Learn how to write Spring Setter Dependency Injection Example. The Setter Injection is a type of Spring Dependency Injection. Spring supports Field Injection, Setter Injection as well as Constructor injection to inject dependencies in a Spring managed bean. Scope of this tutorial is limited to Setter Injections.

More on Spring Dependency Injection:

• Spring Constructor Dependency Injection Example
• Spring Field Dependency Injection Example
• Spring Dependency Injection – Field vs Setter vs Constructor Injection
• Spring Dependency Injection and Inversion of Control

Consider, we have a DogsService, which is a Spring based REST Service.

We will write a DogsController, DogsService, and a DogsDao. This is a dummy service which does nothing. Intention here is to see how the Spring Dependency Injection works with Setter methods.

Don’t know how to write Spring Boot Rest Service?

Read this: Spring Boot Rest Service

Want to learn more about Spring Framework ?

Read this:

• Introduction to Spring Framework
• Spring Framework Architecture
• Spring Dependency Injection and Inversion of Control
• Spring Boot Rest Service

DogsDao.java

There are no fields hence no dependency. We have added a no-argument constructor with a print message.

package com.amitph.spring.dogs.dao;
 
import com.amitph.spring.dogs.repo.Dog;
import org.springframework.stereotype.Component;
 
import java.util.List;
 
@Component
public class DogsDao {
 public DogsDao(){
 System.out.println("DogsDao no-arg constructor called");
 }
 public List<Dog> getAllDogs() {
 System.out.println("DogsDao.getAllDogs called");
 return null;
 }
}

DogsService.java

The DogsService depends on DogsDao. In the class below the setter method is annotated with @Autowired. To see how the Setter Injection works we have added a print statement in the setter method. Along with a setter method, we have also added a no-argument constructor and a parameterised constructor with respective print messages.

package com.amitph.spring.dogs.service;
 
import com.amitph.spring.dogs.dao.DogsDao;
import com.amitph.spring.dogs.repo.Dog;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
 
import java.util.List;
 
@Component
public class DogsService {
 private DogsDao dao;
 
 public List<Dog> getDogs() {
 System.out.println("DogsService.getDogs called");
 return dao.getAllDogs();
 }
 
 @Autowired
 public void setDao(DogsDao dao) {
 System.out.println("DogsService setter called");
 this.dao = dao;
 }
 
 public DogsService(){
 System.out.println("DogsService no-arg constructor called");
 }
 public DogsService(DogsDao dao) {
 System.out.println("DogsService arg constructor called");
 this.dao = dao;
 }
}

DogsController.java

The DogsController depends upon DogsService. The setter is annotated with @Autowired and has a print statement. Along with the setter method, we have also added a no-argument and a parameterised constructor along with respective print messages.

package com.amitph.spring.dogs.web;
 
import com.amitph.spring.dogs.repo.Dog;
import com.amitph.spring.dogs.service.DogsService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
 
import java.util.List;
 
@RestController
@RequestMapping("/dogs")
public class DogsController {
 private DogsService service;
 
 @GetMapping
 public List<Dog> getDogs() {
 return service.getDogs();
 }
 
 @Autowired
 public void setService(DogsService service) {
 System.out.println("DogsController setter called");
 this.service = service;
 }
}

Application Startup

 
 . ____ _ __ _ _
 /\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/ ___)| |_)| | | | | || (_| | ) ) ) )
 ' |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot :: (v2.1.0.RELEASE)
 
