Allow multiple hosts to be set in MongoProperties

Update `MongoProperties` with an `additional-hosts` property which
can be used to configure MongoDB in a multi-host environment.

See gh-32125

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

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.autoconfigure.mongo;
 
+import java.util.List;
+
 import com.mongodb.ConnectionString;
 import org.bson.UuidRepresentation;
 
@@ -33,6 +35,7 @@ import org.springframework.boot.context.properties.ConfigurationProperties;
  * @author Nasko Vasilev
  * @author Mark Paluch
  * @author Artsiom Yudovin
+ * @author Safeer Ansari
  * @since 1.0.0
  */
 @ConfigurationProperties(prefix = "spring.data.mongodb")
@@ -105,6 +108,11 @@ public class MongoProperties {
 	 */
 	private Boolean autoIndexCreation;
 
+	/**
+	 * List of additional hosts to connect with different hosts in a replica set.
+	 */
+	private List<String> additionalHosts;
+
 	public String getHost() {
 		return this.host;
 	}
@@ -208,6 +216,14 @@ public class MongoProperties {
 		this.autoIndexCreation = autoIndexCreation;
 	}
 
+	public List<String> getAdditionalHosts() {
+		return this.additionalHosts;
+	}
+
+	public void setAdditionalHosts(List<String> additionalHosts) {
+		this.additionalHosts = additionalHosts;
+	}
+
 	public static class Gridfs {
 
 		/**

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

@@ -16,7 +16,8 @@
 
 package org.springframework.boot.autoconfigure.mongo;
 
-import java.util.Collections;
+import java.util.ArrayList;
+import java.util.List;
 
 import com.mongodb.ConnectionString;
 import com.mongodb.MongoClientSettings;
@@ -30,6 +31,7 @@ import org.springframework.core.Ordered;
  * {@link MongoProperties} to a {@link MongoClientSettings}.
  *
  * @author Scott Frederick
+ * @author Safeer Ansari
  * @since 2.4.0
  */
 public class MongoPropertiesClientSettingsBuilderCustomizer implements MongoClientSettingsBuilderCustomizer, Ordered {
@@ -63,12 +65,21 @@ public class MongoPropertiesClientSettingsBuilderCustomizer implements MongoClie
 			String host = getOrDefault(this.properties.getHost(), "localhost");
 			int port = getOrDefault(this.properties.getPort(), MongoProperties.DEFAULT_PORT);
 			ServerAddress serverAddress = new ServerAddress(host, port);
-			settings.applyToClusterSettings((cluster) -> cluster.hosts(Collections.singletonList(serverAddress)));
+			List<ServerAddress> serverAddressList = new ArrayList<>(List.of(serverAddress));
+			applyAdditionalHosts(serverAddressList);
+			settings.applyToClusterSettings((cluster) -> cluster.hosts(serverAddressList));
 			return;
 		}
 		settings.applyConnectionString(new ConnectionString(MongoProperties.DEFAULT_URI));
 	}
 
+	private void applyAdditionalHosts(List<ServerAddress> serverAddressList) {
+		if (this.properties.getAdditionalHosts() != null && !this.properties.getAdditionalHosts().isEmpty()) {
+			this.properties.getAdditionalHosts()
+					.forEach((additionalHost) -> serverAddressList.add(new ServerAddress(additionalHost)));
+		}
+	}
+
 	private void applyCredentials(MongoClientSettings.Builder builder) {
 		if (this.properties.getUri() == null && this.properties.getUsername() != null
 				&& this.properties.getPassword() != null) {

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

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.autoconfigure.mongo;
 
+import java.util.Arrays;
 import java.util.List;
 
 import com.mongodb.MongoClientSettings;
@@ -53,6 +54,18 @@ class MongoPropertiesClientSettingsBuilderCustomizerTests {
 		assertServerAddress(allAddresses.get(0), "mongo.example.com", 27017);
 	}
 
+	@Test
+	void additionalHostCanBeAdded() {
+		this.properties.setHost("mongo.example.com");
+		this.properties.setAdditionalHosts(Arrays.asList("mongo.example.com:33", "mongo.example2.com"));
+		MongoClientSettings settings = customizeSettings();
+		List<ServerAddress> allAddresses = getAllAddresses(settings);
+		assertThat(allAddresses).hasSize(3);
+		assertServerAddress(allAddresses.get(0), "mongo.example.com", 27017);
+		assertServerAddress(allAddresses.get(1), "mongo.example.com", 33);
+		assertServerAddress(allAddresses.get(2), "mongo.example2.com", 27017);
+	}
+
 	@Test
 	void credentialsCanBeCustomized() {
 		this.properties.setUsername("user");