Allow Mongo support without Spring Data MongoDB

Closes gh-4049

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/data/mongo/MongoDataAutoConfiguration.java

@@ -140,10 +140,9 @@ public class MongoDataAutoConfiguration implements BeanClassLoaderAware {
 			throws ClassNotFoundException {
 		MongoMappingContext context = new MongoMappingContext();
 		context.setInitialEntitySet(getInitialEntitySet(beanFactory));
-		Class<? extends FieldNamingStrategy> strategyClass = this.properties
-				.getFieldNamingStrategy();
+		Class<?> strategyClass = this.properties.getFieldNamingStrategy();
 		if (strategyClass != null) {
-			context.setFieldNamingStrategy(BeanUtils.instantiate(strategyClass));
+			context.setFieldNamingStrategy((FieldNamingStrategy) BeanUtils.instantiate(strategyClass));
 		}
 		return context;
 	}

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mongo/MongoProperties.java

@@ -22,7 +22,6 @@ import java.util.List;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.core.env.Environment;
-import org.springframework.data.mapping.model.FieldNamingStrategy;
 
 import com.mongodb.MongoClient;
 import com.mongodb.MongoClientOptions;
@@ -91,7 +90,7 @@ public class MongoProperties {
 	/**
 	 * Fully qualified name of the FieldNamingStrategy to use.
 	 */
-	private Class<? extends FieldNamingStrategy> fieldNamingStrategy;
+	private Class<?> fieldNamingStrategy;
 
 	public String getHost() {
 		return this.host;
@@ -133,12 +132,11 @@ public class MongoProperties {
 		this.password = password;
 	}
 
-	public Class<? extends FieldNamingStrategy> getFieldNamingStrategy() {
+	public Class<?> getFieldNamingStrategy() {
 		return this.fieldNamingStrategy;
 	}
 
-	public void setFieldNamingStrategy(
-			Class<? extends FieldNamingStrategy> fieldNamingStrategy) {
+	public void setFieldNamingStrategy(Class<?> fieldNamingStrategy) {
 		this.fieldNamingStrategy = fieldNamingStrategy;
 	}
 

spring-boot-autoconfigure/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -196,6 +196,17 @@
       }
     ]
   },
+  {
+    "name": "spring.data.mongodb.field-naming-strategy",
+    "providers": [
+      {
+        "name": "class-reference",
+        "parameters": {
+          "target": "org.springframework.data.mapping.model.FieldNamingStrategy"
+        }
+      }
+    ]
+  },
   {
     "name": "spring.datasource.driver-class-name",
     "providers": [