diff --git a/.github/workflows/unit-test.yml b/.github/workflows/unit-test.yml
index 0563428..e21f9d8 100644
--- a/.github/workflows/unit-test.yml
+++ b/.github/workflows/unit-test.yml
@@ -7,17 +7,22 @@ on:
     branches: [ main ]
 
 jobs:
-  gradle:
+  build:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        java: [ '11', '17', '21' ]
+    name: Java ${{ matrix.Java }}
+
     steps:
-    - uses: actions/checkout@v3
-    - uses: actions/setup-java@v3
+    - uses: actions/checkout@v4
+    - uses: actions/setup-java@v4
       with:
         distribution: temurin
-        java-version: 17
+        java-version: ${{ matrix.java }}
 
     - name: Setup Gradle
-      uses: gradle/gradle-build-action@v2
+      uses: gradle/actions/setup-gradle@v4
 
     - name: Execute Gradle build
       run: ./gradlew build
diff --git a/lib/build.gradle b/lib/build.gradle
index 4238668..830931d 100644
--- a/lib/build.gradle
+++ b/lib/build.gradle
@@ -33,7 +33,23 @@ java {
 }
 
 compileJava {
-    options.release = 8
+    // Release option was added in JDK 9.
+    if (JavaVersion.current().isJava9Compatible()) {
+        options.release = 8
+    } else {
+        sourceCompatibility = '1.8'
+        targetCompatibility = '1.8'
+    }
+}
+
+sourceSets {
+    test {
+        java {
+            if (JavaVersion.current() < JavaVersion.VERSION_15) {
+                exclude '**/TestCredentialJdk15.java'
+            }
+        }
+    }
 }
 
 publishing {
diff --git a/lib/src/test/java/fi/protonode/certy/TestCredentialJdk15.java b/lib/src/test/java/fi/protonode/certy/TestCredentialJdk15.java
new file mode 100644
index 0000000..fbe5022
--- /dev/null
+++ b/lib/src/test/java/fi/protonode/certy/TestCredentialJdk15.java
@@ -0,0 +1,50 @@
+/*
+ * Copyright Certy 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
+ *
+ *     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 fi.protonode.certy;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.security.cert.X509Certificate;
+import java.security.interfaces.EdECPublicKey;
+
+import org.junit.jupiter.api.Test;
+
+import fi.protonode.certy.Credential.KeyType;
+
+/**
+ * Test cases for Credential class using JDK 15 and later.
+ */
+class TestCredentialJdk15 {
+
+    @Test
+    void testEd25519Certificate() throws Exception {
+        Credential cred = new Credential().subject("CN=joe")
+                .keyType(KeyType.ED25519);
+        X509Certificate cert = cred.getX509Certificate();
+        assertNotNull(cert);
+        EdECPublicKey key = (EdECPublicKey) cert.getPublicKey();
+        assertEquals("Ed25519", key.getAlgorithm());
+    }
+
+    @Test
+    void testInvalidKeySize() {
+        Credential cred3 = new Credential().subject("CN=joe").keyType(KeyType.ED25519).keySize(1);
+        assertThrows(IllegalArgumentException.class, () -> cred3.getX509Certificate());
+    }
+
+}
diff --git a/lib/src/test/java/fi/protonode/certy/TestCredential.java b/lib/src/test/java/fi/protonode/certy/TestCredentialJdk8.java
similarity index 96%
rename from lib/src/test/java/fi/protonode/certy/TestCredential.java
rename to lib/src/test/java/fi/protonode/certy/TestCredentialJdk8.java
index 858b404..3d38423 100644
--- a/lib/src/test/java/fi/protonode/certy/TestCredential.java
+++ b/lib/src/test/java/fi/protonode/certy/TestCredentialJdk8.java
@@ -49,7 +49,6 @@
 import java.security.cert.CertificateFactory;
 import java.security.cert.X509Certificate;
 import java.security.interfaces.ECPublicKey;
-import java.security.interfaces.EdECPublicKey;
 import java.security.interfaces.RSAPublicKey;
 import java.security.spec.ECParameterSpec;
 import java.security.spec.InvalidKeySpecException;
@@ -60,7 +59,10 @@
 import java.util.Arrays;
 import java.util.Date;
 
-class TestCredential {
+/**
+ * Test cases for Credential class using JDK 1.8 and later.
+ */
+class TestCredentialJdk8 {
 
     @Test
     void testSubjectName() throws Exception {
@@ -115,16 +117,6 @@ void testRsaKeySizes() throws Exception {
         expectKey(cred.getX509Certificate(), "RSA", 4096);
     }
 
-    @Test
-    void testEd25519Certificate() throws Exception {
-        Credential cred = new Credential().subject("CN=joe")
-                .keyType(KeyType.ED25519);
-        X509Certificate cert = cred.getX509Certificate();
-        assertNotNull(cert);
-        EdECPublicKey key = (EdECPublicKey) cert.getPublicKey();
-        assertEquals("Ed25519", key.getAlgorithm());
-    }
-
     @Test
     void testExpires() throws Exception {
         Duration hour = Duration.of(1, ChronoUnit.HOURS);
@@ -266,9 +258,6 @@ void testInvalidKeySize() {
 
         Credential cred2 = new Credential().subject("CN=joe").keyType(KeyType.RSA).keySize(1);
         assertThrows(IllegalArgumentException.class, () -> cred2.getX509Certificate());
-
-        Credential cred3 = new Credential().subject("CN=joe").keyType(KeyType.ED25519).keySize(1);
-        assertThrows(IllegalArgumentException.class, () -> cred3.getX509Certificate());
     }
 
     @Test
@@ -423,10 +412,8 @@ void testCrlDistributionPointUri() throws Exception {
         assertNotNull(dps);
 
         DistributionPointName expected = new DistributionPointName(
-            new GeneralNames(
-                new GeneralName(GeneralName.uniformResourceIdentifier, "http://example.com/crl.pem")
-            )
-        );
+                new GeneralNames(
+                        new GeneralName(GeneralName.uniformResourceIdentifier, "http://example.com/crl.pem")));
 
         assertArrayEquals(new DistributionPoint[] { new DistributionPoint(expected, null, null) }, dps);
     }