2019-02-04 19:06:17.058 INFO 68545 --- [ main] com.amitph.spring.dogs.Application : Starting Application on Amitsofficemac.gateway with PID 68545 (/Users/aphaltankar/Workspace/personal/dog-service-jpa/out/production/classes started by aphaltankar in /Users/aphaltankar/Workspace/personal/dog-service-jpa)
2019-02-04 19:06:17.061 INFO 68545 --- [ main] com.amitph.spring.dogs.Application : No active profile set, falling back to default profiles: default
2019-02-04 19:06:17.670 INFO 68545 --- [ main] .s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data repositories in DEFAULT mode.
2019-02-04 19:06:17.724 INFO 68545 --- [ main] .s.d.r.c.RepositoryConfigurationDelegate : Finished Spring Data repository scanning in 48ms. Found 1 repository interfaces.
2019-02-04 19:06:17.992 INFO 68545 --- [ main] trationDelegate$BeanPostProcessorChecker : Bean 'org.springframework.transaction.annotation.ProxyTransactionManagementConfiguration' of type [org.springframework.transaction.annotation.ProxyTransactionManagementConfiguration$$EnhancerBySpringCGLIB$$4a5366ed] is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)
2019-02-04 19:06:18.225 INFO 68545 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port(s): 8080 (http)
2019-02-04 19:06:18.237 INFO 68545 --- [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]
2019-02-04 19:06:18.237 INFO 68545 --- [ main] org.apache.catalina.core.StandardEngine : Starting Servlet Engine: Apache Tomcat/9.0.12
2019-02-04 19:06:18.242 INFO 68545 --- [ main] o.a.catalina.core.AprLifecycleListener : The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [/Users/aphaltankar/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.]
2019-02-04 19:06:18.315 INFO 68545 --- [ main] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
2019-02-04 19:06:18.315 INFO 68545 --- [ main] o.s.web.context.ContextLoader : Root WebApplicationContext: initialization completed in 1205 ms
2019-02-04 19:06:18.339 INFO 68545 --- [ main] o.s.b.w.servlet.ServletRegistrationBean : Servlet dispatcherServlet mapped to [/]
2019-02-04 19:06:18.342 INFO 68545 --- [ main] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'characterEncodingFilter' to: [/*]
2019-02-04 19:06:18.342 INFO 68545 --- [ main] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]
2019-02-04 19:06:18.342 INFO 68545 --- [ main] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'formContentFilter' to: [/*]
2019-02-04 19:06:18.342 INFO 68545 --- [ main] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'requestContextFilter' to: [/*]
2019-02-04 19:06:18.434 INFO 68545 --- [ main] com.zaxxer.hikari.HikariDataSource : HikariPool-1 - Starting...
2019-02-04 19:06:18.524 INFO 68545 --- [ main] com.zaxxer.hikari.HikariDataSource : HikariPool-1 - Start completed.
2019-02-04 19:06:18.645 INFO 68545 --- [ main] o.hibernate.jpa.internal.util.LogHelper : HHH000204: Processing PersistenceUnitInfo [
	name: default
	...]
2019-02-04 19:06:18.690 INFO 68545 --- [ main] org.hibernate.Version : HHH000412: Hibernate Core {5.3.7.Final}
2019-02-04 19:06:18.691 INFO 68545 --- [ main] org.hibernate.cfg.Environment : HHH000206: hibernate.properties not found
2019-02-04 19:06:18.779 INFO 68545 --- [ main] o.hibernate.annotations.common.Version : HCANN000001: Hibernate Commons Annotations {5.0.4.Final}
2019-02-04 19:06:18.868 INFO 68545 --- [ main] org.hibernate.dialect.Dialect : HHH000400: Using dialect: org.hibernate.dialect.MySQL5InnoDBDialect
2019-02-04 19:06:19.279 INFO 68545 --- [ main] j.LocalContainerEntityManagerFactoryBean : Initialized JPA EntityManagerFactory for persistence unit 'default'
DogsDao no-arg constructor called
DogsService no-arg constructor called
DogsService setter called
DogsController no-arg constructor called
DogsController setter called
2019-02-04 19:06:19.650 INFO 68545 --- [ main] o.s.s.concurrent.ThreadPoolTaskExecutor : Initializing ExecutorService 'applicationTaskExecutor'
2019-02-04 19:06:19.681 WARN 68545 --- [ main] aWebConfiguration$JpaWebMvcConfiguration : spring.jpa.open-in-view is enabled by default. Therefore, database queries may be performed during view rendering. Explicitly configure spring.jpa.open-in-view to disable this warning
2019-02-04 19:06:19.856 INFO 68545 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port(s): 8080 (http) with context path ''
2019-02-04 19:06:19.859 INFO 68545 --- [ main] com.amitph.spring.dogs.Application : Started Application in 3.138 seconds (JVM running for 3.647)
 

What happened here is the Spring first tried to instantiate DogsController and found a dependency over DogsService. Hence, it went to instantiate DogsService, which in-turn has a dependency over DogsDao. Hence the DogsDao is the first to get instantiated, then DogsService and finally the DogsController.

Let’s see what happened in detail.

• Line #36: No-argument constructor of DogsDao is called.
• Line #37: No-argument constructor of DogsService is called. Note: the parameterised constructor never got called.
• Line #38: Setter of DogsService is called. Where instance of DogsDao (created at line #36) is injected.
• Line #39: No-argument constructor of DogsController is called.
• Line #40: Setter of DogsController is called. The DogsService instance (created in line #37) is injected.

Summary

You learnt How to write a Setter Injection in a Spring Application. In case of Setter Injection, the setter methods are annotated with @Autowired. Spring will first use the no-argument constructor to instantiate the bean and then call setter methods to inject the dependencies.

In the coming tutorial we will see how the Constructor Based Injection works.