diff --git a/.github/workflows/gradle.yml b/.github/workflows/gradle.yml
index bca2005d1..ef59dc8bd 100644
--- a/.github/workflows/gradle.yml
+++ b/.github/workflows/gradle.yml
@@ -21,7 +21,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: [ '17' ] # We want to test on LTSs.
+        java: # We want to test on LTSs.
+          - '17'
+          - '21'
     steps:
       - name: actions/setup-java@v3 (JDK ${{ matrix.java }})
         uses: actions/setup-java@v4
diff --git a/clients/line-bot-client-base/src/test/java/com/linecorp/bot/client/base/exception/AbstractLineClientExceptionTest.java b/clients/line-bot-client-base/src/test/java/com/linecorp/bot/client/base/exception/AbstractLineClientExceptionTest.java
index 2c8b269ef..a8a273b2a 100644
--- a/clients/line-bot-client-base/src/test/java/com/linecorp/bot/client/base/exception/AbstractLineClientExceptionTest.java
+++ b/clients/line-bot-client-base/src/test/java/com/linecorp/bot/client/base/exception/AbstractLineClientExceptionTest.java
@@ -20,6 +20,7 @@
 import static org.mockito.Mockito.when;
 
 import java.io.IOException;
+import java.net.URI;
 import java.net.URL;
 import java.util.Map;
 
@@ -71,7 +72,7 @@ void constructorNoException() {
     @Test
     void getRequestUrl() throws Exception {
         AbstractLineClientException exception = new AbstractLineClientException(response, "test message");
-        assertThat(exception.getRequestUrl()).isEqualTo(new URL("https://example.com/"));
+        assertThat(exception.getRequestUrl()).isEqualTo(new URI("https://example.com/").toURL());
     }
 
     @Test
diff --git a/spring-boot/line-bot-spring-boot-handler/src/main/java/com/linecorp/bot/spring/boot/handler/support/LineMessageHandlerSupport.java b/spring-boot/line-bot-spring-boot-handler/src/main/java/com/linecorp/bot/spring/boot/handler/support/LineMessageHandlerSupport.java
index 0c58bae46..bb355bcc2 100644
--- a/spring-boot/line-bot-spring-boot-handler/src/main/java/com/linecorp/bot/spring/boot/handler/support/LineMessageHandlerSupport.java
+++ b/spring-boot/line-bot-spring-boot-handler/src/main/java/com/linecorp/bot/spring/boot/handler/support/LineMessageHandlerSupport.java
@@ -20,6 +20,8 @@
 import java.util.Collection;
 import java.util.List;
 
+import jakarta.annotation.PostConstruct;
+
 import org.slf4j.Logger;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
@@ -69,7 +71,10 @@ public LineMessageHandlerSupport(
             final ConfigurableApplicationContext applicationContext) {
         this.returnValueConsumerFactory = returnValueConsumerFactory;
         this.applicationContext = applicationContext;
+    }
 
+    @PostConstruct
+    public void initialize() {
         applicationContext.addApplicationListener(event -> {
             if (event instanceof ContextRefreshedEvent) {
                 refresh();