diff --git a/Jenkinsfile b/Jenkinsfile
index 1e731da27..f82132330 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -6,8 +6,9 @@ buildPlugin(
   useContainerAgent: false,
   // Show failures on all configurations
   failFast: false,
-  // Test Java 11 with minimum Jenkins version, Java 17 with a more recent version
+  // Test Java 11, 17, and 21
   configurations: [
     [platform: 'linux', jdk: 17],
+    [platform: 'linux',   jdk: '21', jenkins: '2.401.3'],
     [platform: 'windows', jdk: 11],
 ])
diff --git a/src/test/java/io/jenkins/docker/connector/DockerComputerAttachConnectorTest.java b/src/test/java/io/jenkins/docker/connector/DockerComputerAttachConnectorTest.java
index d8e81950c..de51bea9a 100644
--- a/src/test/java/io/jenkins/docker/connector/DockerComputerAttachConnectorTest.java
+++ b/src/test/java/io/jenkins/docker/connector/DockerComputerAttachConnectorTest.java
@@ -21,9 +21,9 @@ public void connectAgentViaDirectAttach() throws Exception {
     @Test
     public void connectAgentViaDirectAttachWithCustomCmd() throws Exception {
         final DockerComputerAttachConnector connector = new DockerComputerAttachConnector(COMMON_IMAGE_USERNAME);
-        // We could setJavaExe("/usr/local/openjdk-8/bin/java") too, but that'd mean
-        // we'd break the instant that the public docker image's JVM updates to 9 or
-        // higher; best stick to just using the $PATH.
+        // We could setJavaExe("/opt/jdk-11/bin/java") too, but that'd
+        // mean we'd break the instant that the public docker image's JVM
+        // updates to a newer JDK; best stick to just using the $PATH.
         connector.setJvmArgsString("-Xmx1g" + "\n" + "-Dfoo=bar\n");
         connector.setEntryPointCmdString("java\n"
                 + "${" + ArgumentVariables.JvmArgs.getName() + "}\n"
diff --git a/src/test/java/io/jenkins/docker/connector/DockerComputerConnectorTest.java b/src/test/java/io/jenkins/docker/connector/DockerComputerConnectorTest.java
index 355488dbd..3ed96e415 100644
--- a/src/test/java/io/jenkins/docker/connector/DockerComputerConnectorTest.java
+++ b/src/test/java/io/jenkins/docker/connector/DockerComputerConnectorTest.java
@@ -48,19 +48,12 @@ private static int getJavaVersion() {
         final String systemPropertyName = "java.version";
         final String javaVersion = System.getProperty(systemPropertyName);
         try {
-            if (javaVersion.startsWith("1.")) {
-                // we're using Java 8 or lower so the syntax is 1.x where x is the version we
-                // want.
-                // ... and we know that x will be a single digit.
-                // e.g. 1.8.1 is Java 8, 1.6.3 is Java 6 etc.
-                final String secondNumber = javaVersion.substring(2, 3);
-                return Integer.parseInt(secondNumber);
-            }
-            // otherwise we're using Java 9 or higher so the syntax is x.n...
+            // We're using Java 9 or higher so the syntax is x.n...
             // ... but x might be multiple digits.
             // e.g. 9.0 is Java 9, 11.123.4 is Java 11 etc.
+            // Early access builds report as "21-ea".  Remove all text after "-".
             final int indexOfPeriod = javaVersion.indexOf('.');
-            final String firstNumber = indexOfPeriod < 0 ? javaVersion : javaVersion.substring(0, indexOfPeriod);
+            final String firstNumber = javaVersion.replaceAll("[-.].*", "");
             return Integer.parseInt(firstNumber);
         } catch (RuntimeException ex) {
             throw new IllegalStateException(