Testing Applications

The spring-kafka-test jar contains some useful utilities to assist with testing your applications.

Embedded Kafka Broker

Two implementations are provided:

  • EmbeddedKafkaZKBroker - legacy implementation which starts an embedded Zookeeper instance.

  • EmbeddedKafkaKraftBroker - (default) uses Kraft instead of Zookeeper in combined controller and broker modes (since 3.1).

There are several techniques to configure the broker as discussed in the following sections.

KafkaTestUtils

org.springframework.kafka.test.utils.KafkaTestUtils provides a number of static helper methods to consume records, retrieve various record offsets, and others. Refer to its Javadocs for complete details.

JUnit

org.springframework.kafka.test.utils.KafkaTestUtils also provides some static methods to set up producer and consumer properties. The following listing shows those method signatures:

/**
 * Set up test properties for an {@code <Integer, String>} consumer.
 * @param group the group id.
 * @param autoCommit the auto commit.
 * @param embeddedKafka a {@link EmbeddedKafkaBroker} instance.
 * @return the properties.
 */
public static Map<String, Object> consumerProps(String group, String autoCommit,
                                       EmbeddedKafkaBroker embeddedKafka) { ... }

/**
 * Set up test properties for an {@code <Integer, String>} producer.
 * @param embeddedKafka a {@link EmbeddedKafkaBroker} instance.
 * @return the properties.
 */
public static Map<String, Object> producerProps(EmbeddedKafkaBroker embeddedKafka) { ... }

Starting with version 2.5, the consumerProps method sets the ConsumerConfig.AUTO_OFFSET_RESET_CONFIG to earliest. This is because, in most cases, you want the consumer to consume any messages sent in a test case. The ConsumerConfig default is latest which means that messages already sent by a test, before the consumer starts, will not receive those records. To revert to the previous behavior, set the property to latest after calling the method.

When using the embedded broker, it is generally best practice using a different topic for each test, to prevent cross-talk. If this is not possible for some reason, note that the consumeFromEmbeddedTopics method’s default behavior is to seek the assigned partitions to the beginning after assignment. Since it does not have access to the consumer properties, you must use the overloaded method that takes a seekToEnd boolean parameter to seek to the end instead of the beginning.

A JUnit 4 @Rule wrapper for the EmbeddedKafkaZKBroker is provided to create an embedded Kafka and an embedded Zookeeper server. (See @EmbeddedKafka Annotation for information about using @EmbeddedKafka with JUnit 5). The following listing shows the signatures of those methods:

/**
 * Create embedded Kafka brokers.
 * @param count the number of brokers.
 * @param controlledShutdown passed into TestUtils.createBrokerConfig.
 * @param topics the topics to create (2 partitions per).
 */
public EmbeddedKafkaRule(int count, boolean controlledShutdown, String... topics) { ... }

/**
 *
 * Create embedded Kafka brokers.
 * @param count the number of brokers.
 * @param controlledShutdown passed into TestUtils.createBrokerConfig.
 * @param partitions partitions per topic.
 * @param topics the topics to create.
 */
public EmbeddedKafkaRule(int count, boolean controlledShutdown, int partitions, String... topics) { ... }
The EmbeddedKafkaKraftBroker is not supported with JUnit4.

The EmbeddedKafkaBroker class has a utility method that lets you consume for all the topics it created. The following example shows how to use it:

Map<String, Object> consumerProps = KafkaTestUtils.consumerProps("testT", "false", embeddedKafka);
DefaultKafkaConsumerFactory<Integer, String> cf = new DefaultKafkaConsumerFactory<>(consumerProps);
Consumer<Integer, String> consumer = cf.createConsumer();
embeddedKafka.consumeFromAllEmbeddedTopics(consumer);

The KafkaTestUtils has some utility methods to fetch results from the consumer. The following listing shows those method signatures:

/**
 * Poll the consumer, expecting a single record for the specified topic.
 * @param consumer the consumer.
 * @param topic the topic.
 * @return the record.
 * @throws org.junit.ComparisonFailure if exactly one record is not received.
 */
public static <K, V> ConsumerRecord<K, V> getSingleRecord(Consumer<K, V> consumer, String topic) { ... }

/**
 * Poll the consumer for records.
 * @param consumer the consumer.
 * @return the records.
 */
public static <K, V> ConsumerRecords<K, V> getRecords(Consumer<K, V> consumer) { ... }

The following example shows how to use KafkaTestUtils:

...
template.sendDefault(0, 2, "bar");
ConsumerRecord<Integer, String> received = KafkaTestUtils.getSingleRecord(consumer, "topic");
...

