This tutorial will teach you how to use constructor-based dependency injection(DI) in Spring Framework when building RESTful Web Services.
Read the following tutorial to learn about the different types of dependency injection in Spring and which one may be better suited for your specific use case.
Constructor-Based Dependency Injection Overview
Constructor-based dependency injection is a technique used in the Spring Framework to manage dependencies between components of an application. Dependency injection is a design pattern that allows you to decouple your components and makes your code more modular and testable.
In constructor-based dependency injection, dependencies are injected through the constructor of a class. This means that when a class is instantiated, its dependencies are passed in as arguments to the constructor. The Spring Framework then manages the dependencies and ensures that the correct instances are passed in.
Constructor-based dependency injection has several advantages over other types of dependency injection. Firstly, it ensures that all dependencies are fully initialized when the object is constructed, making it easy to reason about the state of an object. Secondly, it makes it impossible to create an object with missing dependencies, ensuring that your code is always in a valid state. Finally, constructor-based dependency injection makes it easy to use immutable objects, which can help to reduce bugs and make your code more reliable.
Overall, constructor-based dependency injection is a powerful technique for managing dependencies in your Spring application. It allows you to create decoupled, modular components that are easy to test and maintain, and can help to improve the overall quality of your code.
Example
Let’s say we have a UserService interface and an implementation class UserServiceImpl. UserServiceImpl has a dependency on an EncryptionUtils class that is used to encrypt user data. We want to inject an instance of EncryptionUtils into UserServiceImpl using constructor-based dependency injection.
With constructor-based dependency injection, instead of manually creating an instance of EncryptionUtils within UserServiceImpl, we define a constructor for UserServiceImpl that takes an instance of EncryptionUtils as a parameter. We then let the Spring Framework handle the creation and injection of the EncryptionUtils instance into UserServiceImpl.
This approach has a number of benefits:
- It makes the code more modular and easier to test, as we can easily swap out the implementation of
EncryptionUtilswith a mock or stub for testing purposes. - It makes the code more maintainable, as we can easily change the implementation of
EncryptionUtilswithout having to modify the code ofUserServiceImpl. - It promotes loose coupling between classes, as
UserServiceImpldoesn’t need to know howEncryptionUtilsis created or what other dependencies it may have.
Overall, constructor-based dependency injection is a powerful technique for managing dependencies between objects in a Spring application, and can help make the code more modular, testable, and maintainable.
Injecting an Object
Let’s assume we need to inject a Utility class into our Service class. Below is an example of a UserService class which uses a Constructor-Based dependency injection to inject a Utility class.
Note the use of the @Autowired annotation. If your class has a single constructor, using the @Autowired annotation is optional.
package com.appsdeveloperblog.examples.constructordi.ConstructorDI.serivce;
import com.appsdeveloperblog.examples.constructordi.ConstructorDI.UserUtils;
import com.appsdeveloperblog.examples.constructordi.ConstructorDI.model.UserDTO;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class UserServiceImpl implements UserService {
UserUtils utils;
@Autowired
public UserServiceImpl(UserUtils utils)
{
this.utils = utils;
}
@Override
public UserDTO getUser(String userId) {
UserDTO user = new UserDTO();
user.setId(utils.generateUserId());
user.setEmail("[email protected]");
user.setFirstName("Sergey");
user.setLastName("Kargopolov");
return user;
}
}
For a UserUtils class to be injected into a constructor of UserServiceImpl class, the UserUtils class needs to be either a @Service or a @Component. Below is a source code of UserUtils class which is annotated with @Component annotation.
package com.appsdeveloperblog.examples.constructordi.ConstructorDI;
import java.util.UUID;
import org.springframework.stereotype.Component;
@Component
public class UserUtils {
public String generateUserId()
{
return UUID.randomUUID().toString();
}
}
Constructor with Two Strings
Sometimes we need to inject an object with a constructor that takes two arguments, such as strings or primitive data types. Let’s assume we need to inject an object that has a constructor with two string arguments.
Below is an example of the EncryptionUtils class that we will inject into a UserServiceImpl class.
package com.appsdeveloperblog.examples.constructordi.ConstructorDI;
public class EncryptionUtils {
private String keyType;
private String certificate;
public EncryptionUtils(String keyType, String certificate)
{
this.keyType = keyType;
this.certificate = certificate;
}
public String getKeyType() {
return keyType;
}
public void setKeyType(String keyType) {
this.keyType = keyType;
}
public String getCertificate() {
return certificate;
}
public void setCertificate(String certificate) {
this.certificate = certificate;
}
}
We will use the following constructor to inject the above EncryptionUtils class into a UserServiceImpl class.
package com.appsdeveloperblog.examples.constructordi.ConstructorDI.serivce;
import com.appsdeveloperblog.examples.constructordi.ConstructorDI.EncryptionUtils;
import com.appsdeveloperblog.examples.constructordi.ConstructorDI.UserUtils;
import com.appsdeveloperblog.examples.constructordi.ConstructorDI.model.UserDTO;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class UserServiceImpl implements UserService {
UserUtils userUtils;
EncryptionUtils encryptionUtils;
@Autowired
public UserServiceImpl(UserUtils userUtils, EncryptionUtils encryptionUtils)
{
this.userUtils = userUtils;
this.encryptionUtils = encryptionUtils;
}
}
In order for the EncryptionUtils class to be properly instantiated and injected, we need to instantiate it either in a configuration class or in a class marked with the @SpringBootApplication annotation.
Notice the @Bean annotation used above the method. When @Bean is used above a method, Spring Framework will add an object returned by this method to the Spring Application Context. Once the object is added to the Spring Application Context, it can be used for dependency injection in other classes.
package com.appsdeveloperblog.examples.constructordi.ConstructorDI;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
@SpringBootApplication
public class ConstructorDiApplication {
public static void main(String[] args) {
SpringApplication.run(ConstructorDiApplication.class, args);
}
@Bean
public EncryptionUtils getEncryptionUtils()
{
return new EncryptionUtils("3DES", "X509Certificate");
}
}
Video Tutorial
I hope this tutorial was helpful to you. If you would like to learn more about building RESTful Web Services with Spring Framework, please check out the following page on this site: Spring Boot and Spring MVC.
Frequently asked questions
- What is the difference between constructor-based and setter-based dependency injection?
Constructor-based dependency injection involves injecting dependencies through the constructor of a class, while setter-based dependency injection involves injecting dependencies through setter methods. Constructor-based injection is typically preferred because it ensures that all dependencies are available when the object is created, and it makes the class immutable. - How do I decide which type of dependency injection to use?
It’s generally recommended to use constructor-based injection whenever possible, as it makes the code more modular, testable, and maintainable. Setter-based injection can be useful in certain situations, such as when dealing with optional dependencies or when working with legacy code that doesn’t support constructor injection. - Can I use constructor-based injection with circular dependencies?
Yes, you can use constructor-based injection with circular dependencies by using the@Autowiredannotation on the constructor. However, it’s generally recommended to avoid circular dependencies if possible, as they can make the code more difficult to understand and maintain. - Can I mix constructor-based and setter-based injection in the same class?
Yes, you can mix constructor-based and setter-based injection in the same class if necessary. However, it’s generally recommended to use one type of injection consistently throughout the application for consistency and maintainability. - How do I handle dependencies that require additional configuration or setup?
You can use the@Beanannotation to define a method that creates and configures a bean, and then use constructor-based injection to inject the bean into other classes. Alternatively, you can use the@PostConstructannotation to define a method that is called after the bean is initialized, and use that method to perform any necessary configuration or setup.