GH-3466: Optimize KafkaAdmin creation in KafkaTemplate (#3471)

* GH-3466: Optimize KafkaAdmin creation in KafkaTemplate

Fixes: #3466

#3466

Improve bootstrap-server config comparison to avoid unnecessary
KafkaAdmin recreation. This addresses inconsistencies between
List<String> and String configurations for bootstrap servers.

The change ensures that List versions of bootstrap-server configs are
converted to regular Strings by removing brackets. This allows for
consistent comparison between producer and admin configurations.

This optimization is particularly relevant for Spring Boot scenarios
where configs may be provided in different formats but represent the
same underlying values.

* Addressing PR review

**Auto-cherry-pick to `3.2.x` & `3.1.x`**

Author: sobychacko

spring-kafka/src/main/java/org/springframework/kafka/core/KafkaTemplate.java

@@ -80,6 +80,7 @@
 import org.springframework.transaction.support.TransactionSynchronizationManager;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.StringUtils;
 
 import io.micrometer.observation.Observation;
 import io.micrometer.observation.ObservationRegistry;
@@ -484,8 +485,9 @@ public void afterSingletonsInstantiated() {
 			if (this.kafkaAdmin == null) {
 				this.kafkaAdmin = this.applicationContext.getBeanProvider(KafkaAdmin.class).getIfUnique();
 				if (this.kafkaAdmin != null) {
-					Object producerServers = this.producerFactory.getConfigurationProperties()
-							.get(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG);
+					String producerServers = this.producerFactory.getConfigurationProperties()
+							.get(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG).toString();
+					producerServers = removeLeadingAndTrailingBrackets(producerServers);
 					String adminServers = getAdminBootstrapAddress();
 					if (!producerServers.equals(adminServers)) {
 						Map<String, Object> props = new HashMap<>(this.kafkaAdmin.getConfigurationProperties());
@@ -509,16 +511,14 @@ else if (this.micrometerEnabled) {
 	private String getAdminBootstrapAddress() {
 		// Retrieve bootstrap servers from KafkaAdmin bootstrap supplier if available
 		String adminServers = this.kafkaAdmin.getBootstrapServers();
-
 		// Fallback to configuration properties if bootstrap servers are not set
 		if (adminServers == null) {
 			adminServers = this.kafkaAdmin.getConfigurationProperties().getOrDefault(
 					AdminClientConfig.BOOTSTRAP_SERVERS_CONFIG,
 					""
 			).toString();
 		}
-
-		return adminServers;
+		return removeLeadingAndTrailingBrackets(adminServers);
 	}
 
 	@Nullable
@@ -1003,6 +1003,10 @@ public void destroy() {
 		}
 	}
 
+	private static String removeLeadingAndTrailingBrackets(String str) {
+		return StringUtils.trimTrailingCharacter(StringUtils.trimLeadingCharacter(str, '['), ']');
+	}
+
 	@SuppressWarnings("serial")
 	private static final class SkipAbortException extends RuntimeException {
 

spring-kafka/src/test/java/org/springframework/kafka/support/micrometer/ObservationTests.java

@@ -68,6 +68,7 @@
 import org.springframework.lang.Nullable;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+import org.springframework.util.StringUtils;
 
 import io.micrometer.common.KeyValues;
 import io.micrometer.core.instrument.MeterRegistry;
@@ -230,7 +231,7 @@ public KeyValues getLowCardinalityKeyValues(KafkaRecordReceiverContext context)
 		assertThatListenerHasTimerWithNameAndTags(meterRegistryAssert, OBSERVATION_TEST_2, "obs2", "obs2-0");
 
 		assertThat(admin.getConfigurationProperties())
-				.containsEntry(AdminClientConfig.BOOTSTRAP_SERVERS_CONFIG, broker.getBrokersAsString());
+				.containsEntry(AdminClientConfig.BOOTSTRAP_SERVERS_CONFIG, List.of(broker.getBrokersAsString()));
 		// producer factory broker different to admin
 		assertThatAdmin(template, admin, broker.getBrokersAsString() + "," + broker.getBrokersAsString(),
 				"kafkaAdmin");
@@ -386,6 +387,14 @@ void observationErrorException(@Autowired ExceptionListener listener, @Autowired
 				.hasMessage("obs5 error");
 	}
 
+	@Test
+	void kafkaAdminNotRecreatedIfBootstrapServersSameInProducerAndAdminConfig(
+			@Autowired @Qualifier("reuseAdminBeanKafkaTemplate") KafkaTemplate<Integer, String> template,
+			@Autowired KafkaAdmin kafkaAdmin) {
+		// See this issue for more details: https://github.com/spring-projects/spring-kafka/issues/3466
+		assertThat(template.getKafkaAdmin()).isSameAs(kafkaAdmin);
+	}
+
 	@Configuration
 	@EnableKafka
 	public static class Config {
@@ -394,20 +403,30 @@ public static class Config {
 
 		@Bean
 		KafkaAdmin admin(EmbeddedKafkaBroker broker) {
+			String[] brokers = StringUtils.commaDelimitedListToStringArray(broker.getBrokersAsString());
+			List<String> brokersAsList = Arrays.asList(brokers);
 			KafkaAdmin admin = new KafkaAdmin(
-					Map.of(AdminClientConfig.BOOTSTRAP_SERVERS_CONFIG, broker.getBrokersAsString()));
+					Map.of(AdminClientConfig.BOOTSTRAP_SERVERS_CONFIG, brokersAsList));
 			admin.setOperationTimeout(42);
 			return admin;
 		}
 
 		@Bean
+		@Primary
 		ProducerFactory<Integer, String> producerFactory(EmbeddedKafkaBroker broker) {
 			Map<String, Object> producerProps = KafkaTestUtils.producerProps(broker);
 			producerProps.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, broker.getBrokersAsString() + ","
 					+ broker.getBrokersAsString());
 			return new DefaultKafkaProducerFactory<>(producerProps);
 		}
 
+		@Bean
+		ProducerFactory<Integer, String> customProducerFactory(EmbeddedKafkaBroker broker) {
+			Map<String, Object> producerProps = KafkaTestUtils.producerProps(broker);
+			producerProps.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, broker.getBrokersAsString());
+			return new DefaultKafkaProducerFactory<>(producerProps);
+		}
+
 		@Bean
 		ConsumerFactory<Integer, String> consumerFactory(EmbeddedKafkaBroker broker) {
 			Map<String, Object> consumerProps = KafkaTestUtils.consumerProps("obs", "false", broker);
@@ -439,6 +458,14 @@ KafkaTemplate<Integer, String> throwableTemplate(ProducerFactory<Integer, String
 			return template;
 		}
 
+		@Bean
+		KafkaTemplate<Integer, String> reuseAdminBeanKafkaTemplate(
+				@Qualifier("customProducerFactory") ProducerFactory<Integer, String> pf) {
+			KafkaTemplate<Integer, String> template = new KafkaTemplate<>(pf);
+			template.setObservationEnabled(true);
+			return template;
+		}
+
 		@Bean
 		ConcurrentKafkaListenerContainerFactory<Integer, String> kafkaListenerContainerFactory(
 				ConsumerFactory<Integer, String> cf) {