When the embedded Kafka and embedded Zookeeper server are started by the EmbeddedKafkaBroker, a system property named spring.embedded.kafka.brokers is set to the address of the Kafka brokers and a system property named spring.embedded.zookeeper.connect is set to the address of Zookeeper. Convenient constants (EmbeddedKafkaBroker.SPRING_EMBEDDED_KAFKA_BROKERS and EmbeddedKafkaBroker.SPRING_EMBEDDED_ZOOKEEPER_CONNECT) are provided for this property.

Instead of default spring.embedded.kafka.brokers system property, the address of the Kafka brokers can be exposed to any arbitrary and convenient property. 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 Kafka Config for more information about possible broker properties.

Configuring Topics

The following example configuration creates topics called cat and hat with five partitions, a topic called thing1 with 10 partitions, and a topic called thing2 with 15 partitions:

public class MyTests {

    @ClassRule
    private static EmbeddedKafkaRule embeddedKafka = new EmbeddedKafkaRule(1, false, 5, "cat", "hat");

    @Test
    public void test() {
        embeddedKafkaRule.getEmbeddedKafka()
              .addTopics(new NewTopic("thing1", 10, (short) 1), new NewTopic("thing2", 15, (short) 1));
        ...
    }

}

By default, addTopics will throw an exception when problems arise (such as adding a topic that already exists). Version 2.6 added a new version of that method that returns a Map<String, Exception>; the key is the topic name and the value is null for success, or an Exception for a failure.

Using the Same Broker(s) for Multiple Test Classes

You can use the same broker for multiple test classes with something similar to the following:

public final class EmbeddedKafkaHolder {

    private static EmbeddedKafkaBroker embeddedKafka = new EmbeddedKafkaZKBroker(1, false)
            .brokerListProperty("spring.kafka.bootstrap-servers");

    private static boolean started;

    public static EmbeddedKafkaBroker getEmbeddedKafka() {
        if (!started) {
            try {
                embeddedKafka.afterPropertiesSet();
            }
            catch (Exception e) {
                throw new KafkaException("Embedded broker failed to start", e);
            }
            started = true;
        }
        return embeddedKafka;
    }

    private EmbeddedKafkaHolder() {
        super();
    }

}

This assumes a Spring Boot environment and the embedded broker replaces the bootstrap servers property.

Then, in each test class, you can use something similar to the following:

static {
    EmbeddedKafkaHolder.getEmbeddedKafka().addTopics("topic1", "topic2");
}

private static final EmbeddedKafkaBroker broker = EmbeddedKafkaHolder.getEmbeddedKafka();

If you are not using Spring Boot, you can obtain the bootstrap servers using broker.getBrokersAsString().

The preceding example provides no mechanism for shutting down the broker(s) when all tests are complete. This could be a problem if, say, you run your tests in a Gradle daemon. You should not use this technique in such a situation, or you should use something to call destroy() on the EmbeddedKafkaBroker when your tests are complete.

Starting with version 3.0, the framework exposes a GlobalEmbeddedKafkaTestExecutionListener for the JUnit Platform; it is disabled by default. This requires JUnit Platform 1.8 or greater. The purpose of this listener is to start one global EmbeddedKafkaBroker for the whole test plan and stop it at the end of the plan. To enable this listener, and therefore have a single global embedded Kafka cluster for all the tests in the project, the spring.kafka.global.embedded.enabled property must be set to true via system properties or JUnit Platform configuration. In addition, these properties can be provided:

  • spring.kafka.embedded.count - the number of Kafka brokers to manage;

  • spring.kafka.embedded.ports - ports (comma-separated value) for every Kafka broker to start, 0 if random port is preferred; the number of values must be equal to the count mentioned above;

  • spring.kafka.embedded.topics - topics (comma-separated value) to create in the started Kafka cluster;

  • spring.kafka.embedded.partitions - number of partitions to provision for the created topics;

  • spring.kafka.embedded.broker.properties.location - the location of the file for additional Kafka broker configuration properties; the value of this property must follow the Spring resource abstraction pattern;

  • spring.kafka.embedded.kraft - when false, use an EmbeddedKafkaZKBroker instead of an EmbeddedKafkaKraftBroker.

