Add extendMessageConverters to WebMvcConfigurer

Issue: SPR-12450

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/DelegatingWebMvcConfiguration.java

@@ -105,6 +105,11 @@ public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
 		this.configurers.configureMessageConverters(converters);
 	}
 
+	@Override
+	protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
+		this.configurers.extendMessageConverters(converters);
+	}
+
 	@Override
 	protected void addFormatters(FormatterRegistry registry) {
 		this.configurers.addFormatters(registry);

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurationSupport.java

@@ -629,6 +629,7 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 			if (this.messageConverters.isEmpty()) {
 				addDefaultHttpMessageConverters(this.messageConverters);
 			}
+			extendMessageConverters(this.messageConverters);
 		}
 		return this.messageConverters;
 	}
@@ -646,6 +647,9 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 	protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
 	}
 
+	protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
+	}
+
 	/**
 	 * Adds a set of default HttpMessageConverter instances to the given list.
 	 * Subclasses can call this method from {@link #configureMessageConverters(List)}.

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurer.java

@@ -53,14 +53,26 @@ public interface WebMvcConfigurer {
 	void addFormatters(FormatterRegistry registry);
 
 	/**
-	 * Configure the {@link HttpMessageConverter}s to use in argument resolvers
-	 * and return value handlers that support reading and/or writing to the
-	 * body of the request and response. If no message converters are added to
-	 * the list, default converters are added instead.
+	 * Configure the {@link HttpMessageConverter}s to use for reading or writing
+	 * to the body of the request or response. If no converters are added, a
+	 * default list of converters is registered.
+	 * <p><strong>Note</strong> that adding converters to the list, turns off
+	 * default converter registration. To simply add a converter without impacting
+	 * default registration, consider using the method
+	 * {@link #extendMessageConverters(java.util.List)} instead.
 	 * @param converters initially an empty list of converters
 	 */
 	void configureMessageConverters(List<HttpMessageConverter<?>> converters);
 
+	/**
+	 * A hook for extending or modifying the list of converters after it has been
+	 * configured. This may be useful for example to allow default converters to
+	 * be registered and then insert a custom converter through this method.
+	 * @param converters the list of configured converters to extend.
+	 * @since 4.1.3
+	 */
+	void extendMessageConverters(List<HttpMessageConverter<?>> converters);
+
 	/**
 	 * Provide a custom {@link Validator} instead of the one created by default.
 	 * The default implementation, assuming JSR-303 is on the classpath, is:

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurerAdapter.java

@@ -51,6 +51,14 @@ public abstract class WebMvcConfigurerAdapter implements WebMvcConfigurer {
 	public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
 	}
 
+	/**
+	 * {@inheritDoc}
+	 * <p>This implementation is empty.
+	 */
+	@Override
+	public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
+	}
+
 	/**
 	 * {@inheritDoc}
 	 * <p>This implementation returns {@code null}

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurerComposite.java

@@ -78,6 +78,13 @@ class WebMvcConfigurerComposite implements WebMvcConfigurer {
 		}
 	}
 
+	@Override
+	public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
+		for (WebMvcConfigurer delegate : this.delegates) {
+			delegate.extendMessageConverters(converters);
+		}
+	}
+
 	@Override
 	public void addArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers) {
 		for (WebMvcConfigurer delegate : this.delegates) {

spring-webmvc/src/test/java/org/springframework/web/servlet/config/annotation/DelegatingWebMvcConfigurationTests.java

@@ -29,6 +29,7 @@ import org.mockito.MockitoAnnotations;
 
 import org.springframework.core.convert.ConversionService;
 import org.springframework.format.support.FormattingConversionService;
+import org.springframework.http.HttpMessage;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.util.PathMatcher;
@@ -115,18 +116,26 @@ public class DelegatingWebMvcConfigurationTests {
 
 	@Test
 	public void configureMessageConverters() {
+		final HttpMessageConverter customConverter = mock(HttpMessageConverter.class);
+		final StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
 		List<WebMvcConfigurer> configurers = new ArrayList<WebMvcConfigurer>();
 		configurers.add(new WebMvcConfigurerAdapter() {
 			@Override
 			public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
-				converters.add(new StringHttpMessageConverter());
+				converters.add(stringConverter);
+			}
+			@Override
+			public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
+				converters.add(0, customConverter);
 			}
 		});
 		delegatingConfig = new DelegatingWebMvcConfiguration();
 		delegatingConfig.setConfigurers(configurers);
 
 		RequestMappingHandlerAdapter adapter = delegatingConfig.requestMappingHandlerAdapter();
-		assertEquals("Only one custom converter should be registered", 1, adapter.getMessageConverters().size());
+		assertEquals("Only one custom converter should be registered", 2, adapter.getMessageConverters().size());
+		assertSame(customConverter, adapter.getMessageConverters().get(0));
+		assertSame(stringConverter, adapter.getMessageConverters().get(1));
 	}
 
 	@Test

spring-webmvc/src/test/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurationSupportExtensionTests.java

@@ -33,6 +33,7 @@ import org.springframework.format.FormatterRegistry;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
 import org.springframework.mock.web.test.MockHttpServletRequest;
 import org.springframework.mock.web.test.MockServletContext;
@@ -165,9 +166,10 @@ public class WebMvcConfigurationSupportExtensionTests {
 		assertEquals("converted", actual);
 
 		// Message converters
-		assertEquals(1, adapter.getMessageConverters().size());
-		assertEquals(MappingJackson2HttpMessageConverter.class, adapter.getMessageConverters().get(0).getClass());
-		ObjectMapper objectMapper = ((MappingJackson2HttpMessageConverter)adapter.getMessageConverters().get(0)).getObjectMapper();
+		assertEquals(2, adapter.getMessageConverters().size());
+		assertEquals(StringHttpMessageConverter.class, adapter.getMessageConverters().get(0).getClass());
+		assertEquals(MappingJackson2HttpMessageConverter.class, adapter.getMessageConverters().get(1).getClass());
+		ObjectMapper objectMapper = ((MappingJackson2HttpMessageConverter)adapter.getMessageConverters().get(1)).getObjectMapper();
 		assertFalse(objectMapper.getDeserializationConfig().isEnabled(MapperFeature.DEFAULT_VIEW_INCLUSION));
 		assertFalse(objectMapper.getSerializationConfig().isEnabled(MapperFeature.DEFAULT_VIEW_INCLUSION));
 		assertFalse(objectMapper.getDeserializationConfig().isEnabled(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES));
@@ -301,6 +303,11 @@ public class WebMvcConfigurationSupportExtensionTests {
 			converters.add(new MappingJackson2HttpMessageConverter());
 		}
 
+		@Override
+		public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
+			converters.add(0, new StringHttpMessageConverter());
+		}
+
 		@Override
 		public Validator getValidator() {
 			return new Validator() {