diff --git a/handler/src/main/java/io/jeyong/handler/ApplicationTerminator.java b/handler/src/main/java/io/jeyong/handler/ApplicationTerminator.java
index e33d0bc..97d56dd 100644
--- a/handler/src/main/java/io/jeyong/handler/ApplicationTerminator.java
+++ b/handler/src/main/java/io/jeyong/handler/ApplicationTerminator.java
@@ -18,11 +18,15 @@ protected ApplicationTerminator(final String terminationMessagePath, final Strin
 
     public SignalHandler handleTermination() {
         return signal -> {
-            logger.info("Received SIGTERM signal. Initiating termination handler.");
+            logTerminationSignal(signal.getName());
             FileUtils.writeToFile(terminationMessagePath, terminationMessage);
             System.exit(getExitCode());
         };
     }
 
+    private void logTerminationSignal(final String signalName) {
+        logger.info("Received {} signal. Initiating termination handler.", signalName);
+    }
+
     protected abstract int getExitCode();
 }
diff --git a/handler/src/main/java/io/jeyong/handler/FileUtils.java b/handler/src/main/java/io/jeyong/handler/FileUtils.java
index b8007f1..b89bec3 100644
--- a/handler/src/main/java/io/jeyong/handler/FileUtils.java
+++ b/handler/src/main/java/io/jeyong/handler/FileUtils.java
@@ -7,7 +7,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-public class FileUtils {
+public final class FileUtils {
 
     private static final Logger logger = LoggerFactory.getLogger(FileUtils.class);
 
diff --git a/handler/src/main/java/io/jeyong/handler/SignalHandlerRegistrar.java b/handler/src/main/java/io/jeyong/handler/SignalHandlerRegistrar.java
index 569abff..08a6a02 100644
--- a/handler/src/main/java/io/jeyong/handler/SignalHandlerRegistrar.java
+++ b/handler/src/main/java/io/jeyong/handler/SignalHandlerRegistrar.java
@@ -1,9 +1,10 @@
 package io.jeyong.handler;
 
-import jakarta.annotation.PostConstruct;
+import org.springframework.context.event.ContextRefreshedEvent;
+import org.springframework.context.event.EventListener;
 import sun.misc.Signal;
 
-public class SignalHandlerRegistrar {
+public final class SignalHandlerRegistrar {
 
     private final ApplicationTerminator applicationTerminator;
     private final String signalType;
@@ -13,7 +14,7 @@ public SignalHandlerRegistrar(final ApplicationTerminator applicationTerminator,
         this.signalType = signalType;
     }
 
-    @PostConstruct
+    @EventListener(ContextRefreshedEvent.class)
     public void registerHandler() {
         Signal.handle(new Signal(signalType), applicationTerminator.handleTermination());
     }
diff --git a/handler/src/main/java/io/jeyong/handler/SigtermHandlerProperties.java b/handler/src/main/java/io/jeyong/handler/SigtermHandlerProperties.java
index 5fa0bbd..f5812e8 100644
--- a/handler/src/main/java/io/jeyong/handler/SigtermHandlerProperties.java
+++ b/handler/src/main/java/io/jeyong/handler/SigtermHandlerProperties.java
@@ -51,7 +51,10 @@
  */
 // @formatter:on
 @ConfigurationProperties(prefix = "kubernetes.sigterm-handler")
-public class SigtermHandlerProperties {
+public final class SigtermHandlerProperties {
+
+    private static final int MIN_EXIT_CODE = 0;
+    private static final int MAX_EXIT_CODE = 255;
 
     private boolean enabled = true;
     private int exitCode = 0;
@@ -71,6 +74,7 @@ public int getExitCode() {
     }
 
     public void setExitCode(final int exitCode) {
+        validateExitCode(exitCode);
         this.exitCode = exitCode;
     }
 
@@ -89,4 +93,18 @@ public String getTerminationMessage() {
     public void setTerminationMessage(final String terminationMessage) {
         this.terminationMessage = terminationMessage;
     }
+
+    /**
+     * Validates the exit code to ensure it adheres to the POSIX standard.
+     *
+     * @param exitCode the exit code to validate
+     * @throws IllegalArgumentException if the exit code is outside the valid range
+     */
+    private void validateExitCode(final int exitCode) {
+        if (exitCode < MIN_EXIT_CODE || exitCode > MAX_EXIT_CODE) {
+            throw new IllegalArgumentException(
+                    String.format("Exit code must be between %d and %d (inclusive) as per POSIX standard.",
+                            MIN_EXIT_CODE, MAX_EXIT_CODE));
+        }
+    }
 }
diff --git a/handler/src/main/java/io/jeyong/handler/SpringContextTerminator.java b/handler/src/main/java/io/jeyong/handler/SpringContextTerminator.java
index 3053305..0562f08 100644
--- a/handler/src/main/java/io/jeyong/handler/SpringContextTerminator.java
+++ b/handler/src/main/java/io/jeyong/handler/SpringContextTerminator.java
@@ -3,7 +3,7 @@
 import org.springframework.boot.SpringApplication;
 import org.springframework.context.ApplicationContext;
 
-public class SpringContextTerminator extends ApplicationTerminator {
+public final class SpringContextTerminator extends ApplicationTerminator {
 
     private final ApplicationContext applicationContext;
     private final int exitCode;
diff --git a/test/src/test/java/io/jeyong/test/unit/ApplicationTerminatorTest.java b/test/src/test/java/io/jeyong/test/unit/ApplicationTerminatorTest.java
index 0b15005..02a5598 100644
--- a/test/src/test/java/io/jeyong/test/unit/ApplicationTerminatorTest.java
+++ b/test/src/test/java/io/jeyong/test/unit/ApplicationTerminatorTest.java
@@ -8,6 +8,7 @@
 import java.nio.file.Files;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
+import sun.misc.Signal;
 import sun.misc.SignalHandler;
 
 @DisplayName("ApplicationTerminator Unit Test")
@@ -30,7 +31,7 @@ protected int getExitCode() {
         };
 
         // when
-        catchSystemExit(() -> terminator.handleTermination().handle(null));
+        catchSystemExit(() -> terminator.handleTermination().handle(new Signal("TERM")));
 
         // then
         assertThat(Files.readString(tempFile.toPath())).isEqualTo(expectedMessage);
@@ -54,7 +55,7 @@ protected int getExitCode() {
         SignalHandler handler = terminator.handleTermination();
 
         // when
-        int actualExitCode = catchSystemExit(() -> handler.handle(null));
+        int actualExitCode = catchSystemExit(() -> handler.handle(new Signal("TERM")));
 
         // then
         assertThat(actualExitCode).isEqualTo(expectedExitCode);