diff --git a/spring-jdbc/src/main/java/org/springframework/jdbc/datasource/lookup/IsolationLevelDataSourceRouter.java b/spring-jdbc/src/main/java/org/springframework/jdbc/datasource/lookup/IsolationLevelDataSourceRouter.java
index dcfc8a60482..5705c109e35 100644
--- a/spring-jdbc/src/main/java/org/springframework/jdbc/datasource/lookup/IsolationLevelDataSourceRouter.java
+++ b/spring-jdbc/src/main/java/org/springframework/jdbc/datasource/lookup/IsolationLevelDataSourceRouter.java
@@ -16,11 +16,12 @@
 
 package org.springframework.jdbc.datasource.lookup;
 
-import org.springframework.core.Constants;
+import java.util.Map;
+
 import org.springframework.lang.Nullable;
 import org.springframework.transaction.TransactionDefinition;
-import org.springframework.transaction.support.DefaultTransactionDefinition;
 import org.springframework.transaction.support.TransactionSynchronizationManager;
+import org.springframework.util.Assert;
 
 /**
  * DataSource that routes to one of various target DataSources based on the
@@ -82,6 +83,7 @@ import org.springframework.transaction.support.TransactionSynchronizationManager
  * &lt;/bean&gt;</pre>
  *
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 2.0.1
  * @see #setTargetDataSources
  * @see #setDefaultTargetDataSource
@@ -93,8 +95,17 @@ import org.springframework.transaction.support.TransactionSynchronizationManager
  */
 public class IsolationLevelDataSourceRouter extends AbstractRoutingDataSource {
 
-	/** Constants instance for TransactionDefinition. */
-	private static final Constants constants = new Constants(TransactionDefinition.class);
+	/**
+	 * Map of constant names to constant values for the isolation constants
+	 * defined in {@link TransactionDefinition}.
+	 */
+	static final Map<String, Integer> constants = Map.of(
+			"ISOLATION_DEFAULT", TransactionDefinition.ISOLATION_DEFAULT,
+			"ISOLATION_READ_UNCOMMITTED", TransactionDefinition.ISOLATION_READ_UNCOMMITTED,
+			"ISOLATION_READ_COMMITTED", TransactionDefinition.ISOLATION_READ_COMMITTED,
+			"ISOLATION_REPEATABLE_READ", TransactionDefinition.ISOLATION_REPEATABLE_READ,
+			"ISOLATION_SERIALIZABLE", TransactionDefinition.ISOLATION_SERIALIZABLE
+		);
 
