doc: add MutableCredentials Example

Author: ldetmer

samples/install-without-bom/pom.xml

@@ -33,7 +33,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-spanner</artifactId>
-      <version>6.110.0</version>
+      <version>6.111.1</version>
     </dependency>
     <!-- [END spanner_install_without_bom] -->
 

samples/snippets/src/main/java/com/example/spanner/MutableCredentialsExample.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * 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
+ *
+ *       http://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 com.example.spanner;
+
+// [START spanner_mutable_credentials]
+
+import com.google.auth.oauth2.ServiceAccountCredentials;
+import com.google.cloud.spanner.Spanner;
+import com.google.cloud.spanner.SpannerOptions;
+import com.google.cloud.spanner.admin.database.v1.DatabaseAdminClient;
+import com.google.cloud.spanner.connection.MutableCredentials;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.nio.file.attribute.FileTime;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.TimeUnit;
+
+public class MutableCredentialsExample {
+
+  static void createClientWithMutableCredentials() throws IOException {
+    final String credentialsPath = "location_of_service_account_credential_json";
+    Path path = Paths.get(credentialsPath);
+    // Use an array to hold the mutable lastModifiedTime so it can be accessed in the lambda
+    FileTime[] lastModifiedTime = new FileTime[] {Files.getLastModifiedTime(path)};
+
+    // 1 - create service account credentials
+    ServiceAccountCredentials serviceAccountCredentials;
+    try (FileInputStream is = new FileInputStream(credentialsPath)) {
+      serviceAccountCredentials = ServiceAccountCredentials.fromStream(is);
+    }
+
+    // 2 - wrap credentials from step 1 in a MutableCredentials instance
+    MutableCredentials mutableCredentials = new MutableCredentials(serviceAccountCredentials);
+
+    // 3 - set credentials on your SpannerOptions builder to your mutableCredentials
+    SpannerOptions options = SpannerOptions.newBuilder().setCredentials(mutableCredentials).build();
+
+    // 4 - include logic for when/how to update your mutableCredentials
+    // In this example we'll use a SchedulerExecutorService to periodically check for updates
+    ThreadFactory daemonThreadFactory =
+        runnable -> {
+          Thread thread = new Thread(runnable, "spanner-mutable-credentials-rotator");
+          thread.setDaemon(true);
+          return thread;
+        };
+    ScheduledExecutorService executorService =
+        Executors.newSingleThreadScheduledExecutor(daemonThreadFactory);
+    executorService.scheduleAtFixedRate(
+        () -> {
+          try {
+            FileTime currentModifiedTime = Files.getLastModifiedTime(path);
+            if (currentModifiedTime.compareTo(lastModifiedTime[0]) > 0) {
+              lastModifiedTime[0] = currentModifiedTime;
+              try (FileInputStream is = new FileInputStream(credentialsPath)) {
+                ServiceAccountCredentials credentials = ServiceAccountCredentials.fromStream(is);
+                mutableCredentials.updateCredentials(credentials);
+              }
+            }
+          } catch (IOException e) {
+            System.err.println("Failed to check or update credentials: " + e.getMessage());
+          }
+        },
+        15,
+        15,
+        TimeUnit.MINUTES);
+
+    // 5. Use the client
+    try (Spanner spanner = options.getService();
+        DatabaseAdminClient databaseAdminClient = spanner.createDatabaseAdminClient()) {
+      // Perform operations...
+      // long running client operations will always use the latest credentials wrapped in
+      // mutableCredentials
+    } finally {
+      // Ensure the executor is shut down when the application exits or the client is closed
+      executorService.shutdown();
+    }
+  }
+}
+// [END spanner_mutable_credentials]