docs: add samples for transaction isolation level

README.md

@@ -109,6 +109,7 @@ See [Supported Connection Properties](documentation/connection_properties.md) fo
 supported connection properties.
 
 #### Commonly Used Properties
+- default_isolation_level (String): Spanner supports isolation levels REPEATABLE_READ or SERIALIZABLE. SERIALIZABLE is the default. Using isolation level REPEATABLE_READ improves performance by reducing the amount of locks that are taken by transactions that execute a large number of queries in read/write transactions. See https://cloud.google.com/spanner/docs/isolation-levels for more information on the supported isolation levels in Spanner.
 - credentials (String): URL for the credentials file to use for the connection. If you do not specify any credentials at all, the default credentials of the environment as returned by `GoogleCredentials#getApplicationDefault()` is used. Example: `jdbc:cloudspanner:/projects/my-project/instances/my-instance/databases/my-db;credentials=/path/to/credentials.json`
 - autocommit (boolean): Sets the initial autocommit mode for the connection. Default is true.
 - readonly (boolean): Sets the initial readonly mode for the connection. Default is false.

documentation/latency-debugging-guide.md

@@ -5,6 +5,43 @@ queries and transactions and to determine whether transactions or requests are b
 addition, all metrics described in [Latency points in a Spanner request](https://cloud.google.com/spanner/docs/latency-points)
 are also collected by the JDBC driver and can be used for debugging.
 
+## Isolation Level
+
+A common reason for high latency in read/write transactions is lock contention. Spanner by default
+uses isolation level `SERIALIZABLE`. This causes all queries in read/write transactions to take
+locks for all rows that are scanned by a query. Using isolation level `REPEATABLE_READ` reduces the
+number of locks that are taken during a read/write transaction, and can significantly improve
+performance for applications that execute many and/or large queries in read/write transactions.
+
+Enable isolation level `REPEATABLE_READ` by default for all transactions that are executed by the
+JDBC driver by setting the `default_isolation_level` connection property like this in the connection
+URL:
+
+```java
+String projectId = "my-project";
+String instanceId = "my-instance";
+String databaseId = "my-database";
+String isolationLevel = "REPEATABLE_READ";
+
+try (Connection connection =
+    DriverManager.getConnection(
+        String.format(
+            "jdbc:cloudspanner:/projects/%s/instances/%s/databases/%s?default_isolation_level=%s",
+            projectId, instanceId, databaseId, isolationLevel))) {
+  try (Statement statement = connection.createStatement()) {
+    try (ResultSet rs = statement.executeQuery("SELECT CURRENT_TIMESTAMP()")) {
+      while (rs.next()) {
+        System.out.printf(
+            "Connected to Cloud Spanner at [%s]%n", rs.getTimestamp(1).toString());
+      }
+    }
+  }
+}
+```
+
+See https://cloud.google.com/spanner/docs/isolation-levels for more information on the supported
+isolation levels in Spanner.
+
 ## Configuration
 
 You can configure the OpenTelemetry instance that should be used in two ways:

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);
+  }
+
+}

src/test/java/com/google/cloud/spanner/jdbc/JdbcTransactionOptionsTest.java

@@ -27,10 +27,15 @@
 import com.google.cloud.spanner.connection.AbstractMockServerTest;
 import com.google.cloud.spanner.connection.SpannerPool;
 import com.google.spanner.v1.CommitRequest;
+import com.google.spanner.v1.ExecuteSqlRequest;
+import com.google.spanner.v1.TransactionOptions.IsolationLevel;
+import java.sql.Connection;
 import java.sql.DriverManager;
 import java.sql.ResultSet;
 import java.sql.SQLException;
 import java.time.Duration;
+import java.util.Arrays;
+import java.util.stream.Collectors;
 import org.junit.After;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -138,4 +143,63 @@ public void testMaxCommitDelay() throws SQLException {
       assertEquals(Duration.ofMillis(50).toNanos(), request.getMaxCommitDelay().getNanos());
     }
   }
+
+  @Test
+  public void testDefaultIsolationLevel() throws SQLException {
+    for (IsolationLevel isolationLevel :
+        Arrays.stream(IsolationLevel.values())
+            .filter(level -> !level.equals(IsolationLevel.UNRECOGNIZED))
+            .collect(Collectors.toList())) {
+      try (java.sql.Connection connection =
+          DriverManager.getConnection(
+              "jdbc:" + getBaseUrl() + ";default_isolation_level=" + isolationLevel.name())) {
+        connection.setAutoCommit(false);
+        try (ResultSet resultSet =
+            connection.createStatement().executeQuery(SELECT1_STATEMENT.getSql())) {
+          while (resultSet.next()) {
+            // ignore
+          }
+        }
+        connection.commit();
+        assertEquals(1, mockSpanner.countRequestsOfType(ExecuteSqlRequest.class));
+        ExecuteSqlRequest request = mockSpanner.getRequestsOfType(ExecuteSqlRequest.class).get(0);
+        assertTrue(request.hasTransaction());
+        assertTrue(request.getTransaction().hasBegin());
+        assertTrue(request.getTransaction().getBegin().hasReadWrite());
+        assertEquals(isolationLevel, request.getTransaction().getBegin().getIsolationLevel());
+        assertEquals(1, mockSpanner.countRequestsOfType(CommitRequest.class));
+
+        mockSpanner.clearRequests();
+      }
+    }
+  }
+
+  @Test
+  public void testSetIsolationLevel() throws SQLException {
+    try (java.sql.Connection connection = createJdbcConnection()) {
+      connection.setAutoCommit(false);
+      for (int isolationLevel :
+          new int[] {Connection.TRANSACTION_REPEATABLE_READ, Connection.TRANSACTION_SERIALIZABLE}) {
+        connection.setTransactionIsolation(isolationLevel);
+        try (ResultSet resultSet =
+            connection.createStatement().executeQuery(SELECT1_STATEMENT.getSql())) {
+          while (resultSet.next()) {
+            // ignore
+          }
+        }
+        connection.commit();
+        assertEquals(1, mockSpanner.countRequestsOfType(ExecuteSqlRequest.class));
+        ExecuteSqlRequest request = mockSpanner.getRequestsOfType(ExecuteSqlRequest.class).get(0);
+        assertTrue(request.hasTransaction());
+        assertTrue(request.getTransaction().hasBegin());
+        assertTrue(request.getTransaction().getBegin().hasReadWrite());
+        assertEquals(
+            IsolationLevelConverter.convertToSpanner(isolationLevel),
+            request.getTransaction().getBegin().getIsolationLevel());
+        assertEquals(1, mockSpanner.countRequestsOfType(CommitRequest.class));
+
+        mockSpanner.clearRequests();
+      }
+    }
+  }
 }