GH-2750: EmbeddedKafka Set Boot's Bootstrap Prop.

Resolves https://github.com/spring-projects/spring-kafka/issues/2750

Previously, setting the `bootstrapServersProperty` on `@EmbeddedKafka`
(or `brokerListProperty` on the broker itself) caused the broker to
set that property instead of the default property `spring.embedded.kafka.brokers`.

Now, the custom property is set in addition to the default.

Also, the default property name is `spring.kafka.bootstrap-servers`.
This allows easier testing of Spring Boot applications, without the
need for additional configuration.

spring-kafka-docs/src/main/asciidoc/testing.adoc

@@ -133,6 +133,7 @@ Instead of default `spring.embedded.kafka.brokers` system property, the address
 For this purpose a `spring.embedded.kafka.brokers.property` (`EmbeddedKafkaBroker.BROKER_LIST_PROPERTY`) system property can be set before starting an embedded Kafka.
 For example, with Spring Boot a `spring.kafka.bootstrap-servers` configuration property is expected to be set for auto-configuring Kafka client, respectively.
 So, before running tests with an embedded Kafka on random ports, we can set `spring.embedded.kafka.brokers.property=spring.kafka.bootstrap-servers` as a system property - and the `EmbeddedKafkaBroker`  will use it to expose its broker addresses.
+This is now the default value for this property (starting with version 3.0.10).
 
 With the `EmbeddedKafkaBroker.brokerProperties(Map<String, String>)`, you can provide additional properties for the Kafka servers.
 See https://kafka.apache.org/documentation/#brokerconfigs[Kafka Config] for more information about possible broker properties.
@@ -235,7 +236,8 @@ In addition, these properties can be provided:
 Essentially these properties mimic some of the `@EmbeddedKafka` attributes.
 
 See more information about configuration properties and how to provide them in the https://junit.org/junit5/docs/current/user-guide/#running-tests-config-params[JUnit 5 User Guide].
-For example, a `spring.embedded.kafka.brokers.property=spring.kafka.bootstrap-servers` entry (for testing in Spring Boot application) can be added into a `junit-platform.properties` file in the testing classpath.
+For example, a `spring.embedded.kafka.brokers.property=my.bootstrap-servers` entry can be added into a `junit-platform.properties` file in the testing classpath.
+Starting with version 3.0.10, the broker automatically sets this to `spring.kafka.bootstrap-servers`, by default, for testing with Spring Boot applications.
 
 NOTE: It is recommended to not combine a global embedded Kafka and per-test class in a single test suite.
 Both of them share the same system properties, so it is very likely going to lead to unexpected behavior.
@@ -422,7 +424,7 @@ The following example shows how to use an `@EmbeddedKafka` Annotation to create
 ----
 @RunWith(SpringRunner.class)
 @EmbeddedKafka(topics = "someTopic",
-        bootstrapServersProperty = "spring.kafka.bootstrap-servers")
+        bootstrapServersProperty = "spring.kafka.bootstrap-servers") // this is now the default
 public class MyApplicationTests {
 
     @Autowired
@@ -437,6 +439,7 @@ public class MyApplicationTests {
 ----
 ====
 
+NOTE: The `bootstrapServersProperty` is automatically set to `spring.kafka.bootstrap-servers`, by default, starting with version 3.0.10.
 
 ==== Hamcrest Matchers
 

spring-kafka-docs/src/main/asciidoc/whats-new.adoc

@@ -104,3 +104,5 @@ Similarly, `RECEIVED_MESSAGE_KEY` is replaced by `RECEIVED_KEY` and `RECEIVED_PA
 
 Version 3.0.7 introduced a `MockConsumerFactory` and `MockProducerFactory`.
 See <<mock-cons-prod>> for more information.
+
+Starting with version 3.0.10, the embedded Kafka broker, by default, sets the Spring Boot property `spring.kafka.bootstrap-servers` to the address(es) of the embedded broker(s).

spring-kafka-test/src/main/java/org/springframework/kafka/test/EmbeddedKafkaBroker.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018-2022 the original author or authors.
+ * Copyright 2018-2023 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.
@@ -181,7 +181,7 @@ public class EmbeddedKafkaBroker implements InitializingBean, DisposableBean {
 
 	private int zkSessionTimeout = DEFAULT_ZK_SESSION_TIMEOUT;
 
-	private String brokerListProperty;
+	private String brokerListProperty = "spring.kafka.bootstrap-servers";
 
 	private volatile ZooKeeperClient zooKeeperClient;
 
@@ -257,6 +257,8 @@ public class EmbeddedKafkaBroker implements InitializingBean, DisposableBean {
 
