Allow defining default content negotiation strategy

During the HTTP Content Negotiation phase, the ContentNegotiationManager
uses configured ContentNegotiationStrategy(ies) to define the list of
content types accepted by the client.

When HTTP clients don't send Accept headers, nor use a configured
file extension in the request, nor a request param, developers can
define a default content type using the
ContentNegotiationConfigurer.defaultContentType() method.

This change adds a new overloaded defaultContentType method that takes a
ContentNegotiationStrategy as an argument. This strategy will take the
current request as an argument and return a default content type.

Issue: SPR-12286

spring-web/src/main/java/org/springframework/web/accept/ContentNegotiationManagerFactoryBean.java

@@ -63,6 +63,8 @@ public class ContentNegotiationManagerFactoryBean
 
 	private MediaType defaultContentType;
 
+	private ContentNegotiationStrategy defaultNegotiationStrategy;
+
 	private ContentNegotiationManager contentNegotiationManager;
 
 	private ServletContext servletContext;
@@ -187,6 +189,17 @@ public class ContentNegotiationManagerFactoryBean
 		this.defaultContentType = defaultContentType;
 	}
 
+	/**
+	 * Set the {@link ContentNegotiationStrategy} to be used to resolving the default content type.
+	 * <p>This content type will be used when neither the request path extension,
+	 * nor a request parameter, nor the {@code Accept} header could help determine
+	 * the requested content type.
+	 * @since 4.1.2
+	 */
+	public void setDefaultContentType(ContentNegotiationStrategy defaultStrategy) {
+		this.defaultNegotiationStrategy = defaultStrategy;
+	}
+
 	@Override
 	public void setServletContext(ServletContext servletContext) {
 		this.servletContext = servletContext;
@@ -226,6 +239,10 @@ public class ContentNegotiationManagerFactoryBean
 			strategies.add(new FixedContentNegotiationStrategy(this.defaultContentType));
 		}
 
+		if(this.defaultNegotiationStrategy != null) {
+			strategies.add(defaultNegotiationStrategy);
+		}
+
 		this.contentNegotiationManager = new ContentNegotiationManager(strategies);
 	}