Spring in practice : dependency injection

04/11/2023

Java Spring

Table of Contents

Objects and application
Inversion of Control (IOC) Principle
Spring Beans
Using Dependency Injection
Conclusion

In this course you will learn one of the main principle of software design, the dependency injection

Fundamental principle of software design
Introduced by Martin Fowler (famous english computer engineer)
Helps to split responsibilities in your code ⇒ weakly coupled components
Facilitates testing

Spring ecosystem was built on this concept and understand dependency injection is a first step to understand Spring.

Objects and application

When writing an application, as developers, we break down the problem we’re trying to solve into smaller ones, and do our best to comply with the architecture and design principles we’ve chosen for our application: flexible, decoupled, testable, easy to understand, etc.

For that we use a lot of objects

Service contains implementations of your business rules
Components help to resolve a technical problem
Repository interacts with external systems as database, webapi…
Controllers are in front of your app to read and check data sent by users
And you have objects to transport data DTO, entities

When we want to define an object we write for example

public class NameService {

    public String getName() {
        return "Guillaume";
    }
}

And to use this object elsewhere we have to create a new instance with a new instruction

public class WelcomeService {

    public void sayHello() {
        NameService nameService = new NameService();
        System.out.println("Hello " + nameService.getName());
    }
}

We have a strong coupling between these classes WelcomeService and NameService. If we want to change NameService we have a good chance of having to update WelcomeService.

For example, if NameService need to use others objects, you have to update the WelcomeService class constructor

public class NameService {

    private UserService userService;

    public NameService(UserService userService) {
        this.userService = userService;
    }

    public String getName() {
        return "Guillaume";
    }

    // ...
}

As the constructor changed you must update the coupled class

public class WelcomeService {

    public void sayHello() {
        UserService userService = new UserService();
        NameService nameService = new NameService(userService);
        System.out.println("Hello " + nameService.getName());
    }
}

Other consideration: in a web application, you should not implement a service at every call. It’s not efficient if a class has a lot of collaborators, and if a service is called in different points in your application.

In this case the class which not change must be created only once. We often use the Singleton design pattern to do that.

We have to resolve these problems (break coupling and use singleton) and the solution is Inversion of Control (IOC).

To introduce this principle we will use a simpler example

If a class A uses a class B

ioc1

public class B {
    public String name() {
        return "Guillaume";
    }
}

public class A {
    public void hello() {
        B b = new B();
        System.out.println("Hello " + b.name());
    }
}

Inversion of Control (IOC) Principle

To resolve this problem, we can use a client, a factory to instantiate class B and inject it into class A.

ioc2

If an object needs other objects, it does not instantiate itself but they are provided by a factory or a client (a container).

Objects define their collaborators (that is, the other objects they work with) through constructor arguments or properties. Container is responsible for the construction of the objects.

It will provide (inject) the collaborators requested by an object.

The first version of Spring was created to resolve this problem. Spring provides a container to create and inject objects.

You must remember

If an object needs other objects, it does not instantiate itself, but they are provided by a factory (in our case Spring).

Therefore, we no longer have to find the new key word in your code.

The only exception is for objects which contain data : Entity and DTO.

Inversion of Control (IoC) principle is also known as dependency injection (DI).

Spring Beans

In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC container are called beans.

A bean is an object that is instantiated, assembled, and managed by a Spring IoC container

The Java language was named Java in reference to Java coffee, the coffee of Indonesia. The Java logo is a cup of tea. With Spring an app can be seen as a set of Java beans

Figure 1. In this image I play the role of the garbage collector. I clean up the unused Java Beans (beans without reference)

Create a bean

By annotation

In Spring, we can use a stereotype on our classes to defined them as Bean: @Service, @Component, @Repository, @Controller

@Service
public class MyGreetingService {
   // Code ...
}

@Controller
public class MyGreetingController {
   // Code ...
}

Spring Boot is able to scan classpath to auto-detect and auto-configure beans annotated with @Service, @Component, @Repository, or @Controller. Each annotation is equivalent, but a sterotype (@Service, @Repository…) helps to understand the object role in your app

By configuration

Also, we can create a Spring bean in a configuration bean, when we need to configure it.

The first step is to create a Configuration bean annotated with @Configuration. This annotation indicates that the class can be used by the Spring IoC container as a source of bean definitions

@Configuration
public class MyAppConfiguration {

    // ...

