Introduce RestClient.ResponseSpec#requiredBody

Closes gh-36173

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

@@ -813,19 +813,31 @@ final class DefaultRestClient implements RestClient {
 		}
 
 		@Override
-		@SuppressWarnings("NullAway") // See https://github.com/uber/NullAway/issues/1290
 		public <T> @Nullable T body(Class<T> bodyType) {
 			return executeAndExtract((request, response) -> readBody(request, response, bodyType, bodyType, this.hints));
 		}
 
 		@Override
-		@SuppressWarnings("NullAway") // See https://github.com/uber/NullAway/issues/1290
+		public <T> T requiredBody(Class<T> bodyType) {
+			T body = body(bodyType);
+			Assert.state(body != null, "The body must not be null");
+			return body;
+		}
+
+		@Override
 		public <T> @Nullable T body(ParameterizedTypeReference<T> bodyType) {
 			Type type = bodyType.getType();
 			Class<T> bodyClass = bodyClass(type);
 			return executeAndExtract((request, response) -> readBody(request, response, type, bodyClass, this.hints));
 		}
 
+		@Override
+		public <T> T requiredBody(ParameterizedTypeReference<T> bodyType) {
+			T body = body(bodyType);
+			Assert.state(body != null, "The body must not be null");
+			return body;
+		}
+
 		@Override
 		public <T> ResponseEntity<T> toEntity(Class<T> bodyType) {
 			return toEntityInternal(bodyType, bodyType);

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

@@ -1027,9 +1027,24 @@ public interface RestClient {
 		 * response with a status code of 4xx or 5xx. Use
 		 * {@link #onStatus(Predicate, ErrorHandler)} to customize error response
 		 * handling.
+		 * @see #requiredBody(Class)
 		 */
 		<T> @Nullable T body(Class<T> bodyType);
 
+		/**
+		 * Extract the body as an object of the given type.
+		 * @param bodyType the type of return value
+		 * @param <T> the body type
+		 * @return the body
+		 * @throws IllegalStateException if no response body was available
+		 * @throws RestClientResponseException by default when receiving a
+		 * response with a status code of 4xx or 5xx. Use
+		 * {@link #onStatus(Predicate, ErrorHandler)} to customize error response
+		 * handling.
+		 * @since 7.0.4
+		 */
+		<T> T requiredBody(Class<T> bodyType);
+
 		/**
 		 * Extract the body as an object of the given type.
 		 * @param bodyType the type of return value
@@ -1039,9 +1054,24 @@ public interface RestClient {
 		 * response with a status code of 4xx or 5xx. Use
 		 * {@link #onStatus(Predicate, ErrorHandler)} to customize error response
 		 * handling.
+		 * @see #requiredBody(ParameterizedTypeReference)
 		 */
 		<T> @Nullable T body(ParameterizedTypeReference<T> bodyType);
 
+		/**
+		 * Extract the body as an object of the given type.
+		 * @param bodyType the type of return value
+		 * @param <T> the body type
+		 * @return the body
+		 * @throws IllegalStateException if no response body was available
+		 * @throws RestClientResponseException by default when receiving a
+		 * response with a status code of 4xx or 5xx. Use
+		 * {@link #onStatus(Predicate, ErrorHandler)} to customize error response
+		 * handling.
+		 * @since 7.0.4
+		 */
+		<T> T requiredBody(ParameterizedTypeReference<T> bodyType);
+
 		/**
 		 * Return a {@code ResponseEntity} with the body decoded to an Object of
 		 * the given type.

