So since I've been using Spring, if I were to write a service that had dependencies I would do the following:
#Component
public class SomeService {
#Autowired private SomeOtherService someOtherService;
}
I have now run across code that uses another convention to achieve the same goal
#Component
public class SomeService {
private final SomeOtherService someOtherService;
#Autowired
public SomeService(SomeOtherService someOtherService){
this.someOtherService = someOtherService;
}
}
Both of these methods will work, I understand that. But is there some advantage to using option B? To me, it creates more code in the class and unit test. (Having to write constructor and not being able to use #InjectMocks)
Is there something I'm missing? Is there anything else the autowired constructor does besides add code to the unit tests? Is this a more preferred way to do dependency injection?
Yes, option B (which is called constructor injection) is actually recommended over field injection, and has several advantages:
the dependencies are clearly identified. There is no way to forget one when testing, or instantiating the object in any other circumstance (like creating the bean instance explicitly in a config class)
the dependencies can be final, which helps with robustness and thread-safety
you don't need reflection to set the dependencies. InjectMocks is still usable, but not necessary. You can just create mocks by yourself and inject them by simply calling the constructor
See this blog post for a more detailed article, by one of the Spring contributors, Olivier Gierke.
I will explain you in simple words:
In Option(A), you are allowing anyone (in different class outside/inside the Spring container) to create an instance using default constructor (like new SomeService()), which is NOT good as you need SomeOtherService object (as a dependency) for your SomeService.
Is there anything else the autowired constructor does besides add code
to the unit tests? Is this a more preferred way to do dependency
injection?
Option(B) is preferred approach as it does NOT allow to create SomeService object without actually resolving the SomeOtherService dependency.
Please note, that since Spring 4.3 you don't even need an #Autowired on your constructor, so you can write your code in Java style rather than tying to Spring's annotations.
Your snippet would look like that:
#Component
public class SomeService {
private final SomeOtherService someOtherService;
public SomeService(SomeOtherService someOtherService){
this.someOtherService = someOtherService;
}
}
Good to know
If there is only one constructor call, there is no need to include an #Autowired annotation. Then you can use something like this:
#RestController
public class NiceController {
private final DataRepository repository;
public NiceController(ChapterRepository repository) {
this.repository = repository;
}
}
... example of Spring Data Repository injection.
Actually, In my experience, The second option is better. Without the need for #Autowired. In fact, it is wiser to create code that is not too tightly coupled with the framework (as good as Spring is). You want code that tries as much as possible to adopt a deferred decision-making approach. That is as much pojo as possible, so much such that the framework can be swapped out easily.
So I would advise you create a separate Config file and define your bean there, like this:
In SomeService.java file:
public class SomeService {
private final SomeOtherService someOtherService;
public SomeService(SomeOtherService someOtherService){
this.someOtherService = someOtherService;
}
}
In ServiceConfig.java file:
#Config
public class ServiceConfig {
#Bean
public SomeService someService(SomeOtherService someOtherService){
return new SomeService(someOtherService);
}
}
In fact, if you want to get deeply technical about it, there are thread safety questions (among other things) that arise with the use of Field Injection (#Autowired), depending on the size of the project obviously. Check this out to learn more on the advantages and disadvantages of Autowiring. Actually, the pivotal guys actually recommend that you use Constructor injection instead of Field Injection
I hope I won't be downgraded for expressing my opinion, but for me option A better reflects the power of Spring dependency injection, while in the option B you are coupling your class with your dependency, in fact you cannot instantiate an object without passing its dependencies from the constructor. Dependency Injection have been invented for avoid that by implementing Inversion of Control,so for me option B doesn't have any sense.
Autowired constructors provides a hook to add custom code before registering it in the spring container. Suppose SomeService class extends another class named SuperSomeService and it has some constructor which takes a name as its argument. In this case, Autowired constructor works fine. Also, if you have some other members to be initialized, you can do it in the constructor before returning the instance to spring container.
public class SuperSomeService {
private String name;
public SuperSomeService(String name) {
this.name = name;
}
}
#Component
public class SomeService extends SuperSomeService {
private final SomeOtherService someOtherService;
private Map<String, String> props = null;
#Autowired
public SomeService(SomeOtherService someOtherService){
SuperSomeService("SomeService")
this.someOtherService = someOtherService;
props = loadMap();
}
}
I prefer construction injection, just because I can mark my dependency as final which is not possible while injecting properties using property injection.
your dependencies should be final i.e not modified by program.
There are few cases when #Autowired is preferable.
One of them is circular dependency. Imagine the following scenario:
#Service
public class EmployeeService {
private final DepartmentService departmentService;
public EmployeeService(DepartmentService departmentService) {
this.departmentService = departmentService;
}
}
and
#Service
public class DepartmentService {
private final EmployeeService employeeService;
public DepartmentService(EmployeeService employeeService) {
this.employeeService = employeeService;
}
}
Then Spring Bean Factory will throw circular dependency exception. This won't happen if you use #Autowired annotation in both beans. And this is understandable: the constructor injection happens at very early stage of Spring Bean initialization, in createBeanInstance method of Bean Factory, while #Autowired-based injection happens way later, on post processing stage and is done by AutowiredAnnotationBeanPostProcessor.