}

Beans are components instances. A method annotated with @Bean will return an object that should be registered as a bean in the Spring application context.

@Bean is used to explicitly declare a single bean, rather than letting Spring do it automatically as @Component, @Service…

In this example we said to Spring that our UserStore object needs a DataStoreConnectionPool

@Configuration
public class MyAppConfiguration {

  @Bean
  public UserStore userStore(DataStoreConnectionPool connectionPool) {
    return new UserStore(connectionPool.fetchConnection());
  }

}

This way of declaring a bean is used to configure Spring or another library. With these declarations, we can override the default beans configured by Spring.

Add dependencies

When a class need another object, we use @Autowired to inject them via Spring. You have 2 ways to inject a bean in another

Injection by setter

@Component
public class AImpl implements A {

    @Autowired
    private B b;

    public void setB(B b) {
        this.b = b;
    }

    public B getB() {
        return b;
    }
}

Injection by constructor

@Component
public class AImpl implements A {

    private B b;

    @Autowired
    public AImpl(B b) {
        this.b = b;
    }

    public B getB() {
        return b;
    }
}

If you have only one constructor @Autowired is not mandatory for Spring. However, if several constructors are available and there is no primary/default constructor, at least one of the constructors must be annotated with @Autowired in order to instruct the container which one to use.

You have 2 ways of injecting dependencies into an object but injection by constructor is the one recommended by the community

In this example UserStore and CertificateManager are injected into AuthenticationService

@Service
public class AuthenticationService {

  private final UserStore userStore;
  private final CertificateManager certManager;

  public AuthenticationService(UserStore userStore, CertificateManager certManager) {
    this.userStore = userStore;
    this.certManager = certManager;
  }

  public AcccountStatus getAccountStatus(UserAccount account) {
    // here we can use the UserStore with this.userStore
  }
}

How it works ?

Spring looks for components by scanning your application classpath : looking for annotated classes in the app packages or the beans you’ve declared in your configuration beans.

appcontext1

All those components are registered in an application context.

Spring searches a Bean by its type or else by its name

appcontext2

Spring throws a NoSuchBeanDefinitionException if a bean can’t be found

appcontext3

Spring throws a NoUniqueBeanDefinitionException if several beans are found and if it doesn’t know which bean use

appcontext4

Using Dependency Injection

Create a first bean

First, let’s create an interface for our application called GreetingService in package com.emse.spring.automacorp.hello

package com.emse.spring.automacorp.hello;

public interface GreetingService {

  void greet(String name);
}

Don’t forget to commit periodically your work. More information on Git course

Your first job is to output "Hello, Spring! in the console when the application starts.

For that, do the following:

Create in package com.emse.spring.automacorp.hello a class called ConsoleGreetingService. This class has to implement GreetingService interface
Mark it as a service with @Service annotation.
Implement greet method. This method should write to the console using System.out.println.

To check your work you have to create this test in folder src/test

package com.emse.spring.automacorp.hello;

import org.assertj.core.api.Assertions;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.springframework.boot.test.system.CapturedOutput;
import org.springframework.boot.test.system.OutputCaptureExtension;

@ExtendWith(OutputCaptureExtension.class) // (1)
class GreetingServiceTest {

    @Test
    public void testGreeting(CapturedOutput output) {
        GreetingService greetingService = new ConsoleGreetingService(); // (2)
        greetingService.greet("Spring");
        Assertions.assertThat(output.getAll()).contains("Hello, Spring!");
    }
}

(1) We load a Junit5 extension to capture output (log generated by your app)
(2) We’re testing our service implementation without Spring being involved. We create a new instance of this service with a new

You can verify that your implementation is working properly by running ./gradlew test command or by buttons in your IDEA.See this video to see the different solutions

The test source code is valid. If the test execution fails, you have to fix your code.

