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) {