Spring Beans and Dependency Injection
This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Boot 4.1.1!

You are free to use any of the standard Spring Framework techniques to define your beans and their injected dependencies. We generally recommend using constructor injection to wire up dependencies and @ComponentScan to find beans.

If you structure your code as suggested above (locating your application class in a top package), you can add @ComponentScan without any arguments or use the @SpringBootApplication annotation which implicitly includes it. All of your application components (@Component, @Service, @Repository, @Controller, and others) are automatically registered as Spring Beans.

The following example shows a @Service Bean that uses constructor injection to obtain a required RiskAssessor bean:

  • Java
  • Kotlin
import org.springframework.stereotype.Service;

@Service
public class MyAccountService implements AccountService {

    private final RiskAssessor riskAssessor;

    public MyAccountService(RiskAssessor riskAssessor) {
        this.riskAssessor = riskAssessor;
    }

    // ...

}

import org.springframework.stereotype.Service

@Service
class MyAccountService(private val riskAssessor: RiskAssessor) : AccountService

If a bean has more than one constructor, you will need to mark the one you want Spring to use with @Autowired:

  • Java
  • Kotlin
import java.io.PrintStream;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class MyAccountService implements AccountService {

    private final RiskAssessor riskAssessor;

    private final PrintStream out;

    @Autowired
    public MyAccountService(RiskAssessor riskAssessor) {
        this.riskAssessor = riskAssessor;
        this.out = System.out;
    }

    public MyAccountService(RiskAssessor riskAssessor, PrintStream out) {
        this.riskAssessor = riskAssessor;
        this.out = out;
    }

    // ...

}

import org.springframework.beans.factory.annotation.Autowired
import org.springframework.stereotype.Service
import java.io.PrintStream

@Service
class MyAccountService : AccountService {

    private val riskAssessor: RiskAssessor

    private val out: PrintStream

    @Autowired
    constructor(riskAssessor: RiskAssessor) {
        this.riskAssessor = riskAssessor
        out = System.out
    }

    constructor(riskAssessor: RiskAssessor, out: PrintStream) {
        this.riskAssessor = riskAssessor
        this.out = out
    }

    // ...

}
Notice how using constructor injection lets the riskAssessor field be marked as final, indicating that it cannot be subsequently changed.