Fix potential NullPointerException in HalFormsAdaptingResponseBodyAdvice.

Fixes #2123.

Author: odrotbohm

spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/config/HalFormsAdaptingResponseBodyAdvice.java

@@ -30,6 +30,7 @@
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.http.server.ServerHttpRequest;
 import org.springframework.http.server.ServerHttpResponse;
+import org.springframework.lang.Nullable;
 import org.springframework.web.HttpMediaTypeNotAcceptableException;
 import org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor;
 import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;
@@ -55,7 +56,7 @@ public boolean supports(MethodParameter returnType, Class<? extends HttpMessageC
 
 	@Override
 	@SneakyThrows
-	public RepresentationModel<T> beforeBodyWrite(RepresentationModel<T> body, MethodParameter returnType,
+	public RepresentationModel<T> beforeBodyWrite(@Nullable RepresentationModel<T> body, MethodParameter returnType,
 			MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType,
 			ServerHttpRequest request, ServerHttpResponse response) {
 
@@ -66,7 +67,7 @@ public RepresentationModel<T> beforeBodyWrite(RepresentationModel<T> body, Metho
 
 		List<MediaType> accept = request.getHeaders().getAccept();
 
-		boolean hasAffordances = body.getLinks().stream()
+		boolean hasAffordances = body != null && body.getLinks().stream()
 				.anyMatch(it -> !it.getAffordances().isEmpty());
 
 		// Affordances registered -> we're fine as we will render templates

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/config/HalFormsAdaptingResponseBodyAdviceTests.java

@@ -35,6 +35,7 @@
 import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
 import org.springframework.http.server.ServletServerHttpRequest;
 import org.springframework.http.server.ServletServerHttpResponse;
+import org.springframework.lang.Nullable;
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.mock.web.MockHttpServletResponse;
 import org.springframework.web.HttpMediaTypeNotAcceptableException;
@@ -95,7 +96,16 @@ void issues415IfNoCompatibleMediaTypeWasRequested() {
 				.isThrownBy(() -> assertResponseContentType(model, MediaTypes.HAL_JSON));
 	}
 
-	private void assertResponseContentType(RepresentationModel<T> model, MediaType mediaType) {
+	@Test // #2123
+	void handlesNullBodyCorrectly() {
+
+		request.addHeader(HttpHeaders.ACCEPT,
+				MediaType.toString(Arrays.asList(MediaTypes.HAL_JSON)));
+
+		assertThatNoException().isThrownBy(() -> assertResponseContentType(null, MediaTypes.HAL_JSON));
+	}
+
+	private void assertResponseContentType(@Nullable RepresentationModel<T> model, MediaType mediaType) {
 
 		this.response.addHeader(HttpHeaders.CONTENT_TYPE, MediaTypes.HAL_FORMS_JSON_VALUE);
 		ServletServerHttpResponse response = new ServletServerHttpResponse(this.response);