Inject your bean

Your second Job is to create a new interface UserService in package com.emse.spring.automacorp.hello

package com.emse.spring.automacorp.hello;

import java.util.List;

public interface UserService {
    void greetAll(List<String> name);
}

You can now

create an implementation of this interface called DummyUserService
Mark it as a service.
Inject service GreetingService (use interface and not implementation)
Write greetAll method. You have to call greet method of the GreetingService for each name

As for the first service, we’re going to check this new service with a unit test

package com.emse.spring.automacorp.hello;

import org.assertj.core.api.Assertions;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.system.CapturedOutput;
import org.springframework.boot.test.system.OutputCaptureExtension;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.test.context.junit.jupiter.SpringExtension;

@ExtendWith(OutputCaptureExtension.class)
@ExtendWith(SpringExtension.class) // (1)
class DummyUserServiceTest {

    @Configuration // (2)
    @ComponentScan("com.emse.spring.automacorp.hello")
    public static class DummyUserServiceTestConfig{}

    @Autowired // (3)
    public DummyUserService dummyUserService;

    @Test
    public void testGreetingAll(CapturedOutput output) {
        dummyUserService.greetAll(List.of("Elodie", "Charles"));
        Assertions.assertThat(output).contains("Hello, Elodie!", "Hello, Charles!");
    }
}

(1) We use SpringExtension to link our test to Spring. With this annotation a Spring Context will be loaded when this test will run
(2) We have to configure how the context is loaded. In our case we added @ComponentScan("com.emse.spring.automacorp.hello") to help Spring to found our classes. In our app this scan is made by SpringBoot, but in our test SpringBoot is not loaded
(3) As our test has is own Spring Context we can inject inside the bean to test

You can verify that your implementation is working properly by running ./gradlew test command.

Inject your bean in configuration bean

Now, a new class AutomacorpApplicationConfig in com.emse.spring.automacorp package next AutomacorpApplication class. We want to create a new bean of type CommandLineRunner.

CommandLineRunner instances are found by Spring Boot in the Spring context and are executed during the application startup phase.

// (1)
public class AutomacorpApplicationConfig {

  // (2)
  public CommandLineRunner greetingCommandLine() { // (3)
    return args -> {
      // (4)
    };
  }
}

(1) First, annotate this class to mark it as a configuration bean
(2) Add annotation to say that this method return a new Bean Spring
(3) Then, tell Spring that here we need here a GreetingService component, by declaring it as a method argument
(4) Finally, call here some service method to output the "Hello, Spring!" message at startup; since we’re getting GreetingService, no need to instantiate one manually

Starting your application, you should see something like:

2023-08-23T19:59:02.183+02:00  INFO 152677 --- [  restartedMain] o.s.b.d.a.OptionalLiveReloadServer       : LiveReload server is running on port 35729
2023-08-23T19:59:02.210+02:00  INFO 152677 --- [  restartedMain] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8085 (http) with context path ''
2023-08-23T19:59:02.219+02:00  INFO 152677 --- [  restartedMain] c.e.spring.automacorp.AutomacorpApplication  : Started AutomacorpApplication in 1.825 seconds (process running for 2.506)
Hello, Spring!

Other cases

Now, we’re going to test a few cases to understand how a Spring Application reacts to some situations. For each case, try the suggested modifications, restart your application and see what happens.

Of course, after each case, revert those changes, to get "back to normal". (You can use Git for that)

What happens if you comment the @Component / @Service annotation on your ConsoleGreetingService?
Now, try adding AnotherConsoleGreetingService (which says "Bonjour" instead of "Hello"), marked as a component as well. Try again this time after adding a @Primary annotation on ConsoleGreetingService.
Finally, try the following - what happens and why?

@Service
public class ConsoleGreetingService implements GreetingService {

  private final CycleService cycleService;

  @Autowired
  public ConsoleGreetingService(CycleService cycleService) {
    this.cycleService = cycleService;
  }

  @Override
  public void greet(String name) {
    System.out.println("Hello, " + name + "!");
  }
}

@Service
public class CycleService {

  private final ConsoleGreetingService consoleGreetingService;

  @Autowired
  public CycleService(ConsoleGreetingService consoleGreetingService) {
    this.consoleGreetingService = consoleGreetingService;
  }
}

@Primary is not the only way to resolve multiple candidates, you can also use @Qualifier; check its javadoc to see how you could use it.

More information on @Primary here, and qualifiers here.

Conclusion

Does Spring Framework be only Dependency Injection container? The answer is No.

It builds on the core concept of Dependeny Injection but comes with a number of other features (Web, Persistence, etc.) which bring simple abstractions.

Aim of these abstractions is to reduce Boilerplate Code and Duplication Code, promoting Loose Coupling of your application architecture.