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

@@ -22,6 +22,7 @@ import javax.servlet.ServletContext;
 import org.springframework.http.MediaType;
 import org.springframework.web.accept.ContentNegotiationManager;
 import org.springframework.web.accept.ContentNegotiationManagerFactoryBean;
+import org.springframework.web.accept.ContentNegotiationStrategy;
 
 /**
  * Helps with configuring a {@link ContentNegotiationManager}.
@@ -167,6 +168,18 @@ public class ContentNegotiationConfigurer {
 		return this;
 	}
 
+	/**
+	 * 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 ContentNegotiationConfigurer defaultContentType(ContentNegotiationStrategy defaultStrategy) {
+		this.factoryBean.setDefaultContentType(defaultStrategy);
+		return this;
+	}
+
 	/**
 	 * Return the configured {@link ContentNegotiationManager} instance
 	 */