Polishing.

Update javadoc and connection string rendering.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/observability/MongoKeyName.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2025 the original author or authors.
+ * Copyright 2025-present 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.
@@ -30,6 +30,7 @@ import org.springframework.util.StringUtils;
  * {@link KeyValue} and {@link KeyName}.
  *
  * @author Mark Paluch
+ * @since 4.4.9
  */
 record MongoKeyName<C>(String name, boolean required, Function<C, Object> valueFunction) implements KeyName {
 
@@ -43,7 +44,7 @@ record MongoKeyName<C>(String name, boolean required, Function<C, Object> valueF
 	 * @return
 	 * @param <C>
 	 */
-	public static <C> MongoKeyName<C> required(String name, Function<C, Object> valueFunction) {
+	static <C> MongoKeyName<C> required(String name, Function<C, Object> valueFunction) {
 		return required(name, valueFunction, Objects::nonNull);
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/observability/MongoObservation.java

@@ -15,11 +15,17 @@
  */
 package org.springframework.data.mongodb.observability;
 
-import static org.springframework.data.mongodb.observability.MongoKeyName.*;
+import static org.springframework.data.mongodb.observability.MongoKeyName.MongoKeyValue;
+import static org.springframework.data.mongodb.observability.MongoKeyName.just;
 
 import io.micrometer.common.docs.KeyName;
 import io.micrometer.observation.docs.ObservationDocumentation;
 
+import java.net.URLEncoder;
+import java.nio.charset.StandardCharsets;
+import java.util.regex.Pattern;
+
+import org.springframework.lang.Contract;
 import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 
@@ -86,7 +92,7 @@ enum MongoObservation implements ObservationDocumentation {
 		static MongoKeyName<ServerAddress> NET_PEER_PORT = MongoKeyName.required("net.peer.port", ServerAddress::getPort);
 
 		static MongoKeyName<ConnectionString> DB_CONNECTION_STRING = MongoKeyName.requiredString("db.connection_string",
-				Object::toString);
+				MongoObservation::connectionString);
 		static MongoKeyName<ConnectionString> DB_USER = MongoKeyName.requiredString("db.user",
 				ConnectionString::getUsername);
 
@@ -96,7 +102,7 @@ enum MongoObservation implements ObservationDocumentation {
 		 * @param context the context to contribute from, can be {@literal null} if no context is available.
 		 * @return the key value contributor providing low cardinality key names.
 		 */
-		public static Observer observe(@Nullable MongoHandlerContext context) {
+		static Observer observe(@Nullable MongoHandlerContext context) {
 
 			return Observer.fromContext(context, it -> {
 
@@ -115,9 +121,51 @@ enum MongoObservation implements ObservationDocumentation {
 		 *
 		 * @return the key names for low cardinality keys.
 		 */
-		public static KeyName[] getKeyNames() {
+		static KeyName[] getKeyNames() {
 			return observe(null).toKeyNames();
 		}
 	}
 
+	@Contract("null -> null")
+	static @Nullable String connectionString(@Nullable ConnectionString connectionString) {
+
+		if (connectionString == null) {
+			return null;
+		}
+
+		if (!StringUtils.hasText(connectionString.getUsername()) && connectionString.getPassword() == null) {
+			return connectionString.toString();
+		}
+
+		String target = renderPart(connectionString.toString(), "//", connectionString.getUsername());
+
+		if (connectionString.getPassword() != null) {
+
+			String rendered = renderPart(target, ":", new String(connectionString.getPassword()));
+			if (!rendered.equals(target)) {
+				target = rendered;
+			} else {
+				String protocol = connectionString.isSrvProtocol() ? "mongodb+srv" : "mongodb";
+				target = "%s://*****:*****@%s".formatted(protocol,
+						StringUtils.collectionToCommaDelimitedString(connectionString.getHosts()));
+			}
+		}
+
+		return target;
+	}
+
+	private static String renderPart(String source, String prefix, @Nullable String part) {
+
+		if (!StringUtils.hasText(part)) {
+			return source;
+		}
+
+		String intermediate = source.replaceFirst(prefix + Pattern.quote(part), "%s*****".formatted(prefix));
+		if (!intermediate.equals(source)) {
+			return intermediate;
+		}
+
+		String encoded = URLEncoder.encode(part, StandardCharsets.UTF_8);
+		return source.replaceFirst(prefix + Pattern.quote(encoded), "%s*****".formatted(prefix));
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/observability/Observer.java

@@ -30,6 +30,7 @@ import org.springframework.lang.Nullable;
  * observability systems.
  *
  * @author Mark Paluch
+ * @since 4.4.9
  */
 class Observer {
 