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

@@ -0,0 +1,149 @@
+/*
+ * Copyright 2002-present 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.web.client;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.net.URI;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
+import org.springframework.http.client.ClientHttpRequest;
+import org.springframework.http.client.ClientHttpRequestFactory;
+import org.springframework.http.client.ClientHttpResponse;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link DefaultRestClient}.
+ *
+ * @author Sebastien Deleuze
+ */
+class DefaultRestClientTests {
+
+	private final ClientHttpRequestFactory requestFactory = mock();
+
+	private final ClientHttpRequest request = mock();
+
+	private final ClientHttpResponse response = mock();
+
+	private RestClient client;
+
+
+	@BeforeEach
+	void setup() {
+		this.client = RestClient.builder()
+				.requestFactory(this.requestFactory)
+				.build();
+	}
+
+
+	@Test
+	void requiredBodyWithClass() throws IOException {
+		mockSentRequest(HttpMethod.GET, "https://example.org");
+		mockResponseStatus(HttpStatus.OK);
+		mockResponseBody("Hello World", MediaType.TEXT_PLAIN);
+
+		String result = this.client.get()
+				.uri("https://example.org")
+				.retrieve()
+				.requiredBody(String.class);
+
+		assertThat(result).isEqualTo("Hello World");
+	}
+
+	@Test
+	void requiredBodyWithClassAndNullBody() throws IOException {
+		mockSentRequest(HttpMethod.GET, "https://example.org");
+		mockResponseStatus(HttpStatus.OK);
+		mockEmptyResponseBody();
+
+		assertThatIllegalStateException().isThrownBy(() ->
+				this.client.get()
+						.uri("https://example.org")
+						.retrieve()
+						.requiredBody(String.class)
+		);
+	}
+
+	@Test
+	void requiredBodyWithParameterizedTypeReference() throws IOException {
+		mockSentRequest(HttpMethod.GET, "https://example.org");
+		mockResponseStatus(HttpStatus.OK);
+		mockResponseBody("Hello World", MediaType.TEXT_PLAIN);
+
+		String result = this.client.get()
+				.uri("https://example.org")
+				.retrieve()
+				.requiredBody(new ParameterizedTypeReference<>() {});
+
+		assertThat(result).isEqualTo("Hello World");
+	}
+
+	@Test
+	void requiredBodyWithParameterizedTypeReferenceAndNullBody() throws IOException {
+		mockSentRequest(HttpMethod.GET, "https://example.org");
+		mockResponseStatus(HttpStatus.OK);
+		mockEmptyResponseBody();
+
+		assertThatIllegalStateException().isThrownBy(() ->
+				this.client.get()
+						.uri("https://example.org")
+						.retrieve()
+						.requiredBody(new ParameterizedTypeReference<String>() {})
+		);
+	}
+
+
+	private void mockSentRequest(HttpMethod method, String uri) throws IOException {
+		given(this.requestFactory.createRequest(URI.create(uri), method)).willReturn(this.request);
+		given(this.request.getHeaders()).willReturn(new HttpHeaders());
+		given(this.request.getMethod()).willReturn(method);
+		given(this.request.getURI()).willReturn(URI.create(uri));
+	}
+
+	private void mockResponseStatus(HttpStatus responseStatus) throws IOException {
+		given(this.request.execute()).willReturn(this.response);
+		given(this.response.getStatusCode()).willReturn(responseStatus);
+		given(this.response.getStatusText()).willReturn(responseStatus.getReasonPhrase());
+	}
+
+	private void mockResponseBody(String expectedBody, MediaType mediaType) throws IOException {
+		HttpHeaders responseHeaders = new HttpHeaders();
+		responseHeaders.setContentType(mediaType);
+		responseHeaders.setContentLength(expectedBody.length());
+		given(this.response.getHeaders()).willReturn(responseHeaders);
+		given(this.response.getBody()).willReturn(new ByteArrayInputStream(expectedBody.getBytes()));
+	}
+
+	private void mockEmptyResponseBody() throws IOException {
+		HttpHeaders responseHeaders = new HttpHeaders();
+		responseHeaders.setContentLength(0);
+		given(this.response.getHeaders()).willReturn(responseHeaders);
+		given(this.response.getBody()).willReturn(new ByteArrayInputStream(new byte[0]));
+	}
+
+}