Essentially these properties mimic some of the @EmbeddedKafka attributes.

See more information about configuration properties and how to provide them in the JUnit 5 User Guide. 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.

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.
spring-kafka-test has transitive dependencies on junit-jupiter-api and junit-platform-launcher (the latter to support the global embedded broker). If you wish to use the embedded broker and are NOT using JUnit, you may wish to exclude these dependencies.

@EmbeddedKafka Annotation

We generally recommend that you use the rule as a @ClassRule to avoid starting and stopping the broker between tests (and use a different topic for each test). Starting with version 2.0, if you use Spring’s test application context caching, you can also declare a EmbeddedKafkaBroker bean, so a single broker can be used across multiple test classes. For convenience, we provide a test class-level annotation called @EmbeddedKafka to register the EmbeddedKafkaBroker bean. The following example shows how to use it:

@RunWith(SpringRunner.class)
@DirtiesContext
@EmbeddedKafka(partitions = 1,
         topics = {
                 KafkaStreamsTests.STREAMING_TOPIC1,
                 KafkaStreamsTests.STREAMING_TOPIC2 })
public class KafkaStreamsTests {

    @Autowired
    private EmbeddedKafkaBroker embeddedKafka;

    @Test
    public void someTest() {
        Map<String, Object> consumerProps = KafkaTestUtils.consumerProps("testGroup", "true", this.embeddedKafka);
        consumerProps.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
        ConsumerFactory<Integer, String> cf = new DefaultKafkaConsumerFactory<>(consumerProps);
        Consumer<Integer, String> consumer = cf.createConsumer();
        this.embeddedKafka.consumeFromAnEmbeddedTopic(consumer, KafkaStreamsTests.STREAMING_TOPIC2);
        ConsumerRecords<Integer, String> replies = KafkaTestUtils.getRecords(consumer);
        assertThat(replies.count()).isGreaterThanOrEqualTo(1);
    }

    @Configuration
    @EnableKafkaStreams
    public static class KafkaStreamsConfiguration {

        @Value("${" + EmbeddedKafkaBroker.SPRING_EMBEDDED_KAFKA_BROKERS + "}")
        private String brokerAddresses;

        @Bean(name = KafkaStreamsDefaultConfiguration.DEFAULT_STREAMS_CONFIG_BEAN_NAME)
        public KafkaStreamsConfiguration kStreamsConfigs() {
            Map<String, Object> props = new HashMap<>();
            props.put(StreamsConfig.APPLICATION_ID_CONFIG, "testStreams");
            props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, this.brokerAddresses);
            return new KafkaStreamsConfiguration(props);
        }

    }

}

Starting with version 2.2.4, you can also use the @EmbeddedKafka annotation to specify the Kafka ports property.

Starting with version 3.1, set the kraft property to false to use an EmbeddedKafkaZKBroker instead of an EmbeddedKafkaKraftBroker.

The following example sets the topics, brokerProperties, and brokerPropertiesLocation attributes of @EmbeddedKafka support property placeholder resolutions:

@TestPropertySource(locations = "classpath:/test.properties")
@EmbeddedKafka(topics = { "any-topic", "${kafka.topics.another-topic}" },
        brokerProperties = { "log.dir=${kafka.broker.logs-dir}",
                            "listeners=PLAINTEXT://localhost:${kafka.broker.port}",
                            "auto.create.topics.enable=${kafka.broker.topics-enable:true}" },
        brokerPropertiesLocation = "classpath:/broker.properties")

In the preceding example, the property placeholders ${kafka.topics.another-topic}, ${kafka.broker.logs-dir}, and ${kafka.broker.port} are resolved from the Spring Environment. In addition, the broker properties are loaded from the broker.properties classpath resource specified by the brokerPropertiesLocation. Property placeholders are resolved for the brokerPropertiesLocation URL and for any property placeholders found in the resource. Properties defined by brokerProperties override properties found in brokerPropertiesLocation.