Circular dependency is quite common in complex Spring Context application, and it needs not to be just two beans referring one another, it could a complex chain of several beans.
Another use case, where #Autowired is very helpful, is self-injection.
#Service
public class EmployeeService {
#Autowired
private EmployeeService self;
}
This might be needed to invoke an advised method from within the same bean. Self-injection is also discussed here and here.
There is a way to inject the dependencies through constructor using #RequeiredArgsContructor annotation from Lombok
#RequiredArgsConstructor
#Service
class A {
private final B b // needs to be declared final to be injected
}
In this way you don't need to specify a constructor
Following are my code
#RestController
public class EmployeeController {
#Autowired
EmployeeService empService;
public EmployeeController (EmployeeService Impl empServiceImpl) {
super();
this.empService = empServiceImpl;
}
}
#Service
public interface EmployeeService {
public List<EmployeeDTO> getAllEmployeeDetails()
}
public class EmployeeServiceImpl {
public List<EmployeeDTO> getAllEmployeeDetails(){
//methods business logic and repo call goes here
}
}
When I start my server I am getting below error.
Parameter 1 of constructor in
com.app.in.controller.EmployeeController required a bean of type
'com.app.in.service.EmployeeServiceImpl' that could not be found
My understanding might be wrong. If I annotate the EmployeeSeriveImpl class also with #Service then it working.Is that is the correct way to do it ? My question is the service interface is annotated with #Service still why its implementation is also required to annotation. Please let me know if I miss something in that ? What is the standard method to solve this issue ?
You can get your dependency injected using a constructor. And #Autowired is optional in this case.
This is your example, but with a few corrections:
#RestController
public class EmployeeController {
// private final is a good practice. no need in #Autowire
private final EmployeeService empService;
// this constructor will be used to inject your dependency
// #Autowired is optional in this case, but you can put it here
public EmployeeController (EmployeeService empServiceImpl) {
this.empService = empServiceImpl;
}
}
I assume you have an interface EmployeeService and class EmployeeServiceImpl which implements that interface and is Spring Bean.
Something like this:
#Service
public class EmployeeServiceImpl implements EmployeeService {}
Why this #Service is needed? When you put this annotation on your class, Spring knows this is a bean that Spring should manage for you (container will create an instance of it and inject it wherever it is needed).
Check Spring docs to get more details about Dependency Injection.
The Spring team generally advocates constructor injection, as it lets you implement application components as immutable objects and ensures that required dependencies are not null.
I have a controller and a test using #WebMvcTest and its running fine. Now i needed to add a little validation logic and for this i #Autowired an additional bean (a #Component, a MapstructMapper).
As expected now the test is failing due to #WebMvcTest. (No components are discovered)
Is there a way to add one bean to the context created?
Since i am using #MockBeans to mock service layer: is there a way to delegate all mock calls to a real object? With this i could mock the mapper and delegate to real mapper?!
A simple way of getting additional beans in the context is via using nested configuration classes within test classes
#TestConfiguration
static class AdditionalConfig {
#Bean
public SomeBean getSomeBean() {
return new SomeBean());
}
}
Example:
Scenario - If you have some Controller say ProductController and you have the corresponding slice-test for the class say ProductionControllerTest
#RestController
public class ProductController {
#Autowired
private IProductService productService;
#Autowired
private IProductValidator productValidator;
#GetMapping("product")
public Product getProduct(#RequestParam Long id) {
Product product = productService.getProduct(id); // you are using mockBean of productService
productValidator.validateProduct(product); // you need real bean of productValidator
return product;
}
}
Corresponding slide test class with an additional bean configuration
#RunWith(SpringRunner.class)
#WebMvcTest
public class ProductControllerSliceTest {
#Autowired
private MockMvc mockMvc;
#MockBean
private IProductService productService;
#Autowired
private ApplicationContext applicationContext;
#TestConfiguration
static class AdditionalConfig {
#Bean
public IProductValidator productValidator() {
return new ProductValidator();
}
}
#Test
public void testProductGetById() throws Exception {
Product testProductWithID1L = new Product(1L, "testProduct");
when(productService.getProduct(anyLong())).thenReturn(testProductWithID1L);
mockMvc.perform(get("/product")
.param("id", "1")).andDo(print())
.andExpect(status().isOk())
.andExpect(jsonPath("name")
.value("testProduct"))
.andExpect(MockMvcResultMatchers.jsonPath("id")
.value("1"));
}
}
Additional thoughts on your scenario: If you really intend to do the unit testing of the controller class then ideally you should mock all the additional dependencies of class that you are testing.
Ideally, the intention of the unit test is to only test the behavior of the object/class under test. All the dependent classes behavior or external calls should be mocked.