 	/**
 	 * Set the system property with this name to the list of broker addresses.
+	 * Defaults to {@code spring.kafka.bootstrap-servers} for Spring Boot
+	 * compatibility, since 3.0.10.
 	 * @param brokerListProperty the brokerListProperty to set
 	 * @return this broker.
 	 * @since 2.3
@@ -374,10 +376,10 @@ public class EmbeddedKafkaBroker implements InitializingBean, DisposableBean {
 		if (this.brokerListProperty == null) {
 			this.brokerListProperty = System.getProperty(BROKER_LIST_PROPERTY);
 		}
-		if (this.brokerListProperty == null) {
-			this.brokerListProperty = SPRING_EMBEDDED_KAFKA_BROKERS;
+		if (this.brokerListProperty != null) {
+			System.setProperty(this.brokerListProperty, getBrokersAsString());
 		}
-		System.setProperty(this.brokerListProperty, getBrokersAsString());
+		System.setProperty(SPRING_EMBEDDED_KAFKA_BROKERS, getBrokersAsString());
 		System.setProperty(SPRING_EMBEDDED_ZOOKEEPER_CONNECT, getZookeeperConnectionString());
 	}
 
@@ -591,7 +593,8 @@ public class EmbeddedKafkaBroker implements InitializingBean, DisposableBean {
 
 	@Override
 	public void destroy() {
-		System.getProperties().remove(brokerListProperty);
+		System.getProperties().remove(this.brokerListProperty);
+		System.getProperties().remove(SPRING_EMBEDDED_KAFKA_BROKERS);
 		System.getProperties().remove(SPRING_EMBEDDED_ZOOKEEPER_CONNECT);
 		for (KafkaServer kafkaServer : this.kafkaServers) {
 			try {

spring-kafka-test/src/main/java/org/springframework/kafka/test/context/EmbeddedKafka.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2022 the original author or authors.
+ * Copyright 2017-2023 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.
@@ -151,13 +151,14 @@ public @interface EmbeddedKafka {
 	String brokerPropertiesLocation() default "";
 
 	/**
-	 * The property name to set with the bootstrap server addresses instead of the default
+	 * The property name to set with the bootstrap server addresses as well as the default
 	 * {@value org.springframework.kafka.test.EmbeddedKafkaBroker#SPRING_EMBEDDED_KAFKA_BROKERS}.
+	 * Defaults to {@code spring.kafka.bootstrap-servers} since 3.0.10.
 	 * @return the property name.
 	 * @since 2.3
 	 * @see org.springframework.kafka.test.EmbeddedKafkaBroker#brokerListProperty(String)
 	 */
-	String bootstrapServersProperty() default "";
+	String bootstrapServersProperty() default "spring.kafka.bootstrap-servers";
 
 	/**
 	 * Timeout for internal ZK client connection.

spring-kafka-test/src/main/java/org/springframework/kafka/test/junit/GlobalEmbeddedKafkaTestExecutionListener.java

@@ -119,8 +119,10 @@ public class GlobalEmbeddedKafkaTestExecutionListener implements TestExecutionLi
 			this.embeddedKafkaBroker =
 					new EmbeddedKafkaBroker(count, false, partitions, topics)
 							.brokerProperties(brokerProperties)
-							.brokerListProperty(brokerListProperty)
 							.kafkaPorts(ports);
+			if (brokerListProperty != null) {
+				this.embeddedKafkaBroker.brokerListProperty(brokerListProperty);
+			}
 			this.embeddedKafkaBroker.afterPropertiesSet();
 
 			this.logger.info("Started global Embedded Kafka on: " + this.embeddedKafkaBroker.getBrokersAsString());

spring-kafka-test/src/test/java/org/springframework/kafka/test/EmbeddedKafkaBrokerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019 the original author or authors.
+ * Copyright 2019-2023 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.
@@ -30,10 +30,16 @@ public class EmbeddedKafkaBrokerTests {
 	@Test
 	void testUpDown() {
 		EmbeddedKafkaBroker kafka = new EmbeddedKafkaBroker(1);
+		kafka.brokerListProperty("foo.bar");
 		kafka.afterPropertiesSet();
 		assertThat(kafka.getZookeeperConnectionString()).startsWith("127");
+		assertThat(System.getProperty("foo.bar")).isNotNull();
+		assertThat(System.getProperty(EmbeddedKafkaBroker.SPRING_EMBEDDED_KAFKA_BROKERS))
+				.isEqualTo(System.getProperty("foo.bar"));
 		kafka.destroy();
 		assertThat(kafka.getZookeeperConnectionString()).isNull();
+		assertThat(System.getProperty("foo.bar")).isNull();
+		assertThat(System.getProperty(EmbeddedKafkaBroker.SPRING_EMBEDDED_KAFKA_BROKERS)).isNull();
 	}
 
 }

spring-kafka-test/src/test/resources/junit-platform.properties

@@ -1,3 +1,2 @@
 spring.kafka.embedded.count=2
-spring.embedded.kafka.brokers.property=spring.kafka.bootstrap-servers
 spring.kafka.embedded.topics=topic1,topic2