You can use the @EmbeddedKafka annotation with JUnit 4 or JUnit 5.

@EmbeddedKafka Annotation with JUnit5

Starting with version 2.3, there are two ways to use the @EmbeddedKafka annotation with JUnit5. When used with the @SpringJunitConfig annotation, the embedded broker is added to the test application context. You can auto wire the broker into your test, at the class or method level, to get the broker address list.

When not using the spring test context, the EmbdeddedKafkaCondition creates a broker; the condition includes a parameter resolver so you can access the broker in your test method.

@EmbeddedKafka
public class EmbeddedKafkaConditionTests {

    @Test
    public void test(EmbeddedKafkaBroker broker) {
        String brokerList = broker.getBrokersAsString();
        ...
    }

}

A standalone broker (outside the Spring’s TestContext) will be created unless a class annotated @EmbeddedKafka is also annotated (or meta-annotated) with ExtendWith(SpringExtension.class). @SpringJunitConfig and @SpringBootTest are so meta-annotated and the context-based broker will be used when either of those annotations are also present.

When there is a Spring test application context available, the topics and broker properties can contain property placeholders, which will be resolved as long as the property is defined somewhere. If there is no Spring context available, these placeholders won’t be resolved.

Embedded Broker in @SpringBootTest Annotations

Spring Initializr now automatically adds the spring-kafka-test dependency in test scope to the project configuration.

If your application uses the Kafka binder in spring-cloud-stream and if you want to use an embedded broker for tests, you must remove the spring-cloud-stream-test-support dependency, because it replaces the real binder with a test binder for test cases. If you wish some tests to use the test binder and some to use the embedded broker, tests that use the real binder need to disable the test binder by excluding the binder auto configuration in the test class. The following example shows how to do so:

@RunWith(SpringRunner.class)
@SpringBootTest(properties = "spring.autoconfigure.exclude="
    + "org.springframework.cloud.stream.test.binder.TestSupportBinderAutoConfiguration")
public class MyApplicationTests {
    ...
}

There are several ways to use an embedded broker in a Spring Boot application test.

They include:

JUnit4 Class Rule

The following example shows how to use a JUnit4 class rule to create an embedded broker:

@RunWith(SpringRunner.class)
@SpringBootTest
public class MyApplicationTests {

    @ClassRule
    public static EmbeddedKafkaRule broker = new EmbeddedKafkaRule(1, false, "someTopic")
            .brokerListProperty("spring.kafka.bootstrap-servers");

    @Autowired
    private KafkaTemplate<String, String> template;

    @Test
    public void test() {
        ...
    }

}

Notice that, since this is a Spring Boot application, we override the broker list property to set Spring Boot’s property.

@EmbeddedKafka Annotation or EmbeddedKafkaBroker Bean

The following example shows how to use an @EmbeddedKafka Annotation to create an embedded broker:

@RunWith(SpringRunner.class)
@EmbeddedKafka(topics = "someTopic",
        bootstrapServersProperty = "spring.kafka.bootstrap-servers") // this is now the default
public class MyApplicationTests {

    @Autowired
    private KafkaTemplate<String, String> template;

    @Test
    public void test() {
        ...
    }

}
The bootstrapServersProperty is automatically set to spring.kafka.bootstrap-servers by default, starting with version 3.0.10.

Hamcrest Matchers

The org.springframework.kafka.test.hamcrest.KafkaMatchers provides the following matchers:

/**
 * @param key the key
 * @param <K> the type.
 * @return a Matcher that matches the key in a consumer record.
 */
public static <K> Matcher<ConsumerRecord<K, ?>> hasKey(K key) { ... }

/**
 * @param value the value.
 * @param <V> the type.
 * @return a Matcher that matches the value in a consumer record.
 */
public static <V> Matcher<ConsumerRecord<?, V>> hasValue(V value) { ... }

/**
 * @param partition the partition.
 * @return a Matcher that matches the partition in a consumer record.
 */
public static Matcher<ConsumerRecord<?, ?>> hasPartition(int partition) { ... }

