Polish StringOrBytesSerializer

* Use `Assertions#assertThatIllegalStateException`

Signed-off-by: Tran Ngoc Nhan <[email protected]>

Author: ngocnhan-tran1996

spring-kafka/src/main/java/org/springframework/kafka/support/serializer/StringOrBytesSerializer.java

@@ -27,6 +27,7 @@
  * Convenient when used with one of the Json message converters.
  *
  * @author Gary Russell
+ * @author Ngoc Nhan
  * @since 2.3
  *
  */
@@ -42,21 +43,23 @@ public void configure(Map<String, ?> configs, boolean isKey) {
 	@SuppressWarnings("NullAway") // Dataflow analysis limitation
 	@Override
 	public byte[] serialize(String topic, Object data) {
-		if (data instanceof byte[]) {
-			return (byte[]) data;
-		}
-		else if (data instanceof Bytes) {
-			return ((Bytes) data).get();
+		if (data == null) {
+			return null;
 		}
-		else if (data instanceof String) {
-			return this.stringSerializer.serialize(topic, (String) data);
+
+		if (data instanceof byte[] bytes) {
+			return bytes;
 		}
-		else if (data == null) {
-			return null;
+
+		if (data instanceof Bytes bytes) {
+			return bytes.get();
 		}
-		else {
-			throw new IllegalStateException("This serializer can only handle byte[], Bytes or String values");
+
+		if (data instanceof String string) {
+			return this.stringSerializer.serialize(topic, string);
 		}
+
+		throw new IllegalStateException("This serializer can only handle byte[], Bytes or String values");
 	}
 
 	@Override

spring-kafka/src/test/java/org/springframework/kafka/support/serializer/StringOrBytesSerializerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2024 the original author or authors.
+ * Copyright 2019-2025 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.
@@ -26,10 +26,12 @@
 import org.springframework.kafka.test.utils.KafkaTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
 /**
  * @author Gary Russell
  * @author Soby Chacko
+ * @author Ngoc Nhan
  * @since 2.3
  *
  */
@@ -51,6 +53,9 @@ void test() {
 		Map<String, Object> configs = Collections.singletonMap("serializer.encoding", "UTF-16");
 		serializer.configure(configs, false);
 		assertThat(KafkaTestUtils.getPropertyValue(serializer, "stringSerializer.encoding")).isEqualTo(StandardCharsets.UTF_16);
+		assertThat(serializer.serialize("null", null)).isNull();
+		assertThatIllegalStateException().isThrownBy(() -> serializer.serialize("ex", 0))
+				.withMessage("This serializer can only handle byte[], Bytes or String values");
 	}
 
 }