How to mock an exception for compatable future for whenComplete in the below code in junit5 for whenComplete

I am writing an integration test.How to mock an exception for whenComplete in the below code:

                   .send().whenComplete((connAck, exception) -> {
               if (exception != null) {
                   log.error("Failed publishing the  event ")
               } else
         "Successfully published the  event  ")

To mock an exception for the whenComplete block in an integration test using JUnit 5, you would typically want to use a mocking framework like Mockito to simulate the behavior of the publishWith() and send() calls. When using Mockito, you can specify behavior such as throwing an exception when a certain method is called.Here’s an example of how you might mock the exception for the whenComplete block using Mockito and CompletableFuture:1. Create a mock of the Mqtt5AsyncClient.
2. Stub the publishWith() method to return a builder.
3. Stub the send() method to return a CompletableFuture that completes exceptionally.Here is some pseudocode on how you could do this:

import org.mockito.Mockito;
import static org.mockito.Mockito.*;
import com.hivemq.client.mqtt.mqtt5.Mqtt5AsyncClient;
import java.util.concurrent.CompletableFuture;

// ...

// Mock the Mqtt5AsyncClient
Mqtt5AsyncClient mockMqtt5Client = mock(Mqtt5AsyncClient.class);
Mqtt5PublishBuilder.Send> mockSendBuilder = mock(Mqtt5PublishBuilder.Send.class);

when(mockMqtt5Client.publishWith()).thenReturn(mockPublishBuilder); // stub publishWith
when(mockPublishBuilder.topic(anyString())).thenReturn(mockPublishBuilder); // chain the builder methods
when(mockPublishBuilder.send()).thenReturn(CompletableFuture.failedFuture(new RuntimeException("Mock exception"))); // specify to complete exceptionally

// Now run your test logic where mqtt5Client is used...

In this pseudocode, we mock the behavior so that when the send() method is called, it creates a CompletableFuture that already has a failure (RuntimeException("Mock exception") in this case) set. This will ensure that the lambda passed to whenComplete will be invoked with a non-null exception, simulating a failure scenario.Please note, you might need to adjust the code to match the exact builder pattern methods used in your specific implementation. Also, you need to have appropriate imports and the Mockito library included in your project to utilize its functionality.To use the above mocking in the context of your test, you will need to inject this mocked Mqtt5AsyncClient into the component that you’re testing, replacing the actual client that would be used to perform real operations.