Testing Jakarta EE REST APIs with Mockito and JUnit 5

Here’s the revised version using @ExtendWith and @InjectMocks for CDI-style mocking:

Testing Jakarta EE REST APIs with Mockito and JUnit 5
Discover how to test Jakarta EE 10 APIs without a server using Mockito’s dependency injection and JUnit 5 extensions.

1. Maven Dependencies

Add these testing frameworks to your pom.xml:

xml<code><dependencies>
    <em><!-- Jakarta EE 10 API --></em>
    <dependency>
        <groupId>jakarta.platform</groupId>
        <artifactId>jakarta.jakartaee-api</artifactId>
        <version>10.0.0</version>
        <scope>provided</scope>
    </dependency>

    <em><!-- Testing --></em>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter</artifactId>
        <version>5.9.2</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.mockito</groupId>
        <artifactId>mockito-junit-jupiter</artifactId>
        <version>5.4.0</version>
        <scope>test</scope>
    </dependency>
</dependencies>

xml<code><dependencies>

<groupId>jakarta.platform</groupId>

<artifactId>jakarta.jakartaee-api</artifactId>

<scope>provided</scope>

</dependency>

<groupId>org.junit.jupiter</groupId>

<artifactId>junit-jupiter</artifactId>

</dependency>

<groupId>org.mockito</groupId>

<artifactId>mockito-junit-jupiter</artifactId>

</dependency>

</dependencies>

2. REST Resource with CDI

A Jakarta EE resource using @Inject for service layer dependency:

java<code>import jakarta.ws.rs.GET;
import jakarta.ws.rs.Path;
import jakarta.inject.Inject;

@Path("/greet")
public class GreetingResource {
    @Inject
    private GreetingService service;

    @GET
    public String greet() {
        return service.generate();
    }
}

java<code>import jakarta.ws.rs.GET;

import jakarta.ws.rs.Path;

import jakarta.inject.Inject;

@Path("/greet")

public class GreetingResource {

@Inject

private GreetingService service;

@GET

public String greet() {

return service.generate();

}

3. Test Class with Mockito Injection

Using @ExtendWith and @InjectMocks to mock CDI dependencies:

java<code>import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import static org.junit.jupiter.api.Assertions.*;
import static org.mockito.Mockito.*;

@ExtendWith(MockitoExtension.class)
public class GreetingResourceTest {
    
    @Mock
    private GreetingService mockService;
    
    @InjectMocks
    private GreetingResource resource;
    
    @Test
    void testGreet() {
        <em>// Configure mock</em>
        when(mockService.generate()).thenReturn("Mocked greeting");
        
        <em>// Test resource method directly</em>
        String result = resource.greet();
        
        <em>// Verify</em>
        assertEquals("Mocked greeting", result);
        verify(mockService).generate();
    }
}

java<code>import org.junit.jupiter.api.Test;

import org.junit.jupiter.api.extension.ExtendWith;

import org.mockito.InjectMocks;

import org.mockito.Mock;

import org.mockito.junit.jupiter.MockitoExtension;

import static org.junit.jupiter.api.Assertions.*;

import static org.mockito.Mockito.*;

@ExtendWith(MockitoExtension.class)

public class GreetingResourceTest {

@Mock

private GreetingService mockService;

@InjectMocks

private GreetingResource resource;

@Test

void testGreet() {

// Configure mock

when(mockService.generate()).thenReturn("Mocked greeting");

// Test resource method directly

String result = resource.greet();

// Verify

assertEquals("Mocked greeting", result);

verify(mockService).generate();

}

Key Improvements

No HTTP Server: Tests business logic directly without JerseyTest
CDI-Style Mocking: @InjectMocks simulates Jakarta EE’s @Inject behavior
Simplified Setup: Removes Jersey dependencies for pure unit testing

How It Works

@ExtendWith(MockitoExtension.class): Enables Mockito’s JUnit 5 integration
@Mock: Creates a mock instance of GreetingService
@InjectMocks: Injects mocks into fields annotated with @Inject
Direct Method Invocation: Tests resource methods without HTTP layer

Testing CDI Components

For full CDI container testing (including interceptors and producers), use CDI Test:

xml<code><dependency>
    <groupId>org.jboss.weld</groupId>
    <artifactId>weld-junit5</artifactId>
    <version>4.0.3.Final</version>
    <scope>test</scope>
</dependency>

xml<code><dependency>

<groupId>org.jboss.weld</groupId>

<artifactId>weld-junit5</artifactId>

<version>4.0.3.Final</version>

</dependency>

java<code>@ExtendWith(WeldJunit5Extension.class)
class FullCdiTest {
    @Inject
    GreetingResource resource;
    
    @Test
    void testCdiContainer() {
        assertNotNull(resource);
    }
}

java<code>@ExtendWith(WeldJunit5Extension.class)

class FullCdiTest {

@Inject

GreetingResource resource;

@Test

void testCdiContainer() {

assertNotNull(resource);

}

Common Issues & Fixes

Problem: NullPointerException in @InjectMocks
Solution: Ensure all dependencies are properly mocked with @Mock
Problem: CDI features missing
Solution: Use weld-junit5 for full CDI container testing
Problem: Annotation conflicts
Solution: Avoid mixing @InjectMocks with real CDI containers

When to Use This Approach
Unit Testing: Isolated service/resource testing
Fast Feedback: No server startup required
Mock Validation: Verify interactions with dependencies

Implementation Notes

Limited CDI Features: @InjectMocks only handles basic injection
No Interceptors: Use CDI Test or Arquillian for advanced scenarios
Jakarta EE 10: Requires Java 17+ and updated Mockito dependencies

Alternative for REST Endpoints
For HTTP layer testing with dependency injection:

java<code>@ExtendWith(MockitoExtension.class)
public class GreetingResourceHttpTest extends JerseyTest {
    
    @Mock
    private GreetingService service;
    
    @Override
    protected Application configure() {
        <em>// Manual injection workaround</em>
        return new ResourceConfig()
            .register(new GreetingResource() {{
                this.service = service;
            }});
    }
    
    @Test
    void testHttpEndpoint() {
        when(service.generate()).thenReturn("HTTP mock");
        String response = target("greet").request().get(String.class);
        assertEquals("HTTP mock", response);
    }
}

java<code>@ExtendWith(MockitoExtension.class)

public class GreetingResourceHttpTest extends JerseyTest {

@Mock

private GreetingService service;

@Override

protected Application configure() {

// Manual injection workaround

return new ResourceConfig()

.register(new GreetingResource() {{

this.service = service;

}});

}

@Test

void testHttpEndpoint() {

when(service.generate()).thenReturn("HTTP mock");

String response = target("greet").request().get(String.class);

assertEquals("HTTP mock", response);

}

1. Maven Dependencies

2. REST Resource with CDI

3. Test Class with Mockito Injection

Key Improvements

How It Works

Testing CDI Components

Common Issues & Fixes

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