 
 	/**
@@ -104,14 +115,16 @@ public class IsolationLevelDataSourceRouter extends AbstractRoutingDataSource {
 	 */
 	@Override
 	protected Object resolveSpecifiedLookupKey(Object lookupKey) {
-		if (lookupKey instanceof Integer) {
-			return lookupKey;
+		if (lookupKey instanceof Integer isolationLevel) {
+			Assert.isTrue(constants.containsValue(isolationLevel),
+					"Only values of isolation constants allowed");
+			return isolationLevel;
 		}
 		else if (lookupKey instanceof String constantName) {
-			if (!constantName.startsWith(DefaultTransactionDefinition.PREFIX_ISOLATION)) {
-				throw new IllegalArgumentException("Only isolation constants allowed");
-			}
-			return constants.asNumber(constantName);
+			Assert.hasText(constantName, "'lookupKey' must not be null or blank");
+			Integer isolationLevel = constants.get(constantName);
+			Assert.notNull(isolationLevel, "Only isolation constants allowed");
+			return isolationLevel;
 		}
 		else {
 			throw new IllegalArgumentException(
diff --git a/spring-jdbc/src/test/java/org/springframework/jdbc/datasource/lookup/IsolationLevelDataSourceRouterTests.java b/spring-jdbc/src/test/java/org/springframework/jdbc/datasource/lookup/IsolationLevelDataSourceRouterTests.java
new file mode 100644
index 00000000000..f97d77a59cb
--- /dev/null
+++ b/spring-jdbc/src/test/java/org/springframework/jdbc/datasource/lookup/IsolationLevelDataSourceRouterTests.java
@@ -0,0 +1,98 @@
+/*
+ * Copyright 2002-2023 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.jdbc.datasource.lookup;
+
+import java.lang.reflect.Field;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.transaction.TransactionDefinition;
+import org.springframework.util.ReflectionUtils;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.springframework.transaction.TransactionDefinition.ISOLATION_DEFAULT;
+import static org.springframework.transaction.TransactionDefinition.ISOLATION_READ_COMMITTED;
+import static org.springframework.transaction.TransactionDefinition.ISOLATION_READ_UNCOMMITTED;
+import static org.springframework.transaction.TransactionDefinition.ISOLATION_REPEATABLE_READ;
+import static org.springframework.transaction.TransactionDefinition.ISOLATION_SERIALIZABLE;
+
+/**
+ * Tests for {@link IsolationLevelDataSourceRouter}.
+ *
+ * @author Sam Brannen
+ * @since 6.1
+ */
+class IsolationLevelDataSourceRouterTests {
+
+	private final IsolationLevelDataSourceRouter router = new IsolationLevelDataSourceRouter();
+
+
+	@Test
+	void resolveSpecifiedLookupKeyForInvalidTypes() {
+		assertThatIllegalArgumentException().isThrownBy(() -> router.resolveSpecifiedLookupKey(new Object()));
+		assertThatIllegalArgumentException().isThrownBy(() -> router.resolveSpecifiedLookupKey('X'));
+	}
+
+	@Test
+	void resolveSpecifiedLookupKeyByNameForUnsupportedValues() {
+		assertThatIllegalArgumentException().isThrownBy(() -> router.resolveSpecifiedLookupKey(null));
+		assertThatIllegalArgumentException().isThrownBy(() -> router.resolveSpecifiedLookupKey("   "));
+		assertThatIllegalArgumentException().isThrownBy(() -> router.resolveSpecifiedLookupKey("bogus"));
+	}
+
+	/**
+	 * Verify that the internal 'constants' map is properly configured for all
+	 * ISOLATION_ constants defined in {@link TransactionDefinition}.
+	 */
+	@Test
+	void resolveSpecifiedLookupKeyByNameForAllSupportedValues() {
+		Set<Integer> uniqueValues = new HashSet<>();
+		streamIsolationConstants()
+				.forEach(name -> {
+					Integer isolationLevel = (Integer) router.resolveSpecifiedLookupKey(name);
+					Integer expected = IsolationLevelDataSourceRouter.constants.get(name);
+					assertThat(isolationLevel).isEqualTo(expected);
+					uniqueValues.add(isolationLevel);
+				});
+		assertThat(uniqueValues).containsExactlyInAnyOrderElementsOf(IsolationLevelDataSourceRouter.constants.values());
+	}
+
+	@Test
+	void resolveSpecifiedLookupKeyByInteger() {
+		assertThatIllegalArgumentException().isThrownBy(() -> router.resolveSpecifiedLookupKey(999));
+
+		assertThat(router.resolveSpecifiedLookupKey(ISOLATION_DEFAULT)).isEqualTo(ISOLATION_DEFAULT);
+		assertThat(router.resolveSpecifiedLookupKey(ISOLATION_READ_UNCOMMITTED)).isEqualTo(ISOLATION_READ_UNCOMMITTED);
+		assertThat(router.resolveSpecifiedLookupKey(ISOLATION_READ_COMMITTED)).isEqualTo(ISOLATION_READ_COMMITTED);
+		assertThat(router.resolveSpecifiedLookupKey(ISOLATION_REPEATABLE_READ)).isEqualTo(ISOLATION_REPEATABLE_READ);
+		assertThat(router.resolveSpecifiedLookupKey(ISOLATION_SERIALIZABLE)).isEqualTo(ISOLATION_SERIALIZABLE);
+	}
+
+
+	private static Stream<String> streamIsolationConstants() {
+		return Arrays.stream(TransactionDefinition.class.getFields())
+				.filter(ReflectionUtils::isPublicStaticFinal)
+				.map(Field::getName)
+				.filter(name -> name.startsWith("ISOLATION_"));
+	}
+
+}