When you start testing several classes together under one test, you are moving more towards a component test or integration test
A very simple solution is to annotate your test class with #Import specifying the class(es) of the additional bean(s) you want to use in your test, as stated in documentation:
Typically #WebMvcTest is used in combination with #MockBean or #Import
to create any collaborators required by your #Controller beans.
e.g.
#WebMvcTest(MyController.class)
#Import(SomeOtherBean.class)
public class SourcingOrganisationControllerTests {
// The controller bean is made available via #WebMvcTest
#Autowired
private MyController myController;
// Additional beans (only one in this case) are made available via #Import
#Autowired
private SomeOtherBean someOtherBean;
}
While looking at an existing Spring application, I stumbled upon a class with field injection, which we all know isn't recommended for various reasons. I have then decided to refactor it to make use of a more appropriate approach: constructor based DI.
Before refactoring
#Component
public class MaintenanceModeInterceptor implements HandlerInterceptor {
private static final String MAINTENANCE_MODE_VIEW = "common/maintenanceMode";
#Autowired
private ApplicationObject applicationObject;
public MaintenanceModeInterceptor() {
// Required by Spring
}
...
}
After refactoring
#Component
public class MaintenanceModeInterceptor implements HandlerInterceptor {
private static final String MAINTENANCE_MODE_VIEW = "common/maintenanceMode";
private ApplicationObject applicationObject;
public MaintenanceModeInterceptor() {
// Required by Spring
}
#Autowired
public MaintenanceModeInterceptor(ApplicationObject applicationObject) {
this.applicationObject = applicationObject;
}
...
}
Maybe it is related to the fact that a default constructor is present. However, if I remove it, I end up having this exception:
Caused by: java.lang.NoSuchMethodError: my.application.web.interceptor.MaintenanceModeInterceptor: method <init>()V not found
So my understanding is that Spring requires a default constructor for interceptors.
Is there any way to achieve construtor based DI in this scenario?
Thank you.
I think you should remove the non #Autowired constructor and do perform a clean build on your project.
I have a Interface which is registered as part of ServiceLocatorFactoryBean. The main purpose of this Interface is to act as a factory.
http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/beans/factory/config/ServiceLocatorFactoryBean.html
I have "autowired" this Interface in various classes, that I want to test now with Mockito.
The issue is Mockito doesn't support interfaces. How can inject a mock of this interface in the class I am testing.
The only alternative I see is to run the test using SpringJunitRunner and providing an Application Context which has the bean configurations. But this is too verbose.
I take it you'd like to spy on the implementation that Spring generated for your interface?! That's close to impossible to achieve with what you have so far... However there are at least the following alternatives below.
Suppose we have the following setup:
public interface MyService {
String doIt();
}
#Component
public static class ServiceConsumer {
#Autowired
private MyService service;
public String execute() {
return service.doIt();
}
}
0) Later edit: while roaming around, I found that it may be possible to spy and even replace an autowired field with a mock, and fairly easy too, using Springockito-annotations.
#RunWith(SpringJUnit4ClassRunner.class)
#ComponentScan
#ContextConfiguration(loader = SpringockitoAnnotatedContextLoader.class, classes = {SpringockitoConsumerTest.class})
public class SpringockitoConsumerTest {
#WrapWithSpy(beanName = "myService")
#Autowired
private MyService mockService;
#Autowired
private ServiceConsumer consumer;
#Test
public void shouldConsumeService() {
assertEquals("allDone", consumer.execute());
verify(mockService).doIt();
}
}
If Springockito-annotations is out of the question, please see the 2 original suggestions below
1) You could just create your mock of the interface and auto-inject it Mockito in your bean. This is the simplest solution (I could think of at the time of writing) but it does not ensure that the #Autowired annotation was not forgotten in the consumer (perhaps a dedicated test could be added?!):
public class AutoInjectMocksConsumerTest {
#Mock
private MyService serviceMock;
#InjectMocks
private ServiceConsumer consumer = new ServiceConsumer();
#Before
public void initMocks() {
MockitoAnnotations.initMocks(this);
when(serviceMock.doIt()).thenReturn("allDone");
}
#Test
public void shouldConsumeService() {
assertEquals("allDone", consumer.execute());
verify(serviceMock).doIt();
}
}
2) Alternatively as you also said, you could run it with the SpringJunitRunner making a minimum of effort to define and instantiate the necessary Spring context while also providing your own service mock. Albeit people may complain this solution is not that clean, I find it sufficiently elegant and it also validates that the #Autowired annotation was not forgotten in the consumer implementation.
#RunWith(SpringJUnit4ClassRunner.class)
#Configuration
#ComponentScan
#ContextConfiguration(classes = {SpringAutowiringConsumerTest.class})
public class SpringAutowiringConsumerTest {
#Autowired
private MyService mockService;
#Autowired
private ServiceConsumer consumer;
#Test
public void shouldConsumeService() {
assertEquals("allDone", consumer.execute());
verify(mockService).doIt();
}
#Bean
public MyService mockService() {
MyService serviceMock = mock(MyService.class);
when(serviceMock.doIt()).thenReturn("allDone");
return serviceMock;
}
}