ServletServerHttpResponse reflects Content-Type override

Closes gh-25490

spring-web/src/main/java/org/springframework/http/server/ServletServerHttpResponse.java

@@ -20,6 +20,7 @@ import java.io.IOException;
 import java.io.OutputStream;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 
 import javax.servlet.http.HttpServletResponse;
@@ -152,12 +153,14 @@ public class ServletServerHttpResponse implements ServerHttpResponse {
 		@Override
 		@Nullable
 		public String getFirst(String headerName) {
-			String value = servletResponse.getHeader(headerName);
-			if (value != null) {
-				return value;
+			if (headerName.equalsIgnoreCase(CONTENT_TYPE)) {
+				// Content-Type is written as an override so check super first
+				String value = super.getFirst(headerName);
+				return (value != null ? value : servletResponse.getHeader(headerName));
 			}
 			else {
-				return super.getFirst(headerName);
+				String value = servletResponse.getHeader(headerName);
+				return (value != null ? value : super.getFirst(headerName));
 			}
 		}
 
@@ -165,7 +168,13 @@ public class ServletServerHttpResponse implements ServerHttpResponse {
 		public List<String> get(Object key) {
 			Assert.isInstanceOf(String.class, key, "Key must be a String-based header name");
 
-			Collection<String> values1 = servletResponse.getHeaders((String) key);
+			String headerName = (String) key;
+			if (headerName.equalsIgnoreCase(CONTENT_TYPE)) {
+				// Content-Type is written as an override so don't merge
+				return Collections.singletonList(getFirst(headerName));
+			}
+
+			Collection<String> values1 = servletResponse.getHeaders(headerName);
 			if (headersWritten) {
 				return new ArrayList<>(values1);
 			}

spring-web/src/test/java/org/springframework/http/server/ServletServerHttpResponseTests.java

 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -17,7 +17,6 @@
 package org.springframework.http.server;
 
 import java.nio.charset.StandardCharsets;
-import java.util.Collections;
 import java.util.List;
 
 import org.junit.jupiter.api.BeforeEach;
@@ -44,20 +43,20 @@ public class ServletServerHttpResponseTests {
 
 
 	@BeforeEach
-	public void create() throws Exception {
+	void create() {
 		mockResponse = new MockHttpServletResponse();
 		response = new ServletServerHttpResponse(mockResponse);
 	}
 
 
 	@Test
-	public void setStatusCode() throws Exception {
+	void setStatusCode() {
 		response.setStatusCode(HttpStatus.NOT_FOUND);
 		assertThat(mockResponse.getStatus()).as("Invalid status code").isEqualTo(404);
 	}
 
 	@Test
-	public void getHeaders() throws Exception {
+	void getHeaders() {
 		HttpHeaders headers = response.getHeaders();
 		String headerName = "MyHeader";
 		String headerValue1 = "value1";
@@ -77,23 +76,32 @@ public class ServletServerHttpResponseTests {
 	}
 
 	@Test
-	public void preExistingHeadersFromHttpServletResponse() {
+	void preExistingHeadersFromHttpServletResponse() {
 		String headerName = "Access-Control-Allow-Origin";
 		String headerValue = "localhost:8080";
 
 		this.mockResponse.addHeader(headerName, headerValue);
+		this.mockResponse.setContentType("text/csv");
 		this.response = new ServletServerHttpResponse(this.mockResponse);
 
 		assertThat(this.response.getHeaders().getFirst(headerName)).isEqualTo(headerValue);
-		assertThat(this.response.getHeaders().get(headerName)).isEqualTo(Collections.singletonList(headerValue));
-		assertThat(this.response.getHeaders().containsKey(headerName)).isTrue();
-		assertThat(this.response.getHeaders().getFirst(headerName)).isEqualTo(headerValue);
+		assertThat(this.response.getHeaders().get(headerName)).containsExactly(headerValue);
+		assertThat(this.response.getHeaders()).containsKey(headerName);
 		assertThat(this.response.getHeaders().getAccessControlAllowOrigin()).isEqualTo(headerValue);
 	}
 
+	@Test // gh-25490
+	void preExistingContentTypeIsOverriddenImmediately() {
+		this.mockResponse.setContentType("text/csv");
+		this.response = new ServletServerHttpResponse(this.mockResponse);
+		this.response.getHeaders().setContentType(MediaType.APPLICATION_JSON);
+
+		assertThat(response.getHeaders().getContentType()).isEqualTo(MediaType.APPLICATION_JSON);
+	}
+
 	@Test
-	public void getBody() throws Exception {
-		byte[] content = "Hello World".getBytes("UTF-8");
+	void getBody() throws Exception {
+		byte[] content = "Hello World".getBytes(StandardCharsets.UTF_8);
 		FileCopyUtils.copy(content, response.getBody());
 
 		assertThat(mockResponse.getContentAsByteArray()).as("Invalid content written").isEqualTo(content);