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What is the use of contextLoads method in Spring Boot Junit Testcases?

This method is empty in all my JUnit test cases. What is the use of this method?
Sonarqube is complaining
"Add a nested comment explaining why this method is empty, throw an UnsupportedOperationException or complete the implementation."
I can bypass this by adding some comment but I just want to know why it is necessary.

When you build a Spring boot application using Spring Initializer. It auto creates a test class for you with contextLoads empty method.
#SpringBootTest
class ApplicationContextTest {
#Test
void contextLoads() {
}
}
Note the use of #SpringBootTest annotation which tells Spring Boot to look for a main configuration class (one with #SpringBootApplication, for instance) and use that to start a Spring application context. Empty contextLoads() is a test to verify if the application is able to load Spring context successfully or not.
If sonarqube is complaining about the empty method then you can do something like this to verify your controller or service bean context:-
#SpringBootTest
public class ApplicationContextTest {
#Autowired
private MyController myController;
#Autowired
private MyService myService;
#Test
public void contextLoads() throws Exception {
assertThat(myController).isNotNull();
assertThat(myService).isNotNull();
}
}

Use different runner,
if you are using SpringRunner.class, use an alternative one such as MockitoJUnitRunner.class or MockitoJunitRunner.class rather then SpringRunner.class
#Runwith(SpringRunner.class)
#Runwith(JUnit4ClassRunner.class)
#Runwith(MockitoJUnit4Runner.class)
#Runwith(MockitoJUnitRunner.class)

How to Autowire Spring Beans without Application Context

I would like to be able to use a bean via auto-wiring and without having to directly use an ApplicationContext. Below is a dummy example of what I would like to be able to do.
Configuration Class
#Configuration
public class CoffeeConfig
{
#Bean
public CoffeeMachine provideCoffeeMachine()
{
return new CoffeeMachine(provideCoffeeBean());
}
#Bean
public CoffeeBean provideCoffeeBean()
{
return new CoffeeBean(Type.BEST);
}
}
Coffee Shop Class
#Component
public class CoffeeShop
{
#Autowired
private CoffeeMachine cMachine;
public void pourCoffee()
{
System.out.print("Pouring cup of coffee: " + cMachine.pour(Amount.8OZ));
}
}
In order to solve this, I have been reading through spring documentation and spring tutorials. The problem is, I haven't seen anyone attempt to illustrate how to do something as simple as this, and when they do, they end up resorting to using an application context. That being said, I know that if I am running unit tests with Spring, I can do the following:
Test Class
#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration(classes=CoffeeConfig.class, loader=AnnotationConfigContextLoader.class)
public class SpringIOCTests
{
#Autowired
public CoffeeMachine cMachine;
#Test
public void influxDevTest()
{
assertEquals(Type.BEST, cMachine.getBeans());
}
}
The way this configures leads me to believe that using auto-wiring in such a way should be attainable in the actual application instead of using these test-only dependencies such as the ContextConfiguration. I should also note that this unit test does pass.
Does Spring offer a methodology in which one can auto-wire dependencies in a nice and clean way avoiding the direct use of an application contexts?

Initialize database only when running Test using Spring java based configuration

I have the code to create data source and run scripts to create schema and add in data using java based spring configuration. I want to run those scripts only in test mode. Is there any annotation that I need to specify on top of the database initializer bean that would do the job?

You can use #profile annotation for this, like:
#Configuration
#Profile("test_profile")
public class StandaloneDataConfig {
#Bean
public DataSource dataSource() {
// do data source creation/initialization
}
}
Then use:
#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration(loader=AnnotationConfigContextLoader.class,
classes={StandaloneDataConfig.class})
#ActiveProfiles("test_profile")
public class TransferServiceTest {
#Autowired
private TransferService transferService;
#Test
public void testTransferService() {
// test the transferService
}
}
You can read this blog post from the Spring team to learn more details.

