2018-09-11 00:00:00 +0000

Among other things, testing an interface between two systems with
(consumer-driven) contract tests is faster and more stable than doing so with end-to-end tests. This tutorial shows how to create a contract between a message producer and a message consumer using the Pact framework and how to test the producer and consumer against this contract.

The Scenario

As an example to work with, let’s say we have a user service that sends a message to a message broker each time a new user has been created. The message contains a UUID and a user object.

In Java code, the message looks like this:

public class UserCreatedMessage {

  private String messageUuid;

  private User user;


public class User {

  private long id;

  private String name;


In order to reduce boilerplate code, we use Lombok’s @Data annotation to automatically generate getters and setters for us.

Java objects of type UserCreatedMessage are mapped into JSON strings before we send them to the message broker. We use Jackson’s ObjectMapper to do the mapping from Java objects to JSON strings and back, since it’s included in Spring Boot projects by default.

Note the @NotNull annotations on the fields. This annotation is part of the standard Java Bean Validation annotations we’ll be using to validate message objects later on.

Consumer and Producer Architecture

Before diving into the consumer and producer tests, let’s have a look at the architecture. Having a clean architecture is important since we don’t want to test the whole conglomerate of classes, but only those classes that are responsible for consuming and producing messages.

The figure below shows the data flow through our consumer and provider code base.


  1. In the domain logic on the producer side, something happens that triggers a message.
  2. The message is passed as a Java object to the MessageProducer class which transforms it into a JSON string.
  3. The JSON string is passed on to the MessagePublisher, whose single responsibility is to send it to the message broker.
  4. On the consumer side, the MessageListener class receives the message as a string from the broker.
  5. The string message is passed to the MessageConsumer, which transforms it back into a Java object.
  6. The Java object is passed into the domain logic on the consumer side to be processed.

In the contract between consumer and producer, we want to define the structure of the exchanged JSON message. So, to verify the contract, we actually only need to check that

  • MessageProducer correctly transforms Java objects into JSON strings
  • MessageConsumer correctly transforms JSON strings into Java objects.

Since we’re testing the MessageProducer and MessageConsumer classes in isolation, we don’t care what message broker we’re using. We’re just verifying that these two classes speak the same (JSON) language and can be sure that the contract between producer and consumer is met.

Testing the Message Consumer

Since we’re doing consumer-driven contract testing, we’re starting with the consumer side. You can find the code for the consumer in my github repo.

Our MessageConsumer class looks like this:

public class MessageConsumer {

  private ObjectMapper objectMapper;
  public MessageConsumer(ObjectMapper objectMapper) {
    this.objectMapper = objectMapper;

  public void consumeStringMessage(String messageString) throws IOException {
    UserCreatedMessage message = 
        objectMapper.readValue(messageString, UserCreatedMessage.class);
    Validator validator = 
    Set<ConstraintViolation<UserCreatedMessage>> violations = 
      throw new ConstraintViolationException(violations);
    // pass message into business use case


It takes a string message as input, interprets it as JSON and transforms it into a UserCreatedMessage object with the help of ObjectMapper.

To check if all fields are valid, we use a Java Bean Validator. In our case, the validator will check if all fields are set since we used the @NotNull annotation on all fields in the message class.

If the validation fails, we throw an exception. This is important since we need some kind of signal if the incoming string message is invalid.

If everything looks good, we pass the message object into the business logic.

To test the consumer, we create a unit test similar as we would for a plain REST consumer test:

public class MessageConsumerTest {

  public MessagePactProviderRule mockProvider = 
    new MessagePactProviderRule(this);
  private byte[] currentMessage;

  private MessageConsumer messageConsumer;

  @Pact(provider = "userservice", consumer = "userclient")
  public MessagePact userCreatedMessagePact(MessagePactBuilder builder) {
    PactDslJsonBody body = new PactDslJsonBody();
            .numberType("id", 42L)
            .stringType("name", "Zaphod Beeblebrox")

    return builder
            .expectsToReceive("a user created message")

  public void verifyCreatePersonPact() throws IOException {
    messageConsumer.consumeStringMessage(new String(this.currentMessage));

   * This method is called by the Pact framework.
  public void setMessage(byte[] message) {
    this.currentMessage = message;


We use @SpringBootTest so we can let Spring create a MessageConsumer and @Autowire it into our test. We could do without Spring and just create the MessageConsumer ourselves, though.

The MessageProviderRule takes care of starting up a mock provider that accepts a message and validates if it matches the contract.

The contract itself is defined in the method annotated with @Pact. The method annotated with @PactVerification verifies that our MessageConsumer can read the message.

For the verification, we simply pass the string message provided by Pact into the consumer and if there is no exception, we assume that the consumer can handle the message. This is why it’s important that the MessageConsumer class does all the JSON parsing and validation.

Testing the Message Producer

Let’s look at the producer side. You can find the producer source code in my github repo.

The MessageProducer class looks something like this:

class MessageProducer {

    private ObjectMapper objectMapper;

    private MessagePublisher messagePublisher;

        ObjectMapper objectMapper,
        MessagePublisher messagePublisher) {
      this.objectMapper = objectMapper;
      this.messagePublisher = messagePublisher;

    void produceUserCreatedMessage(UserCreatedMessage message)
        throws IOException {
      String stringMessage = 
      messagePublisher.publishMessage(stringMessage, "user.created");


The central part is the method produceUserCreatedMessage(). It takes a UserCreatedMessage object, transforms it into a JSON string, and then passes that string to the MessagePublisher who will send it to the message broker.

The Java-to-JSON mapping is done with an ObjectMapper instance.

The test for the MessageProducer class looks like this:

public class UserCreatedMessageProviderTest {

    public final Target target = 
        new AmqpTarget(Collections.singletonList("io.reflectoring"));

    private MessagePublisher publisher = 

    private MessageProducer messageProvider = 
        new MessageProducer(new ObjectMapper(), publisher);

    @PactVerifyProvider("a user created message")
    public String verifyUserCreatedMessage() throws IOException {
      // given
        .publishMessage(any(String.class), eq("user.created"));

      // when
      UserCreatedMessage message = UserCreatedMessage.builder()
              .name("Zaphod Beeblebrox")

      // then
      ArgumentCaptor<String> messageCapture = 
      verify(publisher, times(1))
        .publishMessage(messageCapture.capture(), eq("user.created"));

      return messageCapture.getValue();

With the @PactFolder and @Provider annotation, we tell Pact to load the contracts for the provider named userservice from a certain folder. The contract must have been created earlier by the consumer.

For each interaction in those contracts, we need a method annotated with @PactVerifyProvider, in our case only one. In this method, we use Mockito to mock all dependencies of our MessageProducer away and then pass to it an object of type UserCreatedMessage.

The MessageProducer will dutifully transform that message object into a JSON string and pass that string to the mocked MessagePublisher. We capture the JSON string that is passed to the MessagePublisher and return it.

Pact will automatically send the produced string message to the Target field annotated with @TestTarget (in this case an instance of AmqpTarget) where it will be checked against the contract.

Classpath Issues

I couldn’t quite get the AmqpTarget class to work due to classpath issues. Hence, I created a subclass that overrides some of the reflection magic. Have a look at the code if you run into the same problem.


Due to a clean architecture with our components having single responsibilities, we can reduce the contract test between a message producer and a message consumer to verifying that the mapping between Java objects and JSON strings works as expected.

We don’t have to deal with the actual or even a simulated message broker to verify that message consumer and message provider speak the same language.

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