/**
 * Matcher testing the timestamp of a {@link ConsumerRecord} assuming the topic has been set with
 * {@link org.apache.kafka.common.record.TimestampType#CREATE_TIME CreateTime}.
 *
 * @param ts timestamp of the consumer record.
 * @return a Matcher that matches the timestamp in a consumer record.
 */
public static Matcher<ConsumerRecord<?, ?>> hasTimestamp(long ts) {
  return hasTimestamp(TimestampType.CREATE_TIME, ts);
}

/**
 * Matcher testing the timestamp of a {@link ConsumerRecord}
 * @param type timestamp type of the record
 * @param ts timestamp of the consumer record.
 * @return a Matcher that matches the timestamp in a consumer record.
 */
public static Matcher<ConsumerRecord<?, ?>> hasTimestamp(TimestampType type, long ts) {
  return new ConsumerRecordTimestampMatcher(type, ts);
}

AssertJ Conditions

You can use the following AssertJ conditions:

/**
 * @param key the key
 * @param <K> the type.
 * @return a Condition that matches the key in a consumer record.
 */
public static <K> Condition<ConsumerRecord<K, ?>> key(K key) { ... }

/**
 * @param value the value.
 * @param <V> the type.
 * @return a Condition that matches the value in a consumer record.
 */
public static <V> Condition<ConsumerRecord<?, V>> value(V value) { ... }

/**
 * @param key the key.
 * @param value the value.
 * @param <K> the key type.
 * @param <V> the value type.
 * @return a Condition that matches the key in a consumer record.
 * @since 2.2.12
 */
public static <K, V> Condition<ConsumerRecord<K, V>> keyValue(K key, V value) { ... }

/**
 * @param partition the partition.
 * @return a Condition that matches the partition in a consumer record.
 */
public static Condition<ConsumerRecord<?, ?>> partition(int partition) { ... }

/**
 * @param value the timestamp.
 * @return a Condition that matches the timestamp value in a consumer record.
 */
public static Condition<ConsumerRecord<?, ?>> timestamp(long value) {
  return new ConsumerRecordTimestampCondition(TimestampType.CREATE_TIME, value);
}

/**
 * @param type the type of timestamp
 * @param value the timestamp.
 * @return a Condition that matches the timestamp value in a consumer record.
 */
public static Condition<ConsumerRecord<?, ?>> timestamp(TimestampType type, long value) {
  return new ConsumerRecordTimestampCondition(type, value);
}

Example

The following example brings together most of the topics covered in this chapter:

public class KafkaTemplateTests {

    private static final String TEMPLATE_TOPIC = "templateTopic";

    @ClassRule
    public static EmbeddedKafkaRule embeddedKafka = new EmbeddedKafkaRule(1, true, TEMPLATE_TOPIC);

    @Test
    public void testTemplate() throws Exception {
        Map<String, Object> consumerProps = KafkaTestUtils.consumerProps("testT", "false",
            embeddedKafka.getEmbeddedKafka());
        DefaultKafkaConsumerFactory<Integer, String> cf =
                            new DefaultKafkaConsumerFactory<>(consumerProps);
        ContainerProperties containerProperties = new ContainerProperties(TEMPLATE_TOPIC);
        KafkaMessageListenerContainer<Integer, String> container =
                            new KafkaMessageListenerContainer<>(cf, containerProperties);
        final BlockingQueue<ConsumerRecord<Integer, String>> records = new LinkedBlockingQueue<>();
        container.setupMessageListener(new MessageListener<Integer, String>() {

            @Override
            public void onMessage(ConsumerRecord<Integer, String> record) {
                System.out.println(record);
                records.add(record);
            }

        });
        container.setBeanName("templateTests");
        container.start();
        ContainerTestUtils.waitForAssignment(container,
                            embeddedKafka.getEmbeddedKafka().getPartitionsPerTopic());
        Map<String, Object> producerProps =
                            KafkaTestUtils.producerProps(embeddedKafka.getEmbeddedKafka());
        ProducerFactory<Integer, String> pf =
                            new DefaultKafkaProducerFactory<>(producerProps);
        KafkaTemplate<Integer, String> template = new KafkaTemplate<>(pf);
        template.setDefaultTopic(TEMPLATE_TOPIC);
        template.sendDefault("foo");
        assertThat(records.poll(10, TimeUnit.SECONDS), hasValue("foo"));
        template.sendDefault(0, 2, "bar");
        ConsumerRecord<Integer, String> received = records.poll(10, TimeUnit.SECONDS);
        assertThat(received, hasKey(2));
        assertThat(received, hasPartition(0));
        assertThat(received, hasValue("bar"));
        template.send(TEMPLATE_TOPIC, 0, 2, "baz");
        received = records.poll(10, TimeUnit.SECONDS);
        assertThat(received, hasKey(2));
        assertThat(received, hasPartition(0));
        assertThat(received, hasValue("baz"));
    }

