Better empty response body support in RestTemplate

Prior to this change, RestTemplate returned an empty response body if:

* HTTP return status 204 or 304
* Content-length header equals 0

This change adds a new condition for this, better supporting RFC7230
section 3.4, for connections that are closed without response body:

* No Content-length header
* No Transfer-encoding: chunked header value
* a Connection: close header value

See SPR-7911 for previous efforts in that space.

Issue: SPR-8016

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

@@ -23,6 +23,7 @@ import java.util.List;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.client.ClientHttpResponse;
@@ -122,9 +123,14 @@ public class HttpMessageConverterExtractor<T> implements ResponseExtractor<T> {
 	}
 
 	/**
-	 * Indicates whether the given response has a message body. <p>Default implementation
-	 * returns {@code false} for a response status of {@code 204} or {@code 304}, or a {@code
-	 * Content-Length} of {@code 0}.
+	 * Indicates whether the given response has a message body.
+	 * <p>Default implementation returns {@code false} for:
+	 * <ul>
+	 *     <li>a response status of {@code 204} or {@code 304}</li>
+	 *     <li>a {@code Content-Length} of {@code 0}</li>
+	 *     <li>no indication of content (no {@code Content-Length} nor {@code Transfer-encoding: chunked}) and
+	 *     a ({@code Connection: closed}) header. See rfc7230 section 3.4</li>
+	 * </ul>
 	 *
 	 * @param response the response to check for a message body
 	 * @return {@code true} if the response has a body, {@code false} otherwise
@@ -136,8 +142,19 @@ public class HttpMessageConverterExtractor<T> implements ResponseExtractor<T> {
 				responseStatus == HttpStatus.NOT_MODIFIED) {
 			return false;
 		}
-		long contentLength = response.getHeaders().getContentLength();
-		return contentLength != 0;
+		HttpHeaders headers = response.getHeaders();
+		long contentLength = headers.getContentLength();
+		if(contentLength == 0) {
+			return false;
+		}
+		boolean chunked = headers.containsKey(HttpHeaders.TRANSFER_ENCODING)
+				&& headers.get(HttpHeaders.TRANSFER_ENCODING).contains("chunked");
+		boolean closed = headers.containsKey(HttpHeaders.CONNECTION)
+				&& headers.getConnection().contains("close");
+		if(!chunked && contentLength == -1 && closed) {
+			return false;
+		}
+		return true;
 	}
 
 }

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

@@ -108,7 +108,7 @@ public class HttpMessageConverterExtractorTests {
 		assertEquals(expected, result);
 	}
 
-	@Test
+	@Test(expected = RestClientException.class)
 	@SuppressWarnings("unchecked")
 	public void cannotRead() throws IOException {
 		HttpMessageConverter<String> converter = mock(HttpMessageConverter.class);
@@ -122,13 +122,22 @@ public class HttpMessageConverterExtractorTests {
 		given(response.getHeaders()).willReturn(responseHeaders);
 		given(converter.canRead(String.class, contentType)).willReturn(false);
 
-		try {
-			extractor.extractData(response);
-			fail("RestClientException expected");
-		}
-		catch (RestClientException expected) {
-			// expected
-		}
+		extractor.extractData(response);
+	}
+
+	@Test
+	public void connectionClose() throws IOException {
+		HttpMessageConverter<String> converter = mock(HttpMessageConverter.class);
+		List<HttpMessageConverter<?>> converters = new ArrayList<HttpMessageConverter<?>>();
+		converters.add(converter);
+		HttpHeaders responseHeaders = new HttpHeaders();
+		responseHeaders.setConnection("close");
+		extractor = new HttpMessageConverterExtractor<String>(String.class, createConverterList(converter));
+		given(response.getStatusCode()).willReturn(HttpStatus.OK);
+		given(response.getHeaders()).willReturn(responseHeaders);
+
+		Object result = extractor.extractData(response);
+		assertNull(result);
 	}
 
 	@Test