How do I write a test case using Mockito and JUnit 5 extensions

Step 1: Setup Your Project

First, ensure you have JUnit 5 and Mockito dependencies in your project. If you're using Maven, add these dependencies to your `pom.xml`:

xml

    org.junit.jupiter
    junit-jupiter-api
    5.9.0
    test


    org.junit.jupiter
    junit-jupiter-engine
    5.9.0
    test


    org.mockito
    mockito-core
    4.1.0
    test


    org.mockito
    mockito-junit-jupiter
    4.1.0
    test

For Gradle, add these to your `build.gradle`:

groovy
testImplementation 'org.junit.jupiter:junit-jupiter-api:5.9.0'
testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.9.0'
testImplementation 'org.mockito:mockito-core:4.1.0'
testImplementation 'org.mockito:mockito-junit-jupiter:4.1.0'

Step 2: Create a Class to Test

Let's say you have a simple `Service` class that depends on a `Database` interface:

java
public interface Database {
    boolean isAvailable();
    int getUniqueId();
}

public class Service {
    private Database database;

    public Service(Database database) {
        this.database = database;
    }

    public boolean query(String query) {
        return database.isAvailable();
    }

    @Override
    public String toString() {
        return "Using database with id: " + String.valueOf(database.getUniqueId());
    }
}

Step 3: Write the Test Case

Now, you can write a test case using Mockito and JUnit 5 extensions. Use the `@ExtendWith(MockitoExtension.class)` annotation to enable Mockito support in JUnit 5.

java
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

import static org.junit.jupiter.api.Assertions.assertNotNull;
import static org.junit.jupiter.api.Assertions.assertTrue;
import static org.mockito.Mockito.when;

@ExtendWith(MockitoExtension.class)
public class ServiceTest {

    @Mock
    Database databaseMock;

    @Test
    public void testQuery() {
        assertNotNull(databaseMock);
        
        // Configure the mock to return true when isAvailable() is called
        when(databaseMock.isAvailable()).thenReturn(true);
        
        // Create an instance of Service with the mock database
        Service service = new Service(databaseMock);
        
        // Call the method under test
        boolean result = service.query("* from t");
        
        // Assert the result
        assertTrue(result);
    }
}

Explanation:

- `@ExtendWith(MockitoExtension.class)`: This annotation tells JUnit 5 to use the Mockito extension, which automatically initializes mock objects annotated with `@Mock`.
- `@Mock Database databaseMock;`: This creates a mock object for the `Database` interface.
- `when(databaseMock.isAvailable()).thenReturn(true);`: This configures the mock to return `true` when `isAvailable()` is called.
- `Service service = new Service(databaseMock);`: Creates an instance of `Service` using the mock database.
- `boolean result = service.query("* from t");`: Calls the method under test.
- `assertTrue(result);`: Asserts that the result is as expected.

This setup allows you to easily write unit tests for classes with dependencies by mocking those dependencies using Mockito.