Injecting Mockito Mock objects using Spring JavaConfig and #Autowired

I'm trying to replace an #Autowired object with a Mockito mock object. The usual way of doing this was with xml using Springockito:
<mockito:mock id="SomeMock" class="com.package.MockInterface" />
Currently I'm trying to move over to using Spring's JavaConfig to do the job. All of a sudden the Java expressions are a whole lot more verbose than xml:
#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration
public class MyTestClass {
#Configuration
static class Config {
#Bean
public MockInterface somethingSpecial() {
return Mockito.mock(MockInterface.class);
}
}
#Autowired MockInterface mockObj;
// test code
}
I discovered a library called Springockito-annotations, which allows you to do the following:
#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration(loader=SpringockitoContextLoader.class)
public class MyTestClass {
#Autowired #ReplaceWithMock MockInterface mockObj;
// test code
}
Clearly, a whole lot prettier :) The only problem is that this context loader doesn't allow me to use #Configuration and JavaConfig for other beans (if I do, Spring complains that there are no candidates that match those autowired fields).
Do you guys know of a way to get Spring's JavaConfig and Springockito-annotations to play nice? Alternatively, is there another shorthand for creating mocks?
As a nice bonus, using Springockito and xml config, I was able to mock out concrete classes without providing autowiring candidates to its dependencies (if it had any). Is this not possible without xml?

Moving away from the now unmaintained (as of this writing) Spingockito-annotations and to Mockito, we have a way of doing this very simply:
#RunWith(MockitoJUnitRunner.class)
#ContextConfiguration
public class MyTestClass {
#Mock MockInterface mockObj;
// test code
}
If you're using a real object, but would like to mock a dependency within it, for instance testing a service layer with DAO:
#RunWith(MockitoJUnitRunner.class)
#ContextConfiguration
public class MyTestClass {
#InjectMocks RealService;
#Mock MockDAO mockDAO;
// test code
}
Finally, this can also be applied to Spring-boot, but using annotation initialization within setUp() until multiple class runners are supported:
#RunWith(SpringJUnit4ClassRunner.class)
#SpringApplicationConfiguration(classes = MyMainSpringBootClass.class)
public class MyTestClass {
#InjectMocks RealService;
#Mock MockDAO mockDAO;
#Before
public final void setUp() throws Exception{
MockitoAnnotations.initMocks(this);
}
// test code
}

Outdated and deprecated!
Read about mocking and spying in Spring Boot 1.4
Please read also #ethesx answer,
Springockito is unmaintaned
Old answer
This is possible now to mock Spring application without any XML file with Springockito-annotations.. This solution works also with Spring Boot.
import static org.mockito.BDDMockito.*;
import org.kubek2k.springockito.annotations.*;
#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration(classes = Application.class,
loader = SpringockitoAnnotatedContextLoader.class)
#DirtiesContext(classMode = DirtiesContext.ClassMode.AFTER_CLASS)
public class MainControllerTest {
#Autowired
MainController mainController;
#Autowired
#ReplaceWithMock
FooService fooService;
#Test
public void shouldGetBar() {
//given
given(fooService.result("foo")).willReturn("bar");
//when
Bar bar build = fooService.getBar("foo");
//then
assertThat(bar).isNotNull();
}
}
Dependencies: org.kubek2k:springockito-annotations:1.0.9

It appears that SpringockitoContextLoader extends GenericXmlContextLoader which is described as:
Concrete implementation of AbstractGenericContextLoader that reads bean definitions from XML resources.
So you are limited to xml bean definitions at the moment.
You could write your own context loader, taking relevant parts from the SpringockitoContextLoader class. Take a look here to get started, perhaps you could extend AnnotationConfigContextLoader for example?

integration testing spring service layer based on migrated data

#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration(locations={"/applicationContext-test.xml"})
#Transactional
public class MyServiceTest {
#Resource(name="myService")
public MyService myService;
#Test
public void testSeomthing() {
//do some asserts using myService.whatever()
}
}
However the tests are based on data I migrate in, so every time I run my suite of tests I want to execute my unrelated migration code. I don't want to run a #Before in each test class. I want to run it once at beginning of complete test process, where can I put this ?

I would advice you to create a test bean somewhere with startup logic invoked in #PostConstruct:
#Service
public class TestBean {
#PostConstruct
public void init() {
//startup logic here
}
}
Obviously this bean should only be created for tests, the easiest way to achieve this is to place it in src/test/java in a package that is component-scanned by Spring for #Service-annotated classes.
Note: you must remember that #PostConstruct is not running in a transaction! See How to call method on spring proxy once initialised.

JUnit also offers a #BeforeClass annotation which you can place on a static method to initialize resources just once.