("Reading to [" + bodyClass.getName() + "] as \"" + contentType + "\"");
 			}
-						return (T) messageConverter.read((Class)bodyClass, this.clientResponse);
+			catch (IOException ex) {
+				throw new UncheckedIOException(ex);
 			}
 		}
-				throw new UnknownContentTypeException(bodyType, contentType,
-						this.clientResponse.getStatusCode(), this.clientResponse.getStatusText(),
-						this.clientResponse.getHeaders(), RestClientUtils.getBody(this.clientResponse));
 	}
-			catch (IOException | HttpMessageNotReadableException ex) {
-				throw new RestClientException("Error while extracting response for type [" +
-						ResolvableType.forType(bodyType) + "] and content type [" + contentType + "]", ex);
+
+
+	private class DefaultConvertibleClientHttpResponse implements RequestHeadersSpec.ConvertibleClientHttpResponse {
+
+		private final ClientHttpResponse delegate;
+
+
+		public DefaultConvertibleClientHttpResponse(ClientHttpResponse delegate) {
+			this.delegate = delegate;
 		}
+
+
+		@Nullable
+		@Override
+		public <T> T bodyTo(Class<T> bodyType) {
+			return readWithMessageConverters(this.delegate, () -> {} , bodyType, bodyType);
 		}
 
-		private MediaType getContentType() {
-			MediaType contentType = this.clientResponse.getHeaders().getContentType();
-			if (contentType == null) {
-				contentType = MediaType.APPLICATION_OCTET_STREAM;
+		@Nullable
+		@Override
+		public <T> T bodyTo(ParameterizedTypeReference<T> bodyType) {
+			Type type = bodyType.getType();
+			Class<T> bodyClass = bodyClass(type);
+			return readWithMessageConverters(this.delegate, () -> {} , type, bodyClass);
 		}
-			return contentType;
+
+		@Override
+		public InputStream getBody() throws IOException {
+			return this.delegate.getBody();
 		}
 
-		private void applyStatusHandlers(HttpRequest request, ClientHttpResponse response) throws IOException {
-			for (StatusHandler handler : this.statusHandlers) {
-				if (handler.test(response)) {
-					handler.handle(request, response);
-					return;
+		@Override
+		public HttpHeaders getHeaders() {
+			return this.delegate.getHeaders();
 		}
+
+		@Override
+		public HttpStatusCode getStatusCode() throws IOException {
+			return this.delegate.getStatusCode();
 		}
+
+		@Override
+		public String getStatusText() throws IOException {
+			return this.delegate.getStatusText();
+		}
+
+		@Override
+		public void close() {
+			this.delegate.close();
 		}
+
 	}
 
+
 }

spring-web/src/main/java/org/springframework/web/client/RestClient.java

@@ -623,7 +623,33 @@ public interface RestClient {
 			 * @return the exchanged type
 			 * @throws IOException in case of I/O errors
 			 */
-			T exchange(HttpRequest clientRequest, ClientHttpResponse clientResponse) throws IOException;
+			T exchange(HttpRequest clientRequest, ConvertibleClientHttpResponse clientResponse) throws IOException;
+		}
+
+
+		/**
+		 * Extension of {@link ClientHttpResponse} that can convert the body.
+		 */
+		interface ConvertibleClientHttpResponse extends ClientHttpResponse {
+
+			/**
+			 * Extract the response body as an object of the given type.
+			 * @param bodyType the type of return value
+			 * @param <T> the body type
+			 * @return the body, or {@code null} if no response body was available
+			 */
+			@Nullable
+			<T> T bodyTo(Class<T> bodyType);
+
+			/**
+			 * Extract the response body as an object of the given type.
+			 * @param bodyType the type of return value
+			 * @param <T> the body type
+			 * @return the body, or {@code null} if no response body was available
+			 */
+			@Nullable
+			<T> T bodyTo(ParameterizedTypeReference<T> bodyType);
+
 		}
 	}
 

spring-web/src/test/java/org/springframework/web/client/RestClientIntegrationTests.java

@@ -661,6 +661,55 @@ class RestClientIntegrationTests {
 		});
 	}
 
+	@ParameterizedRestClientTest
+	void exchangeForJson(ClientHttpRequestFactory requestFactory) {
+		startServer(requestFactory);
+
+		prepareResponse(response -> response
+				.setHeader("Content-Type", "application/json")
+				.setBody("{\"bar\":\"barbar\",\"foo\":\"foofoo\"}"));
+
+		Pojo result = this.restClient.get()
+				.uri("/pojo")
+				.accept(MediaType.APPLICATION_JSON)
+				.exchange((request, response) -> response.bodyTo(Pojo.class));
+
+		assertThat(result.getFoo()).isEqualTo("foofoo");
+		assertThat(result.getBar()).isEqualTo("barbar");
+
+		expectRequestCount(1);
+		expectRequest(request -> {
+			assertThat(request.getPath()).isEqualTo("/pojo");
+			assertThat(request.getHeader(HttpHeaders.ACCEPT)).isEqualTo("application/json");
+		});
+	}
+
+	@ParameterizedRestClientTest
+	void exchangeForJsonArray(ClientHttpRequestFactory requestFactory) {
+		startServer(requestFactory);
+
+		prepareResponse(response -> response
+				.setHeader("Content-Type", "application/json")
+				.setBody("[{\"bar\":\"bar1\",\"foo\":\"foo1\"},{\"bar\":\"bar2\",\"foo\":\"foo2\"}]"));
+
+		List<Pojo> result = this.restClient.get()
+				.uri("/pojo")
+				.accept(MediaType.APPLICATION_JSON)
+				.exchange((request, response) -> response.bodyTo(new ParameterizedTypeReference<>() {}));
+
+		assertThat(result).hasSize(2);
+		assertThat(result.get(0).getFoo()).isEqualTo("foo1");
+		assertThat(result.get(0).getBar()).isEqualTo("bar1");
+		assertThat(result.get(1).getFoo()).isEqualTo("foo2");
+		assertThat(result.get(1).getBar()).isEqualTo("bar2");
+
+		expectRequestCount(1);
+		expectRequest(request -> {
+			assertThat(request.getPath()).isEqualTo("/pojo");
+			assertThat(request.getHeader(HttpHeaders.ACCEPT)).isEqualTo("application/json");
+		});
+	}
+
 	@ParameterizedRestClientTest
 	void exchangeFor404(ClientHttpRequestFactory requestFactory) {
 		startServer(requestFactory);