@@ -40,7 +41,7 @@ class Observer {
 	 *
 	 * @return a new {@link Observer}.
 	 */
-	public static Observer create() {
+	static Observer create() {
 		return new Observer();
 	}
 
@@ -53,7 +54,7 @@ class Observer {
 	 * @return the stateful {@link Observer}.
 	 * @param <C> context type.
 	 */
-	public static <C> Observer fromContext(@Nullable C context, Consumer<? super ContextualObserver<C>> consumer) {
+	static <C> Observer fromContext(@Nullable C context, Consumer<? super ContextualObserver<C>> consumer) {
 
 		Observer contributor = create();
 
@@ -68,7 +69,7 @@ class Observer {
 	 * @param keyValue
 	 * @return
 	 */
-	public Observer contribute(MongoKeyName.MongoKeyValue keyValue) {
+	Observer contribute(MongoKeyName.MongoKeyValue keyValue) {
 
 		keyValues.add(keyValue);
 
@@ -83,7 +84,7 @@ class Observer {
 	 * @return the nested contextual {@link ContextualObserver} that can contribute key-value tuples.
 	 * @param <C>
 	 */
-	public <C> ContextualObserver<C> contextual(@Nullable C context) {
+	<C> ContextualObserver<C> contextual(@Nullable C context) {
 
 		if (context == null) {
 			return new EmptyContextualObserver<>(keyValues);
@@ -92,15 +93,11 @@ class Observer {
 		return new DefaultContextualObserver<>(context, keyValues);
 	}
 
-	public <T> ContextualObserver<T> empty(Class<T> targetType) {
-		return new EmptyContextualObserver<>(this.keyValues);
-	}
-
-	public KeyValues toKeyValues() {
+	KeyValues toKeyValues() {
 		return KeyValues.of(keyValues);
 	}
 
-	public KeyName[] toKeyNames() {
+	KeyName[] toKeyNames() {
 
 		KeyName[] keyNames = new KeyName[keyValues.size()];
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/observability/MongoObservationUnitTests.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2025-present 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.observability;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.nio.charset.StandardCharsets;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import com.mongodb.ConnectionString;
+
+/**
+ * @author Christoph Strobl
+ */
+class MongoObservationUnitTests {
+
+	@ParameterizedTest // GH-5020
+	@MethodSource("connectionStrings")
+	void connectionStringRendering(ConnectionString source, String expected) {
+		assertThat(MongoObservation.connectionString(source)).isEqualTo(expected);
+	}
+
+	private static Stream<Arguments> connectionStrings() {
+
+		return Stream.of(Arguments.of(new ConnectionString(
+				"mongodb+srv://m0ngP%40oUser:m0ngP%40o@cluster0.example.mongodb.net/?retryWrites=true&w=majority"),
+				"mongodb+srv://*****:*****@cluster0.example.mongodb.net/?retryWrites=true&w=majority"), //
+				Arguments.of(new ConnectionString(
+						"mongodb://mongodb:m0ngP%40o@cluster0.example.mongodb.net,cluster1.example.com:1234/?retryWrites=true"),
+						"mongodb://*****:*****@cluster0.example.mongodb.net,cluster1.example.com:1234/?retryWrites=true"), //
+				Arguments.of(
+						new ConnectionString("mongodb://myDatabaseUser@cluster0.example.mongodb.net/?authMechanism=MONGODB-X509"),
+						"mongodb://*****@cluster0.example.mongodb.net/?authMechanism=MONGODB-X509"), //
+				Arguments.of(
+						new ConnectionString("mongodb+srv://myDatabaseUser:mongodb@cluster0.example.mongodb.net/?w=acknowledged"),
+						"mongodb+srv://*****:*****@cluster0.example.mongodb.net/?w=acknowledged"), //
+				Arguments.of(
+						new ConnectionString(
+								new String("mongodb://mongodb:mongodb@localhost:27017".getBytes(), StandardCharsets.US_ASCII)),
+						"mongodb://*****:*****@localhost:27017"),
+				Arguments.of(new ConnectionString("mongodb+srv://cluster0.example.mongodb.net/?retryWrites=true&w=majority"),
+						"mongodb+srv://cluster0.example.mongodb.net/?retryWrites=true&w=majority"), //
+				Arguments.of(
+						new ConnectionString(
+								"mongodb+srv://mongodb:mongodb@cluster0.example.mongodb.net/?retryWrites=true&w=majority"),
+						"mongodb+srv://*****:*****@cluster0.example.mongodb.net/?retryWrites=true&w=majority"));
+	}
+
+}