}

The preceding example uses the Hamcrest matchers. With AssertJ, the final part looks like the following code:

assertThat(records.poll(10, TimeUnit.SECONDS)).has(value("foo"));
template.sendDefault(0, 2, "bar");
ConsumerRecord<Integer, String> received = records.poll(10, TimeUnit.SECONDS);
// using individual assertions
assertThat(received).has(key(2));
assertThat(received).has(value("bar"));
assertThat(received).has(partition(0));
template.send(TEMPLATE_TOPIC, 0, 2, "baz");
received = records.poll(10, TimeUnit.SECONDS);
// using allOf()
assertThat(received).has(allOf(keyValue(2, "baz"), partition(0)));

Mock Consumer and Producer

The kafka-clients library provides MockConsumer and MockProducer classes for testing purposes.

If you wish to use these classes in some of your tests with listener containers or KafkaTemplate respectively, starting with version 3.0.7, the framework now provides MockConsumerFactory and MockProducerFactory implementations.

These factories can be used in the listener container and template instead of the default factories, which require a running (or embedded) broker.

Here is an example of a simple implementation returning a single consumer:

@Bean
ConsumerFactory<String, String> consumerFactory() {
    MockConsumer<String, String> consumer = new MockConsumer<>(OffsetResetStrategy.EARLIEST);
    TopicPartition topicPartition0 = new TopicPartition("topic", 0);
    List<TopicPartition> topicPartitions = Collections.singletonList(topicPartition0);
    Map<TopicPartition, Long> beginningOffsets = topicPartitions.stream().collect(Collectors
            .toMap(Function.identity(), tp -> 0L));
    consumer.updateBeginningOffsets(beginningOffsets);
    consumer.schedulePollTask(() -> {
        consumer.addRecord(
                new ConsumerRecord<>("topic", 0, 0L, 0L, TimestampType.NO_TIMESTAMP_TYPE, 0, 0, null, "test1",
                        new RecordHeaders(), Optional.empty()));
        consumer.addRecord(
                new ConsumerRecord<>("topic", 0, 1L, 0L, TimestampType.NO_TIMESTAMP_TYPE, 0, 0, null, "test2",
                        new RecordHeaders(), Optional.empty()));
    });
    return new MockConsumerFactory(() -> consumer);
}

If you wish to test with concurrency, the Supplier lambda in the factory’s constructor would need create a new instance each time.

With the MockProducerFactory, there are two constructors; one to create a simple factory, and one to create factory that supports transactions.

Here are examples:

@Bean
ProducerFactory<String, String> nonTransFactory() {
    return new MockProducerFactory<>(() ->
            new MockProducer<>(true, new StringSerializer(), new StringSerializer()));
}

@Bean
ProducerFactory<String, String> transFactory() {
    MockProducer<String, String> mockProducer =
            new MockProducer<>(true, new StringSerializer(), new StringSerializer());
    mockProducer.initTransactions();
    return new MockProducerFactory<String, String>((tx, id) -> mockProducer, "defaultTxId");
}

Notice in the second case, the lambda is a BiFunction<Boolean, String> where the first parameter is true if the caller wants a transactional producer; the optional second parameter contains the transactional id. This can be the default (as provided in the constructor), or can be overridden by the KafkaTransactionManager (or KafkaTemplate for local transactions), if so configured. The transactional id is provided in case you wish to use a different MockProducer based on this value.

If you are using producers in a multi-threaded environment, the BiFunction should return multiple producers (perhaps thread-bound using a ThreadLocal).

Transactional MockProducers must be initialized for transactions by calling initTransaction().