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Use Factory Method Injection in @Configuration Classes in Java

Medium
maintainabilityperformance optimization

What is it?​

This practice recommends the use of constructor or method parameter injection instead of field injection in @Configuration classes.

Why apply it?​

Field injection can cause early initialization of beans or unnecessary context searching. By using method parameter injection, dependencies are requested as late as possible, optimizing the way the application context loads.

How to fix it?​

Define dependencies as method parameters in @Bean factory methods instead of annotating fields with @Autowired.

Examples​

Example 1:​

Negative​

The negative example uses field injection, causing early or undesirable initialization of beans.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import javax.sql.DataSource;

@Configuration
public class FooConfiguration {

  @Autowired
  private DataSource dataSource;  // Noncompliant

  @Bean
  public MyService myService() {
    return new MyService(this.dataSource);
  }
}

Example 2:​

Positive​

The positive example uses method parameter injection, optimizing when dependencies are resolved within the Spring context.

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import javax.sql.DataSource;

@Configuration
public class FooConfiguration {

  @Bean
  public MyService myService(DataSource dataSource) {  // Compliant
    return new MyService(dataSource);
  }
}

Negative​

The negative example uses field injection for multiple dependencies, which may lead to unintended behavior.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class BarConfiguration {

  @Autowired
  private Service1 service1;  // Noncompliant

  @Autowired
  private Service2 service2;  // Noncompliant

  @Bean
  public AnotherService anotherService() {
    return new AnotherService(service1, service2);
  }
}

Example 3:​

Positive​

The positive example leverages method parameter injection for multiple dependencies.

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class BarConfiguration {

  @Bean
  public AnotherService anotherService(Service1 service1, Service2 service2) {  // Compliant
    return new AnotherService(service1, service2);
  }
}