Mockito @InjectMocks - Mocks Dependency Injection

Mockito tries to inject mocked dependencies using one of the three approaches, in the specified order.

1. Constructor Based Injection - when there is a constructor defined for the class, Mockito tries to inject dependencies using the biggest constructor.
2. Setter Methods Based - when there are no constructors defined, Mockito tries to inject dependencies using setter methods.
3. Field Based - if there are no constructors or field-based injection possible, then mockito tries to inject dependencies into the field itself.

If there is only one matching mock object, then mockito will inject that into the object. If there is more than one mocked object of the same class, then mock object name is used to inject the dependencies.

Let’s create some services and classes with dependencies so that we can see Mockito dependency injection of mocks in action. Service Classes

package com.journaldev.injectmocksservices;

public interface Service {

	public boolean send(String msg);
}

package com.journaldev.injectmocksservices;

public class EmailService implements Service {

	@Override
	public boolean send(String msg) {
		System.out.println("Sending email");
		return true;
	}

}

package com.journaldev.injectmocksservices;

public class SMSService implements Service {

	@Override
	public boolean send(String msg) {
		System.out.println("Sending SMS");
		return true;
	}
}

App Service Classes with Dependencies

package com.journaldev.injectmocksservices;

//For Constructor Based @InjectMocks injection
public class AppServices {

	private EmailService emailService;
	private SMSService smsService;

	public AppServices(EmailService emailService, SMSService smsService) {
		this.emailService = emailService;
		this.smsService = smsService;
	}

	public boolean sendSMS(String msg) {
		return smsService.send(msg);
	}

	public boolean sendEmail(String msg) {
		return emailService.send(msg);
	}
}

package com.journaldev.injectmocksservices;

//For Property Setter Based @InjectMocks injection
public class AppServices1 {

	private EmailService emailService;
	private SMSService smsService;

	public void setEmailService(EmailService emailService) {
		this.emailService = emailService;
	}

	public void setSmsService(SMSService smsService) {
		this.smsService = smsService;
	}

	public boolean sendSMS(String msg) {
		return smsService.send(msg);
	}

	public boolean sendEmail(String msg) {
		return emailService.send(msg);
	}

}

package com.journaldev.injectmocksservices;

//For Field Based @InjectMocks injection
public class AppServices2 {

	private EmailService emailService;
	private SMSService smsService;

	public boolean sendSMS(String msg) {
		return smsService.send(msg);
	}

	public boolean sendEmail(String msg) {
		return emailService.send(msg);
	}

}

@InjectMocks Constructor Injection Example

package com.journaldev.mockito.injectmocks;

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

import org.junit.jupiter.api.Test;
import org.mockito.InjectMocks;
import org.mockito.Mock;

import com.journaldev.injectmocksservices.AppServices;
import com.journaldev.injectmocksservices.AppServices1;
import com.journaldev.injectmocksservices.AppServices2;
import com.journaldev.injectmocksservices.EmailService;
import com.journaldev.injectmocksservices.SMSService;

class MockitoInjectMocksExamples extends BaseTestCase {

	@Mock EmailService emailService;
	
	@Mock SMSService smsService;
	
	@InjectMocks AppServices appServicesConstructorInjectionMock;
	
	@InjectMocks AppServices1 appServicesSetterInjectionMock;
	
	@InjectMocks AppServices2 appServicesFieldInjectionMock;
	
	@Test
	void test_constructor_injection_mock() {
		when(appServicesConstructorInjectionMock.sendEmail("Email")).thenReturn(true);
		when(appServicesConstructorInjectionMock.sendSMS(anyString())).thenReturn(true);
		
		assertTrue(appServicesConstructorInjectionMock.sendEmail("Email"));
		assertFalse(appServicesConstructorInjectionMock.sendEmail("Unstubbed Email"));
		
		assertTrue(appServicesConstructorInjectionMock.sendSMS("SMS"));
		
	}
}

Did you noticed that my test class is extending BaseTestCase. This is to initialize Mockito mocks before the tests, here is the code of the class.

package com.journaldev.mockito.injectmocks;

import org.junit.jupiter.api.BeforeEach;
import org.mockito.MockitoAnnotations;

class BaseTestCase {

	@BeforeEach
	void init_mocks() {
		MockitoAnnotations.initMocks(this);
	}

}

If you won’t call MockitoAnnotations.initMocks(this); then you will get NullPointerException. Also, I am using JUnit 5 to run the test cases. If you are not familiar with it, have a look at JUnit 5 Tutorial.

@InjectMocks Setter Methods Injection Example

@Test
void test_setter_injection_mock() {
	when(appServicesSetterInjectionMock.sendEmail("New Email")).thenReturn(true);
	when(appServicesSetterInjectionMock.sendSMS(anyString())).thenReturn(true);
	
	assertTrue(appServicesSetterInjectionMock.sendEmail("New Email"));
	assertFalse(appServicesSetterInjectionMock.sendEmail("Unstubbed Email"));
	
	assertTrue(appServicesSetterInjectionMock.sendSMS("SMS"));
	
}

@InjectMocks Field Based Injection Example

@Test
void test_field_injection_mock() {
	when(appServicesFieldInjectionMock.sendEmail(anyString())).thenReturn(true);
	when(appServicesFieldInjectionMock.sendSMS(anyString())).thenReturn(true);
	
	assertTrue(appServicesFieldInjectionMock.sendEmail("Email"));
	assertTrue(appServicesFieldInjectionMock.sendEmail("New Email"));
	
	assertTrue(appServicesFieldInjectionMock.sendSMS("SMS"));
	
}

You can check out complete code and more Mockito examples from our GitHub Repository.

Reference: API Doc