docs: add samples for transaction isolation level

samples/snippets/pom.xml

@@ -23,6 +23,13 @@
   <!-- [START spanner-jdbc_install_with_bom] -->
   <dependencyManagement>
     <dependencies>
+      <dependency>
+        <groupId>com.google.cloud</groupId>
+        <artifactId>google-cloud-spanner-bom</artifactId>
+        <version>6.91.1</version>
+        <type>pom</type>
+        <scope>import</scope>
+      </dependency>
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>libraries-bom</artifactId>
@@ -37,6 +44,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-spanner-jdbc</artifactId>
+      <version>2.29.1</version>
       <exclusions>
         <exclusion>
           <groupId>com.google.api.grpc</groupId>

samples/snippets/src/main/java/com/example/spanner/jdbc/IsolationLevel.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2025 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.jdbc;
+
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.PreparedStatement;
+import java.sql.ResultSet;
+import java.sql.SQLException;
+import java.util.Properties;
+
+final class IsolationLevel {
+
+  static void isolationLevel(
+      final String project,
+      final String instance,
+      final String database,
+      final Properties properties)
+      throws SQLException {
+    String url = String.format(
+        "jdbc:cloudspanner:/projects/%s/instances/%s/databases/%s",
+        project, instance, database);
+    try (Connection connection = DriverManager.getConnection(url, properties)) {
+      connection.setAutoCommit(false);
+
+      // Spanner supports setting the isolation level to:
+      // 1. TRANSACTION_SERIALIZABLE (this is the default)
+      // 2. TRANSACTION_REPEATABLE_READ
+
+      // The following line sets the default isolation level that will be used
+      // for all read/write transactions on this connection.
+      connection.setTransactionIsolation(
+          Connection.TRANSACTION_REPEATABLE_READ);
+
+      // This query will not take any locks when using
+      // isolation level repeatable read.
+      try (ResultSet resultSet = connection
+          .createStatement()
+          .executeQuery("SELECT SingerId, Active "
+              + "FROM Singers "
+              + "ORDER BY LastName")) {
+        while (resultSet.next()) {
+          try (PreparedStatement statement = connection.prepareStatement(
+              "INSERT OR UPDATE INTO SingerHistory "
+                  + "(SingerId, Active, CreatedAt) "
+                  + "VALUES (?, ?, CURRENT_TIMESTAMP)")) {
+            statement.setLong(1, resultSet.getLong(1));
+            statement.setBoolean(2, resultSet.getBoolean(2));
+            statement.executeUpdate();
+          }
+        }
+      }
+      connection.commit();
+    }
+  }
+
+  private IsolationLevel() {
+  }
+}

samples/snippets/src/test/java/com/example/spanner/jdbc/IsolationLevelTest.java

@@ -0,0 +1,97 @@
+/*
+ * Copyright 2025 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.jdbc;
+
+import static com.example.spanner.jdbc.IsolationLevel.isolationLevel;
+import static org.junit.Assume.assumeTrue;
+
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.SQLException;
+import java.sql.Statement;
+import java.util.Properties;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+import org.testcontainers.DockerClientFactory;
+import org.testcontainers.containers.GenericContainer;
+import org.testcontainers.containers.wait.strategy.Wait;
+import org.testcontainers.images.PullPolicy;
+import org.testcontainers.utility.DockerImageName;
+
+@RunWith(JUnit4.class)
+public class IsolationLevelTest {
+
+  private static GenericContainer<?> emulator;
+
+  private static final String PROJECT = "emulator-project";
+  private static final String INSTANCE = "my-instance";
+  private static final String DATABASE = "my-database";
+  private static final Properties PROPERTIES = new Properties();
+
+  @BeforeClass
+  public static void setupEmulator() throws Exception {
+    assumeTrue("This test requires Docker", DockerClientFactory.instance().isDockerAvailable());
+
+    emulator =
+        new GenericContainer<>(DockerImageName.parse("gcr.io/cloud-spanner-emulator/emulator"));
+    emulator.withImagePullPolicy(PullPolicy.alwaysPull());
+    emulator.addExposedPort(9010);
+    emulator.setWaitStrategy(Wait.forListeningPorts(9010));
+    emulator.start();
+
+    PROPERTIES.setProperty("endpoint",
+        String.format("localhost:%d", emulator.getMappedPort(9010)));
+    PROPERTIES.setProperty("autoConfigEmulator", "true");
+
+    String url = String.format(
+        "jdbc:cloudspanner:/projects/%s/instances/%s/databases/%s",
+        PROJECT, INSTANCE, DATABASE);
+    try (Connection connection = DriverManager.getConnection(url, PROPERTIES)) {
+      try (Statement statement = connection.createStatement()) {
+        statement.addBatch(
+            "CREATE TABLE Singers ("
+                + "SingerId INT64 PRIMARY KEY, "
+                + "FirstName STRING(MAX), "
+                + "LastName STRING(MAX), "
+                + "Active BOOL)");
+        statement.addBatch(
+            "CREATE TABLE SingerHistory ("
+                + "SingerId INT64, "
+                + "Active BOOL, "
+                + "CreatedAt TIMESTAMP) "
+                + "PRIMARY KEY (SingerId, CreatedAt)");
+        statement.executeBatch();
+      }
+    }
+  }
+
+  @AfterClass
+  public static void stopEmulator() {
+    if (emulator != null) {
+      emulator.stop();
+    }
+  }
+
+  @Test
+  public void testIsolationLevel() throws SQLException {
+    isolationLevel("emulator-project", "my-instance", "my-database", PROPERTIES);
+  }
+
+}