Ensure that MongoClient's EventLoopGroup is shut down during context close

See gh-16087

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

@@ -21,11 +21,15 @@ import java.util.stream.Collectors;
 import javax.annotation.PreDestroy;
 
 import com.mongodb.MongoClientSettings;
+import com.mongodb.MongoClientSettings.Builder;
 import com.mongodb.connection.netty.NettyStreamFactoryFactory;
 import com.mongodb.reactivestreams.client.MongoClient;
+import io.netty.channel.EventLoopGroup;
+import io.netty.channel.nio.NioEventLoopGroup;
 import io.netty.channel.socket.SocketChannel;
 import reactor.core.publisher.Flux;
 
+import org.springframework.beans.factory.DisposableBean;
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
@@ -77,23 +81,51 @@ public class MongoReactiveAutoConfiguration {
 	}
 
 	@Configuration
-	@ConditionalOnClass(SocketChannel.class)
+	@ConditionalOnClass({ SocketChannel.class, NioEventLoopGroup.class })
 	static class NettyDriverConfiguration {
 
 		@Bean
 		@Order(Ordered.HIGHEST_PRECEDENCE)
 		public MongoClientSettingsBuilderCustomizer nettyDriverCustomizer(
 				ObjectProvider<MongoClientSettings> settings) {
-			return (builder) -> {
-				if (!isStreamFactoryFactoryDefined(settings.getIfAvailable())) {
-					builder.streamFactoryFactory(
-							NettyStreamFactoryFactory.builder().build());
-				}
-			};
+			return new EventLoopGroupMongoClientSettingsBuilderCustomizer(settings);
 		}
 
-		private boolean isStreamFactoryFactoryDefined(MongoClientSettings settings) {
-			return settings != null && settings.getStreamFactoryFactory() != null;
+		private static final class EventLoopGroupMongoClientSettingsBuilderCustomizer
+				implements MongoClientSettingsBuilderCustomizer, DisposableBean {
+
+			private final ObjectProvider<MongoClientSettings> settings;
+
+			private EventLoopGroup eventLoopGroup;
+
+			private EventLoopGroupMongoClientSettingsBuilderCustomizer(
+					ObjectProvider<MongoClientSettings> settings) {
+				this.settings = settings;
+			}
+
+			@Override
+			public void customize(Builder builder) {
+				if (!isStreamFactoryFactoryDefined(this.settings.getIfAvailable())) {
+					NioEventLoopGroup eventLoopGroup = new NioEventLoopGroup();
+					this.eventLoopGroup = eventLoopGroup;
+					builder.streamFactoryFactory(NettyStreamFactoryFactory.builder()
+							.eventLoopGroup(eventLoopGroup).build());
+				}
+			}
+
+			@Override
+			public void destroy() {
+				EventLoopGroup eventLoopGroup = this.eventLoopGroup;
+				if (eventLoopGroup != null) {
+					eventLoopGroup.shutdownGracefully().awaitUninterruptibly();
+					this.eventLoopGroup = null;
+				}
+			}
+
+			private boolean isStreamFactoryFactoryDefined(MongoClientSettings settings) {
+				return settings != null && settings.getStreamFactoryFactory() != null;
+			}
+
 		}
 
 	}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/mongo/MongoReactiveAutoConfigurationTests.java

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 package org.springframework.boot.autoconfigure.mongo;
 
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
 
 import com.mongodb.MongoClientSettings;
 import com.mongodb.ReadPreference;
@@ -25,6 +26,7 @@ import com.mongodb.connection.StreamFactory;
 import com.mongodb.connection.StreamFactoryFactory;
 import com.mongodb.connection.netty.NettyStreamFactoryFactory;
 import com.mongodb.reactivestreams.client.MongoClient;
+import io.netty.channel.EventLoopGroup;
 import org.junit.Test;
 
 import org.springframework.boot.autoconfigure.AutoConfigurations;
@@ -89,11 +91,17 @@ public class MongoReactiveAutoConfigurationTests {
 
 	@Test
 	public void nettyStreamFactoryFactoryIsConfiguredAutomatically() {
+		AtomicReference<EventLoopGroup> capture = new AtomicReference<>();
 		this.contextRunner.run((context) -> {
 			assertThat(context).hasSingleBean(MongoClient.class);
-			assertThat(getSettings(context).getStreamFactoryFactory())
-					.isInstanceOf(NettyStreamFactoryFactory.class);
+			StreamFactoryFactory factory = getSettings(context).getStreamFactoryFactory();
+			assertThat(factory).isInstanceOf(NettyStreamFactoryFactory.class);
+			capture.set((EventLoopGroup) ReflectionTestUtils.getField(factory,
+					"eventLoopGroup"));
+			assertThat(capture.get()).isNotNull();
+			assertThat(capture.get().isShutdown()).isFalse();
 		});
+		assertThat(capture.get().isShutdown()).isTrue();
 	}
 
 	@Test