Do not process conditional requests for non-GET

Prior to this commit, HttpEntityMethodProcessor would process
conditional requests even if those aren't GET requests.
This is an issue for POST requests with "If-None-Match: *" headers and
many other use cases, which should not receive an HTTP 304 Not Modified
status in response.

This commit only triggers ETag/Last-Modified conditional requests bits
for GET requests.

Issue: SPR-13496

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

@@ -172,7 +172,8 @@ public class HttpEntityMethodProcessor extends AbstractMessageConverterMethodPro
 		Object body = responseEntity.getBody();
 		if (responseEntity instanceof ResponseEntity) {
 			outputMessage.setStatusCode(((ResponseEntity<?>) responseEntity).getStatusCode());
-			if (isResourceNotModified(inputMessage, outputMessage)) {
+			if (inputMessage.getServletRequest().getMethod() == "GET"
+					&& isResourceNotModified(inputMessage, outputMessage)) {
 				outputMessage.setStatusCode(HttpStatus.NOT_MODIFIED);
 				// Ensure headers are flushed, no body should be written.
 				outputMessage.flush();

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

@@ -112,7 +112,7 @@ public class HttpEntityMethodProcessorMockTests {
 		returnTypeInt = new MethodParameter(getClass().getMethod("handle3"), -1);
 
 		mavContainer = new ModelAndViewContainer();
-		servletRequest = new MockHttpServletRequest("POST", "/foo");
+		servletRequest = new MockHttpServletRequest("GET", "/foo");
 		servletResponse = new MockHttpServletResponse();
 		webRequest = new ServletWebRequest(servletRequest, servletResponse);
 	}
@@ -182,6 +182,7 @@ public class HttpEntityMethodProcessorMockTests {
 	@Test(expected = HttpMediaTypeNotSupportedException.class)
 	public void resolveArgumentNotReadable() throws Exception {
 		MediaType contentType = MediaType.TEXT_PLAIN;
+		servletRequest.setMethod("POST");
 		servletRequest.addHeader("Content-Type", contentType.toString());
 
 		given(messageConverter.getSupportedMediaTypes()).willReturn(Collections.singletonList(contentType));
@@ -194,6 +195,7 @@ public class HttpEntityMethodProcessorMockTests {
 
 	@Test(expected = HttpMediaTypeNotSupportedException.class)
 	public void resolveArgumentNoContentType() throws Exception {
+		servletRequest.setMethod("POST");
 		servletRequest.setContent("some content".getBytes(Charset.forName("UTF-8")));
 		processor.resolveArgument(paramHttpEntity, mavContainer, webRequest, null);
 		fail("Expected exception");
@@ -455,6 +457,32 @@ public class HttpEntityMethodProcessorMockTests {
 		assertEquals(0, servletResponse.getContentAsByteArray().length);
 	}
 
+	// SPR-13496
+	@Test
+	public void handleReturnTypePostRequestWithIfNotModified() throws Exception {
+		String wildcardValue = "*";
+		String etagValue = "\"some-etag\"";
+		servletRequest.setMethod("POST");
+		servletRequest.addHeader(HttpHeaders.IF_NONE_MATCH, wildcardValue);
+		HttpHeaders responseHeaders = new HttpHeaders();
+		responseHeaders.set(HttpHeaders.ETAG, etagValue);
+		ResponseEntity<String> returnValue = new ResponseEntity<String>("body", responseHeaders, HttpStatus.OK);
+
+		given(messageConverter.canWrite(String.class, null)).willReturn(true);
+		given(messageConverter.getSupportedMediaTypes()).willReturn(Collections.singletonList(MediaType.TEXT_PLAIN));
+		given(messageConverter.canWrite(String.class, MediaType.TEXT_PLAIN)).willReturn(true);
+
+
+		processor.handleReturnValue(returnValue, returnTypeResponseEntity, mavContainer, webRequest);
+
+		assertTrue(mavContainer.isRequestHandled());
+		assertEquals(HttpStatus.OK.value(), servletResponse.getStatus());
+		assertEquals(1, servletResponse.getHeaderValues(HttpHeaders.ETAG).size());
+		assertEquals(etagValue, servletResponse.getHeader(HttpHeaders.ETAG));
+		ArgumentCaptor<HttpOutputMessage> outputMessage = ArgumentCaptor.forClass(HttpOutputMessage.class);
+		verify(messageConverter).write(eq("body"), eq(MediaType.TEXT_PLAIN),  outputMessage.capture());
+	}
+
 	@SuppressWarnings("unused")
 	public ResponseEntity<String> handle1(HttpEntity<String> httpEntity, ResponseEntity<String> entity,
 			int i, RequestEntity<String> requestEntity) {